THE DEMAND SIDE 需求侧

1.1 OVERVIEW 1.1 概述

In the late 2000s, households and firms across the world cut back their spending and the global economy went into recession. There is little disagreement among economists that the dramatic fall in global GDP in 2008-09, which is now known as the global financial crisis, was a negative shock to aggregate demand. The Covid-19 pandemic led to a much larger contraction of global economic activity in the first half of 2020 than had occurred during the financial crisis. A combination of government mandated lock-downs of businesses and travel, and decisions by citizens to reduce their face-to-face interactions produced a collapse in demand for goods and services.
在 2000 年代末，全球的家庭和企业减少了支出，全球经济陷入衰退。经济学家们几乎没有分歧，认为 2008-09 年全球 GDP 的剧烈下降，即现在所称的全球金融危机，是对总需求的负面冲击。新冠疫情导致 2020 年上半年全球经济活动的收缩远远超过金融危机期间的情况。政府强制实施的商业和旅行封锁，以及公民减少面对面互动的决定，导致了对商品和服务需求的崩溃。

This chapter begins the task of building a macroeconomic model with the demand side of the economy, which concerns the spending decisions of households, firms and government. As we shall see in Chapter 2, the supply side refers to the production activities in the economy. Dramatic changes in aggregate demand-that is, spending in the economy as a whole-as occurred in 2020 or in 2008-09 are unusual. In more normal times, shifts in aggregate spending decisions are the main sources of the irregular fluctuations from recession to boom that form the business cycle.
本章开始构建一个宏观经济模型，侧重于经济的需求侧，涉及家庭、企业和政府的支出决策。正如我们将在第二章中看到的，供给侧指的是经济中的生产活动。2020 年或 2008-09 年发生的总需求剧烈变化——即整个经济的支出——是非常不寻常的。在更正常的时期，总支出决策的变化是导致经济周期中从衰退到繁荣的不规则波动的主要来源。

If a period of depressed spending is forecast, there is much discussion in the financial media about whether the central bank will intervene and offset the likely recession by loosening monetary policy, for example, by cutting the interest rate. The central bank would cut the interest rate because it expects this to encourage spending and help return the economy to stability. Similarly, if a boom in spending were forecast, the central bank would try to dampen it down by raising the interest rate. As well as the central bank, the government also needs to know how spending patterns are likely to evolve and affect output. A recession will depress tax revenue and increase spending on unemployment benefits. Forecasting patterns of spending is therefore a priority, not only for businesses, but also for the monetary and fiscal policy makers.
如果预测到消费支出将出现一段时间的低迷，金融媒体上会有很多讨论，关于中央银行是否会干预，通过放松货币政策来抵消可能的经济衰退，例如，降低利率。中央银行会降低利率，因为它预计这将鼓励消费支出，并帮助经济恢复稳定。同样，如果预测到消费支出将出现繁荣，中央银行会试图通过提高利率来抑制这种情况。除了中央银行，政府也需要了解消费模式可能如何演变并影响产出。经济衰退将压低税收收入，并增加失业救济支出。因此，预测消费模式是一个优先事项，不仅对企业而言，对货币和财政政策制定者也是如此。

Aggregate demand and spending decisions
总需求和支出决策

In this chapter, we focus on what lies behind the spending decisions in the economy and how they influence the level of economic activity. By the level of activity, we mean output or income. When output changes, employment also changes. For example, when output rises, more workers or longer hours are needed to produce the higher level of output. With more hours worked, the wage bill is larger and with higher sales, total profits are higher. This is why changes in economic activity are thought of as both changes in output and in income (i.e. wages and profits). When we think of a real-world
在本章中，我们关注经济中支出决策背后的因素以及它们如何影响经济活动水平。我们所说的活动水平是指产出或收入。当产出变化时，就业也会发生变化。例如，当产出上升时，需要更多的工人或更长的工作时间来生产更高水平的产出。随着工作时间的增加，工资支出也会增加，而随着销售额的提高，总利润也会增加。这就是为什么经济活动的变化被视为产出和收入（即工资和利润）的变化。当我们考虑现实世界时

economy, output is normally growing and recessions and booms produce fluctuations around a trend growth rate. However, we will often simplify by working with levels of output rather than its rate of growth.
经济中，产出通常在增长，衰退和繁荣会导致围绕趋势增长率的波动。然而，我们通常会通过关注产出水平而不是其增长率来简化分析。
Spending decisions are complex and forward-looking. For consumers they involve questions like ‘how do I allocate my spending over time given my current income and wealth, and how I expect my income to evolve in the future?’. Firms make decisions to purchase machinery and equipment and to build new premises based on a business plan that includes forecasts about how input costs and demand for their products will evolve over time. The government must also forecast demographic trends when making plans for building new schools and hospitals.
支出决策是复杂且具有前瞻性的。对于消费者来说，这涉及到诸如“在考虑到我当前的收入和财富，以及我对未来收入变化的预期时，我该如何分配我的支出？”的问题。企业根据商业计划做出购买机械设备和建设新场所的决策，该计划包括对输入成本和产品需求随时间变化的预测。政府在制定新学校和医院建设计划时，也必须预测人口趋势。
Decisions about spending by the entities that make up the economy-firms, families and government bodies-lie behind the demand side of the economy. Macroeconomics is concerned with the aggregate sum of spending decisions of these groups, and the consequences of those decisions for economy-wide outcomes such as the rate of unemployment or inflation. Household spending decisions add up to aggregate consumption, C; firms’ investment decisions add up to aggregate investment, I (note that I refers to spending on machinery, equipment and new houses and other buildings); and government spending on different goods and services adds up to a single number, G. Including the purchase of new business premises as part of investment highlights the difference between on the one hand, aggregate demand, which refers to spending on goods and services and on the other hand, the purchase of assets such as company shares or second-hand property. Buying a second-hand factory building does not contribute to aggregate demand. It is the transfer of the ownership of an asset from one business to another. In contrast, the sale of new business premises reflects the goods and services used in its construction and sale.
关于经济中各个实体（企业、家庭和政府机构）支出的决策构成了经济的需求侧。宏观经济学关注这些群体支出决策的总和，以及这些决策对经济整体结果（如失业率或通货膨胀）的影响。家庭的支出决策汇总为总消费 C；企业的投资决策汇总为总投资 I（注意，I 指的是对机械、设备、新房和其他建筑的支出）；政府在不同商品和服务上的支出汇总为一个数字 G。将新商业场所的购买视为投资的一部分，突显了总需求与资产购买之间的区别，后者包括公司股票或二手房产的购买。购买二手工厂建筑并不贡献于总需求。这是资产从一个企业转移到另一个企业的所有权转移。与此相反，新商业场所的销售反映了其建设和销售中使用的商品和服务。
Although the first chapters in the book concentrate on a closed economy (a single nation without links with others) as noted above, in the fuller model (set out in Chapters 11 and 12) the demand side includes foreign spending on home goods and services, exports ( X ), and domestic spending on foreign goods and services, imports

(M)

. Aggregate demand is real expenditure on goods and services produced in the home economy. This can be summarized by an equation relating real expenditure, which is called

y^{D}

, to its individual components:
尽管本书的前几章集中于封闭经济（一个没有与其他国家联系的单一国家），如上所述，在更完整的模型（在第 11 章和第 12 章中阐述）中，需求侧包括对本国商品和服务的外国支出、出口（X）以及对外国商品和服务的国内支出、进口

(M)

。总需求是对本国经济中生产的商品和服务的实际支出。这可以通过一个方程来总结，该方程将实际支出（称为

y^{D}

）与其各个组成部分联系起来：

y^{D} = C + I + G + (X - M), (aggregate demand)

where we add to the trio of

C, I

and G , the expenditure of foreigners on home output, X and subtract the spending of home agents on output produced abroad, M .
我们在

C, I

和 G 的三重奏中增加外国对国内产出的支出 X，并减去国内代理对国外生产的产出的支出 M。

Aggregate demand and government policy
总需求与政府政策

Two of the major tools of macroeconomic policy, monetary and fiscal policy, work by influencing different elements on the demand side. Policy makers worry about fluctuations in aggregate demand because they affect household wellbeing via unemployment and inflation.
宏观经济政策的两个主要工具，货币政策和财政政策，通过影响需求侧的不同要素来发挥作用。政策制定者担心总需求的波动，因为它们通过失业和通货膨胀影响家庭福祉。
Monetary policy seeks to stabilize aggregate demand by changing interest rates, which affect the investment decisions of firms and the purchase of new houses and
货币政策旨在通过改变利率来稳定总需求，这会影响企业的投资决策和新房的购买
durable goods like new cars and furniture by households. A rise in the interest rate increases the cost of financing investment projects, and projects that would have gone ahead with lower interest rates are postponed or cancelled. Monetary policy also has indirect effects because the interest rate affects incentives to save and therefore shifts spending decisions over time. For example, by making new borrowing more expensive and increasing the return on saving, a higher interest rate will encourage households to postpone consumption.
耐用消费品，如新车和家具，由家庭购买。利率上升会增加投资项目的融资成本，而本来在较低利率下会进行的项目则被推迟或取消。货币政策也有间接影响，因为利率影响储蓄的激励，从而改变消费决策的时间。例如，通过使新借款变得更昂贵并提高储蓄的回报，较高的利率将鼓励家庭推迟消费。

Through changes in government spending on goods and services (G), fiscal policy affects aggregate demand directly. Fiscal policy can also be used to affect demand indirectly through its influence on household incomes and through that channel, on household spending. Changes in taxation and in the transfers made by the government to households in the form of pensions, disability and unemployment benefits feed through into the spending decisions of households, i.e. into C , to affect aggregate demand.
通过政府对商品和服务（G）的支出变化，财政政策直接影响总需求。财政政策还可以通过对家庭收入的影响以及通过这一渠道对家庭支出的影响，间接影响需求。税收变化以及政府以养老金、残疾和失业救济金形式向家庭转移的资金，会影响家庭的消费决策，即影响 C，从而影响总需求。

An important reason to study the demand side is to construct a model of the transmission mechanism by which monetary and fiscal policy, via the spending decisions of households, firms and the government, affect the economy. Before explaining the first building block of the macro model, which is the IS curve, we provide some facts about the demand side.
研究需求侧的一个重要原因是构建一个传导机制模型，通过家庭、企业和政府的支出决策，货币和财政政策如何影响经济。在解释宏观模型的第一个构建块，即 IS 曲线之前，我们提供一些关于需求侧的事实。

1.1.1 Facts about the demand side and business cycles
1.1.1 关于需求侧和商业周期的事实

Shares of GDP 国内生产总值的份额

Table 1.1 sets out the average composition of gross domestic product (GDP) in five major economies for the period between the financial crisis and the pandemic, from 2010 to 2018. GDP is the national accounts measure of national output. We discuss the calculation of GDP in the next subsection. Table 1.1 shows that consumption makes up the largest proportion of GDP in all the economies with the exception of China. In particular, the importance of consumption in GDP ranges from 37% in China to 68% in the US. A large part of this cross-country variation can be explained by differences in the contribution of investment. For example, an exceptionally high proportion of Chinese GDP arises from investment spending; the UK’s share is low. The table also
表 1.1 列出了 2010 年至 2018 年金融危机与疫情之间五大主要经济体的国内生产总值（GDP）平均构成。GDP 是国家账户衡量国家产出的指标。我们将在下一小节讨论 GDP 的计算。表 1.1 显示，消费在所有经济体中占 GDP 的最大比例，唯独中国例外。特别是，消费在 GDP 中的重要性在中国为 37%，在美国为 68%。这种跨国差异的很大一部分可以用投资贡献的差异来解释。例如，中国 GDP 中异常高的比例来自投资支出；而英国的比例则较低。表格还显示

Consumption 消费

投资包括政府。

Investment

incl. gov.

政府消费支出

Government

cons. spending

Net exports 净出口

China 中国

36.7

44.9

15.9

2.4

Germany 德国

53.6

20.5

19.6

6.3

Japan 日本

57.3

23.2

19.9

-0.4

United Kingdom 英国

64.7

16.7

19.9

-1.4

United States 美国

67.9

20.2

15.0

-3.1

Table 1.1 Shares of GDP (in %), current prices, average 2010-18.
表 1.1 国内生产总值（GDP）份额（%），现行价格，2010-2018 年平均。
Note: GDP in current prices is a measure of nominal GDP; shares might not add up to 100% due to statistical discrepancies.
注意：当前价格的 GDP 是名义 GDP 的一个衡量标准；由于统计差异，份额可能无法加总到 100%。
Source: OECD National Accounts (data accessed July 2020).
来源：经济合作与发展组织国家账户（数据访问于 2020 年 7 月）。
highlights the variation across countries in the contribution to GDP from net exports. On average, between 2010 and 2018, the US, the UK and Japan ran trade deficits with imports in excess of exports, whereas Germany and China were in trade surplus (see Chapter 12 for further discussion and definitions).
强调了各国在净出口对 GDP 贡献方面的差异。2010 年至 2018 年间，美国、英国和日本的贸易出现赤字，进口超过出口，而德国和中国则处于贸易顺差状态（详见第 12 章以获取更多讨论和定义）。

The upper panel of Figure 1.1 shows how the shares of consumption, investment and government spending in GDP changed over time for the UK from 1948 to 2019. From 1948 until the end of the 1960s the share of consumption falls and is mirrored by a rise in the share of investment. The share of investment peaks in 1989, falls sharply in the
图 1.1 的上半部分显示了 1948 年至 2019 年间，英国消费、投资和政府支出在 GDP 中所占份额随时间的变化。从 1948 年到 1960 年代末，消费的份额下降，而投资的份额则相应上升。投资的份额在 1989 年达到峰值，随后急剧下降。

Figure 1.1 Components of GDP in the UK: as a percentage of GDP between 1948 and 2019 (upper panel) and volatility of growth rates between 1958 Q3 and 2017 Q3 (lower panel).
图 1.1 英国 GDP 的组成部分：1948 年至 2019 年间占 GDP 的百分比（上图）以及 1958 年第三季度至 2017 年第三季度的增长率波动性（下图）。
Note: Volatility has been calculated as the standard deviation of the GDP growth rate over a rolling 21-quarter period. The upper graph uses data from the series ‘Gross Domestic Product by category of expenditure, current prices’ and the lower graph uses data from the series ‘Gross Domestic Product by category of expenditure, chained volume measures’.
注意：波动性是通过对 21 个季度滚动期间的 GDP 增长率计算标准差得出的。上图使用的是“按支出类别划分的国内生产总值（现价）”系列的数据，下图使用的是“按支出类别划分的国内生产总值（链式量度）”系列的数据。

Source: UK Office for National Statistics (data accessed July 2020).
来源：英国国家统计局（数据访问于 2020 年 7 月）。
financial crisis and recovers only weakly thereafter. The government’s consumption spending increases sharply in the crisis but then declines as austerity policies were introduced from 2010.
金融危机后仅微弱复苏。政府的消费支出在危机中急剧增加，但在 2010 年实施紧缩政策后则下降。

National accounts 国民账户

The national accounts are used to measure the output of an economy. The most commonly used measure for calculating national output is gross domestic product, or GDP. GDP can be measured in three different ways. All three equations for calculating GDP are identities, which means that because of the way the variables are defined, the left-hand side must always equal the right-hand side. This special feature of an identity is signalled by the use of the equals sign with three bars.
国民账户用于衡量一个经济体的产出。计算国民产出的最常用指标是国内生产总值，或称 GDP。GDP 可以通过三种不同的方式进行测量。计算 GDP 的这三种方程都是恒等式，这意味着由于变量的定义方式，左侧总是等于右侧。恒等式的这一特殊特征通过使用三条横线的等号来表示。

First, is the expenditure method, which is the one we use in the model of the demand side. This method measures GDP as the total expenditure on the economy’s output of goods and services:
首先是支出法，这是我们在需求侧模型中使用的方法。该方法将 GDP 衡量为对经济产出商品和服务的总支出：

y \equiv C + I + G + (X - M)

(GDP identity, expenditure method)
（GDP 恒等式，支出法）
where

y \equiv

output (or GDP),

C \equiv

consumption,

I \equiv

investment (including changes in stocks of raw materials and finished goods),

G \equiv

government spending and (

X - M

)

\equiv

net exports. This is an identity, as it simply breaks down GDP into its constituent components. A fall in one of the components on the right-hand side, for example consumption, will therefore always result in an equivalent reduction in measured GDP on the left-hand side.
其中

y \equiv

输出（或 GDP），

C \equiv

消费，

I \equiv

投资（包括原材料和成品库存的变化），

G \equiv

政府支出和（

X - M

）

\equiv

净出口。这是一个恒等式，因为它只是将 GDP 分解为其组成部分。右侧某个组成部分的下降，例如消费，必然会导致左侧测量的 GDP 相应减少。

When a new house is bought, just like a new piece of machinery, this is an investment decision-the house provides a flow of services to the household over many yearsand is included in the national accounts under the term, I. The house and the piece of machinery form part of the economy’s capital stock. National accountants have to make many tricky classification decisions: although households buy a variety of durable goods that provide services over many years, by convention it is only housing that is considered as investment in the national accounts. Cars and furniture, for example, are treated in the national accounts as consumption.
当购买一栋新房子时，就像购买一台新机器一样，这是一项投资决策——房子在许多年内为家庭提供服务，并在国家账户中被归类为 I。房子和机器都是经济资本存量的一部分。国家会计师必须做出许多棘手的分类决策：尽管家庭购买各种耐用消费品，这些商品在许多年内提供服务，但根据惯例，只有住房被视为国家账户中的投资。例如，汽车和家具在国家账户中被视为消费。

It is important to recall that only government spending on goods and services is part of aggregate demand; G does not include government expenditure on transfers (e.g. pensions or social security payments). When the recipients of transfers spend their income from benefits or pensions, this is then recorded as consumption.
重要的是要记住，只有政府对商品和服务的支出才是总需求的一部分；G 不包括政府对转移支付（例如养老金或社会保障支付）的支出。当转移支付的接受者用他们的福利或养老金收入进行消费时，这被记录为消费。

The expenditure method only includes final goods. For example, during the production process, firms buy raw materials and intermediate goods to make their final goods. The only purchases that are counted in GDP are when the firm sells the finished goods to the consumer. This avoids any double counting. In this way, GDP captures only the value added created in the economy. This leads onto the second approach for calculating GDP; the value added method. This method measures GDP as the value added created in all sectors of the economy, such that:
支出法仅包括最终商品。例如，在生产过程中，企业购买原材料和中间商品以制造最终商品。计入 GDP 的唯一购买是企业将成品出售给消费者时。这避免了任何重复计算。通过这种方式，GDP 仅捕捉到经济中创造的附加值。这引出了计算 GDP 的第二种方法：附加值法。该方法将 GDP 衡量为经济各个部门创造的附加值，具体如下：

y \equiv value of output sold - cost of raw materials and intermediate goods

(GDP identity, value added method)
（GDP 恒等式，增加值法）

The third and final approach for calculating GDP is the income method. This method measures GDP as the total income of all agents in the economy, such that:
计算 GDP 的第三种也是最后一种方法是收入法。该方法将 GDP 视为经济中所有经济主体的总收入，具体如下：

y \equiv

salaries of workers + profits of the owners of capital (GDP identity, income method)

y \equiv

工资 + 资本所有者的利润（GDP 恒等式，收入法）
The three methods of calculating GDP are all identities and each holds at each point in time. It makes intuitive sense that the total income in the economy is equal to the total expenditure on goods and services produced at home, because for every transaction there is both a buyer and a seller. What is expenditure for the buyer is income to the seller. In practice, however, GDP calculated using the three methods may differ. This is a matter of measurement error, which can arise for several reasons. Measurement error may be more serious for one rather than another of the three methods. For example, tax evasion or the existence of a substantial black market are likely to produce a greater problem of underestimation of GDP when measured by incomes than for the other methods. If the underlying components could be measured completely accurately, then each of the three approaches would yield exactly the same estimate of GDP.
计算 GDP 的三种方法都是恒等式，并且在每个时间点上都成立。经济中的总收入等于国内生产的商品和服务的总支出，这在直观上是合理的，因为每一笔交易都有买方和卖方。对买方而言的支出对卖方而言就是收入。然而，在实践中，使用三种方法计算的 GDP 可能会有所不同。这是一个测量误差的问题，可能由于多种原因而产生。测量误差在三种方法中可能对某一种方法更为严重。例如，逃税或存在大量黑市在通过收入测量 GDP 时，可能会导致更严重的低估问题，而对其他方法的影响则较小。如果基础组成部分能够被完全准确地测量，那么这三种方法将会得出完全相同的 GDP 估算值。

Relative volatility 相对挥发性

The lower panel of Figure 1.1 shows the relative volatility of growth rates for the three components of aggregate demand: a higher standard deviation means higher volatility. We can see that investment is much more volatile than consumption and government spending: the line showing the standard deviation for investment lies above that for consumption and government spending. Investment depends on expected post-tax profits and is very dependent on how optimistic firms are, so it tends to flourish in boom periods and collapse in recessions, making it more volatile than the other components of GDP, or GDP itself. In addition to this, investment can also be postponed in recessions, whereas government current spending and consumption cannot be as easily delayed. For example, a household still has to spend money on food and drink in a recession, whereas a firm may choose to wait until the economy has recovered before it undertakes investment. We shall also see-in Section 1.2.6that fixed investment decisions are often bunched, further accounting for the greater volatility of investment.
图 1.1 的下半部分显示了总需求三个组成部分的增长率相对波动性：标准差越大，波动性越高。我们可以看到，投资的波动性远高于消费和政府支出：投资的标准差线位于消费和政府支出之上。投资依赖于预期的税后利润，并且非常依赖于企业的乐观程度，因此在经济繁荣时期往往蓬勃发展，而在经济衰退时则会崩溃，使其比 GDP 的其他组成部分或 GDP 本身更具波动性。此外，投资在经济衰退时也可以被推迟，而政府的当前支出和消费则不容易被延迟。例如，家庭在经济衰退时仍然需要花钱购买食品和饮料，而企业可能选择等到经济复苏后再进行投资。我们还将在 1.2.6 节中看到，固定投资决策往往是集中在一起的，这进一步解释了投资的更大波动性。

Growth and cycles 增长与周期

The idea that economies fluctuate between phases of boom and recession is confirmed by looking at the data for the US. Figure 1.2 illustrates the long-run growth and business cycles in the US economy. In the top panel, the log of GDP in constant prices is plotted and shows the rather steady growth rate of GDP over the long run. Using the log scale, a straight line would represent a constant growth rate from 1948 to 2022.

^{1}

In fact, average annual growth rates in each decade from 1948 were:

4.3 %, 4.5 %, 3.2 %, 3.1 %

3.2 %, 1.9 %, 2.3 %

, and

1.5 %

between 2020 and the first quarter of 2022.
经济在繁荣和衰退阶段之间波动的观点通过观察美国的数据得到了证实。图 1.2 展示了美国经济的长期增长和商业周期。在顶部面板中，以不变价格计算的 GDP 的对数被绘制出来，显示了 GDP 在长期内相对稳定的增长率。使用对数刻度，1948 年至 2022 年期间的直线将代表一个恒定的增长率。实际上，从 1948 年开始，每个十年的平均年增长率分别为：

4.3 %, 4.5 %, 3.2 %, 3.1 %

、

3.2 %, 1.9 %, 2.3 %

和

1.5 %

，在 2020 年到 2022 年第一季度之间。

Figure 1.2 Business cycles in the United States between 1948 and 2022: Log of real GDP (upper panel) and real GDP growth (lower panel)
图 1.2 1948 年至 2022 年美国的商业周期：实际 GDP 的对数（上图）和实际 GDP 增长（下图）
Note: The shaded grey areas represent recessions as defined by NBER. Both graphs use data from the series ‘Real GDP, chained dollars, billions of chained (2012) dollars.’
注意：阴影灰色区域表示 NBER 定义的经济衰退。两个图表使用的数据来自“实际 GDP，链式美元，十亿链式（2012）美元”系列。
Source: US Bureau of Economic Analysis (data accessed September 2022).
来源：美国经济分析局（数据访问于 2022 年 9 月）。

By plotting the annual growth rate of GDP in the lower panel, the fluctuations of the economy around the long-run trend are highlighted. The dotted line represents the average growth rate between 1948 and 2019 of

3.2 %

. Peaks and troughs of growth occur about twice a decade. In the US, an independent body called the National Bureau of Economic Research (NBER) establishes the dates of US business cycles and states there have been twelve separate contractions (i.e. recessions) over this period, which are represented by the shaded grey areas on the graphs. Recessions start at the peak of a business cycle and end at the trough: during recessions, economic activity is contracting. In the period we are analysing, the recessions range from 6 to 18 months in duration. This is why the annual GDP growth rate shown in Figure 1.2 is not negative in all of the recessions (e.g. the 2001 recession). The longest of these recessions was the 2007-09 global financial crisis, which spanned a year and a half.
通过在下方图表中绘制 GDP 的年增长率，突显了经济围绕长期趋势的波动。虚线表示 1948 年至 2019 年间的平均增长率

3.2 %

。增长的高峰和低谷大约每十年出现两次。在美国，一个名为国家经济研究局（NBER）的独立机构确定美国商业周期的日期，并指出在此期间发生了十二次独立的经济收缩（即衰退），这些衰退在图表上用阴影灰色区域表示。衰退始于商业周期的高峰，结束于低谷：在衰退期间，经济活动正在收缩。在我们分析的期间，衰退的持续时间从 6 个月到 18 个月不等。这就是为什么图 1.2 中显示的年 GDP 增长率在所有衰退中并不都是负值（例如 2001 年的衰退）。其中持续时间最长的衰退是 2007-09 年的全球金融危机，持续了一年半。

Before the global financial crisis many macroeconomists claimed that better policy making had improved macro stability. This led to the use of the term ‘the Great Moderation’ to describe the calmer macroeconomic conditions from the mid-1980s (fewer and shorter recessions as illustrated in Figure 1.2).
在全球金融危机之前，许多宏观经济学家声称更好的政策制定改善了宏观稳定。这导致了“伟大的温和期”这一术语的使用，用以描述从 1980 年代中期开始的更平稳的宏观经济条件（如图 1.2 所示，经济衰退更少且持续时间更短）。

Figure 1.3 Volatility of real GDP growth in the United States: 1950 Q3 to 2019 Q4.
图 1.3 美国实际 GDP 增长的波动性：1950 年第三季度至 2019 年第四季度。
Note: Volatility has been calculated as the standard deviation of the GDP growth rate over a rolling 21-quarter period. Graph uses data from the series ‘Real GDP, chained dollars, billions of chained (2012) dollars.’
注意：波动性是通过对 21 个季度滚动期间的 GDP 增长率计算标准差得出的。图表使用的数据来自“实际 GDP，链式美元，十亿链式（2012）美元”系列。

Source: US Bureau of Economic Analysis (data accessed September 2022).
来源：美国经济分析局（数据访问于 2022 年 9 月）。

A more precise way to document the Great Moderation is to look at the volatility of GDP growth. Figure 1.3 shows the volatility of US GDP growth since the late 1950s. By plotting the moving average of the standard deviation of the growth rate, changes in volatility can be seen easily. Figure 1.3 uses a 21-quarter moving average of the standard deviation of the annual growth rate. There is a noticeable fall in the volatility of GDP growth from the mid 1980s. However, the spike in volatility experienced during the recession brought the era of the Great Moderation to an end. Volatility was low again between the end of the financial crisis and the Covid-19 crisis.
记录“大缓和”的一种更精确的方法是观察 GDP 增长的波动性。图 1.3 显示了自 20 世纪 50 年代末以来美国 GDP 增长的波动性。通过绘制增长率标准差的移动平均，可以很容易地看到波动性的变化。图 1.3 使用了 21 个季度的年度增长率标准差的移动平均。从 20 世纪 80 年代中期开始，GDP 增长的波动性明显下降。然而，在经济衰退期间经历的波动性激增使“大缓和”时代结束。在金融危机结束和新冠疫情之间，波动性再次较低。

1.1.2 Introducing the IS curve
1.1.2 介绍 IS 曲线

In this chapter, we begin to capture essential elements of the complex real-world macro-economy in a model. The model is called the 3-equation model because there are three core elements, representing the demand side, the supply side and the policy maker. The IS is the curve that represents the demand side. It is called the IS curve because it refers to planned investment and savings decisions; hence the I and the S .
在本章中，我们开始在模型中捕捉复杂现实宏观经济的基本要素。该模型被称为三方程模型，因为它有三个核心要素，分别代表需求方、供给方和政策制定者。IS 曲线代表需求方。它被称为 IS 曲线，因为它涉及计划投资和储蓄决策；因此有 I 和 S。

It was originally formulated as a simple shorthand version of John Maynard Keynes’s description of the demand side in his General Theory.

^{2}

它最初被制定为约翰·梅纳德·凯恩斯在其《通论》中对需求侧描述的简单速记版本。

By modelling the goods market we can answer interesting questions about the demand side that we might otherwise struggle with, such as the following:
通过对商品市场建模，我们可以回答一些关于需求方面的有趣问题，否则我们可能会很难解决，例如以下问题：

If you give an individual a $$ 100$ bonus, how much of it will they spend?
如果你给一个人一个 $$ 100$ 奖金，他们会花掉多少？
How much will output increase following a rise in government spending or a boost in the confidence of households and firms about their future prospects?
政府支出增加或家庭和企业对未来前景信心提升后，产出将增加多少？
To what extent will a rise in the interest rate curtail investment in new housing, machinery and equipment?
利率上升在多大程度上会抑制对新住房、机械和设备的投资？

The IS curve summarizes how aggregate output in the economy is affected by changes in the spending decisions of families, firms and government bodies. For example, when firms increase their spending on new equipment, this triggers increased production in the capital goods producing sector of the economy. More people are employed to produce the extra capital goods and as they spend their wages, demand for consumer goods goes up and employment and output expand in those sectors as well. As a result of this process, the economy will move to a higher level of output and employment. We shall see that in response to the initial expansion of spending, output will grow until the extra saving households want to make just balances the extra spending on investment. At that point, the process by which the impact of the initial increase in aggregate demand is multiplied through the economy in subsequent rounds of higher employment, spending by newly employed workers, higher demand for consumer goods, etc. comes to an end.
IS 曲线总结了家庭、企业和政府支出决策变化如何影响经济中的总产出。例如，当企业增加对新设备的支出时，这会引发经济中资本品生产部门的生产增加。更多的人被雇用来生产额外的资本品，随着他们花费工资，消费品的需求上升，相关部门的就业和产出也随之扩大。由于这一过程，经济将达到更高的产出和就业水平。我们将看到，作为对初始支出扩张的反应，产出将增长，直到家庭希望额外储蓄的金额恰好平衡对投资的额外支出。在这一点上，初始总需求增加的影响通过随后的更高就业、刚被雇用工人的支出、对消费品的更高需求等在经济中被放大的过程结束。

Aggregate demand in the private sector
私营部门的总需求

Focusing attention on decisions by the private sector, the level of aggregate demand will be affected by current income and by the following factors.
关注私营部门的决策，整体需求水平将受到当前收入和以下因素的影响。

Expectations about the future: The plans of firms to invest in new IT, equipment and premises depend on their expectations of future post-tax profits. If firms experience high levels of capacity utilization and strong order books, they will increase investment in new capacity. Households prefer to have smooth rather than fluctuating consumption, which means they need to save and borrow in order to spread their consumption more evenly over time. To make their saving and borrowing decisions, they must form a view about the future growth of their income. The life-cycle motive for saving refers to the planning of saving as the person takes into account the projected pattern of income during their working life and retirement. Households will revise upward their estimate of how much they can spend each period if they have new information that leads them to expect their income to grow more strongly. Firms and households form their expectations in
对未来的期望：企业投资新信息技术、设备和场所的计划取决于它们对未来税后利润的预期。如果企业经历高水平的产能利用率和强劲的订单簿，它们将增加对新产能的投资。家庭更倾向于平稳而非波动的消费，这意味着他们需要储蓄和借款，以便在时间上更均匀地分配消费。为了做出储蓄和借款的决策，他们必须对未来收入的增长形成看法。生命周期储蓄动机是指个人在考虑其工作生涯和退休期间的收入预期模式时进行储蓄规划。如果家庭获得新的信息，导致他们预期收入将更强劲增长，他们将上调每个时期的支出估计。企业和家庭在形成预期时会考虑这些因素。

the face of uncertainty. For example, an increase in the unemployment rate in the economy could be a signal to households that uncertainty about their future income has risen, which would trigger an increase in saving for precautionary reasons.
不确定性的面貌。例如，经济中失业率的上升可能向家庭发出信号，表明他们对未来收入的不确定性增加，这将引发出于预防原因的储蓄增加。
2. The extent of credit constraints: These arise because of problems faced by banks in assessing the creditworthiness of households and firms. It is impossible for banks to have full information about borrowers’ projects and actions. Borrowing by households and by small and medium-sized firms is therefore often restricted by banks. Households and business that cannot borrow as much money as they would like are said to be credit constrained. The information problems inherent in bank lending mean that access to credit is often highly dependent on the amount of collateral the borrower has with which to secure the loan. The most common form of collateral for households is the value of their house. This means that changes in the value of collateral, which occur when house prices change, affect consumption and investment because they either tighten or relax credit constraints. People with higher wealth have more collateral and are less likely to be credit-constrained.
2. 信贷约束的程度：这些约束源于银行在评估家庭和企业信用 worthiness 时面临的问题。银行不可能对借款人的项目和行为拥有全面的信息。因此，家庭和中小型企业的借款往往受到银行的限制。无法借到足够资金的家庭和企业被称为信贷受限。银行贷款中固有的信息问题意味着，获得信贷往往高度依赖借款人拥有的抵押品数量。家庭最常见的抵押品形式是他们房屋的价值。这意味着抵押品价值的变化（当房价变化时发生）会影响消费和投资，因为它们要么收紧，要么放松信贷约束。财富较高的人拥有更多的抵押品，因此不太可能受到信贷约束。
3. The interest rate: There are a number of channels through which the interest rate will affect aggregate demand. When interest rates go up, households find it more expensive to get a mortgage. This reduces the demand for new houses and for furnishings and other consumer durables that go along with moving house. A higher interest rate will lead firms to rein in their spending plans on new capital equipment and buildings. Finally, households will tend to postpone consumption spending because of the improved returns from saving, which we call the substitution effect. However, creditor and debtor households would be expected to react differently due to income changes. A creditor household will find their income has gone up when the interest rate rises and this will boost consumption spending through the income effect. For debtor households, the effect will be the opposite. The first and third effects (substitution effects, and income effect for debtors) are normally stronger than the second one (income effect for creditors) and thus a higher interest rate is associated with postponed consumption.
3. 利率：利率通过多种渠道影响总需求。当利率上升时，家庭发现获得抵押贷款的成本更高。这减少了对新房屋以及与搬家相关的家具和其他耐用消费品的需求。更高的利率将导致企业缩减对新资本设备和建筑的支出计划。最后，家庭往往会因为储蓄带来的更高回报而推迟消费支出，这被称为替代效应。然而，债权人家庭和债务人家庭由于收入变化的不同，反应也会有所不同。债权人家庭会发现，当利率上升时，他们的收入增加，这将通过收入效应促进消费支出。对于债务人家庭，效果则相反。第一和第三种效应（替代效应和债务人的收入效应）通常比第二种效应（债权人的收入效应）更强，因此更高的利率与推迟消费相关。

The IS curve IS 曲线

The IS curve summarizes in an equation and diagram the demand side in the macroeconomic model. It shows the combinations of the interest rate and output at which aggregate demand (i.e. spending) in the economy is equal to output.
IS 曲线在宏观经济模型中用一个方程和图表总结了需求侧。它显示了利率和产出之间的组合，在这些组合中，经济中的总需求（即支出）等于产出。

Figure 1.4 shows the IS curve. This is a downward-sloping relationship. To see this, think of the combination of a high interest rate and low output. When the interest rate is high, spending on housing, consumer durables, machinery and equipment will be low. This means aggregate demand is low and a low level of output will satisfy the low demand. Hence, we have the first point on the IS curve at A. Now, take a combination of a low interest rate and high output. Here the situation is the opposite: buoyant spending on new houses, consumer durables and investment goods generates a high level of output and high incomes for households. This second point gives us the downward-sloping IS curve at B.
图 1.4 显示了 IS 曲线。这是一条向下倾斜的关系。要理解这一点，可以考虑高利率和低产出的组合。当利率高时，住房、耐用消费品、机械和设备的支出将会很低。这意味着总需求低，低水平的产出将满足低需求。因此，我们在 A 点得到了 IS 曲线的第一个点。现在，考虑低利率和高产出的组合。在这里，情况正好相反：对新房、耐用消费品和投资品的强劲支出产生了高水平的产出和家庭的高收入。这个第二个点给我们在 B 点得到了向下倾斜的 IS 曲线。

Figure 1.4 The IS curve-the effects of changes in optimism and economic policy. Visit www.oup.com/he/carlin-soskice to explore the IS curve with Animated Analytical Diagram 1.4.
图 1.4 IS 曲线——乐观情绪和经济政策变化的影响。访问 www.oup.com/he/carlin-soskice 探索带有动画分析图 1.4 的 IS 曲线。

To show how changes in profit or income growth expectations, uncertainty and the value of collateral can be captured in the diagram, we hold the interest rate constant and look at shifts in the IS curve. As an example, in a situation of depressed profit expectations, we would expect firms to postpone new investment. The result is lower investment spending at any interest rate. The IS curve shifts to the left (shown by the curve labelled IS (pessimistic expectations)).
为了展示利润或收入增长预期、经济不确定性和抵押品价值如何在图表中体现，我们保持利率不变，观察 IS 曲线的移动。例如，在利润预期低迷的情况下，我们预计企业会推迟新的投资。结果是在任何利率下投资支出减少。IS 曲线向左移动（由标记为 IS（悲观预期）的曲线表示）。

Using the IS diagram, we can show how the central bank or the government can affect the demand side. Following up the situation where the business environment becomes more pessimistic, the central bank could lower the interest rate to stimulate investment. This would be shown as a move along the IS curve from point

C

to point

D

. As an example, there was much discussion about the decision by the US central bank, the Federal Reserve, to reduce the policy interest rate and keep it low as a way of stimulating investment in the aftermath of the collapse of the Dotcom bubble in 2001. The long period of low interest rates stimulated investment in new house construction and we shall see in Chapter 9 the role this played in the background to the global financial crisis of 2008.
使用 IS 图，我们可以展示中央银行或政府如何影响需求侧。在商业环境变得更加悲观的情况下，中央银行可以降低利率以刺激投资。这将表现为 IS 曲线从点

C

移动到点

D

。例如，关于美国中央银行——联邦储备委员会——在 2001 年互联网泡沫崩溃后决定降低政策利率并保持低位以刺激投资的讨论很多。长期的低利率刺激了新房建设的投资，我们将在第 9 章看到这在 2008 年全球金融危机背景中所起的作用。

Another response to a leftward shift of the IS curve due to pessimistic profit expectations could be action by the government rather than central bank. If the government decides to launch a major expenditure programme, such as to improve healthcare provision or to install an information super-highway, this will shift the IS curve to the right: at a given interest rate, the government purchases a larger amount of goods and services. Under our assumption that suppliers will respond to the higher demand, the economy moves to a higher level of output and employment. In the diagram, this would shift the IS curve to the right back towards its initial position.
另一个对由于悲观利润预期导致的 IS 曲线向左移动的反应可能是政府而非中央银行采取行动。如果政府决定启动一项重大支出计划，例如改善医疗服务或建设信息高速公路，这将使 IS 曲线向右移动：在给定的利率下，政府购买更多的商品和服务。在我们假设供应商会对更高的需求做出反应的情况下，经济将达到更高的产出和就业水平。在图中，这将使 IS 曲线向右移动，回到其初始位置。

The demand side is only part of the macroeconomic model. To understand how the spending decisions of households, firms and the government fit into the bigger picture,
需求方只是宏观经济模型的一部分。要理解家庭、企业和政府的支出决策如何融入更大的图景，
we need to include the supply side and the motivations and behaviour of the policy makers. Chapter 2 addresses the supply side and Chapter 3 brings in the policy maker. Before moving on, we set out the modelling of the demand side in more detail.
我们需要考虑供应方以及政策制定者的动机和行为。第二章讨论供应方，第三章引入政策制定者。在继续之前，我们更详细地阐述需求方的建模。

1.2 MODELLING 1.2 建模

The Modelling sections of the chapters in the book provide more details of the models and their components introduced in the Overview sections. Detailed derivations of some of the results are available in the appendix.
本书各章的建模部分提供了概述部分中介绍的模型及其组成部分的更多细节。某些结果的详细推导可在附录中找到。

As a first step in this chapter, we introduce the concept of goods market equilibrium and the definition and mechanics of the multiplier process. Throughout this section, we assume that firms are willing to meet the higher demand for their goods and services and workers are willing to take the extra jobs or work the extra hours that are offered.
在本章的第一步中，我们介绍了商品市场均衡的概念以及乘数过程的定义和机制。在本节中，我们假设企业愿意满足对其商品和服务的更高需求，工人也愿意接受额外的工作或加班。

1.2.1 Goods market equilibrium
1.2.1 商品市场均衡

We begin our modelling of the goods market by considering each component of aggregate demand in the closed economy. The aggregate-or economy-wide-demand for goods and services consists of:
我们通过考虑封闭经济中总需求的每个组成部分来开始对商品市场的建模。商品和服务的总需求或经济整体需求包括：

consumption: Expenditure by individuals on goods and services. Spending is on both durable products such as a car, laptop or sofa and on non-durable products such as a theatre show, child-care services or groceries.
消费：个人在商品和服务上的支出。支出包括耐用产品，如汽车、笔记本电脑或沙发，以及非耐用产品，如戏剧表演、儿童保育服务或杂货。
investment: Expenditure on goods that produce a stream of utility in the form of future services. When discussing investment as a component of aggregate demand, it includes spending on capital goods (machinery, equipment, and buildings). In addition, most spending on R&D is treated as investment in national accounts. Spending is by households on new houses, by firms on new capital goods, including structures, and building up inventories of materials or finished goods, and by government (on public infrastructure projects, such as building a new high speed rail line).
投资：对能够产生未来服务流的商品的支出。当讨论投资作为总需求的一个组成部分时，它包括对资本货物（机械、设备和建筑物）的支出。此外，大多数研发支出在国民账户中被视为投资。支出包括家庭对新房的支出，企业对新资本货物（包括建筑物）的支出，以及对材料或成品库存的积累，政府则在公共基础设施项目（如建设新的高速铁路）上进行支出。
demand stemming from government purchases: Government expenditure on salaries, goods and services. Spending includes public sector wages (e.g. civil servants, teachers), purchases of goods (e.g. educational supplies, ammunition for the army) and purchases of services (e.g. waste disposal and contract cleaning).
来自政府采购的需求：政府在薪资、商品和服务上的支出。支出包括公共部门工资（例如，公务员、教师）、商品采购（例如，教育用品、军队弹药）和服务采购（例如，废物处理和合同清洁）。

The model of goods market equilibrium
商品市场均衡模型

We now set out a model of the goods market. In the closed economy, aggregate demand,

y^{D}

, is given by
我们现在提出一个商品市场模型。在封闭经济中，总需求

y^{D}

由以下公式给出：

y^{D} = C + I + G

and we ignore imports and exports. All variables are in real terms, which is also referred to as constant price terms. Equilibrium in the goods market requires that planned real expenditure on goods and services (i.e. aggregate demand) is equal to real output
我们忽略进出口。所有变量都是以实际价格计的，也称为恒定价格计。商品市场的均衡要求计划的商品和服务的实际支出（即总需求）等于实际产出。

y^{D} = y, (goods market equilibrium)

where

y

is output.

y

is also income: spending on the output of the economy in turn becomes the income of those producing it (wages and profits). This circular flow of income to expenditure and output, and back to the incomes of producers means we can use the terms output, income and expenditure interchangeably. We shall come back to the goods market equilibrium condition after providing some more detail about consumption and investment behaviour.
其中

y

是产出。

y

也是收入：对经济产出的支出反过来成为生产者的收入（工资和利润）。这种收入、支出和产出之间的循环流动，以及回到生产者收入的过程，意味着我们可以将产出、收入和支出这几个术语互换使用。在提供一些关于消费和投资行为的更多细节后，我们将回到商品市场均衡条件。
To start, we assume that aggregate consumption is a simple linear function of aftertax or disposable aggregate income
首先，我们假设总消费是税后或可支配总收入的一个简单线性函数

C = c_{0} + c_{1} (y - T)

where T is total taxes.
T 是总税收。
To get the consumption function we make the additional assumption that taxes are a fixed proportion of income; i.e.

T = t y

, where

0 < t < 1

. The consumption function then becomes
为了得到消费函数，我们额外假设税收是收入的固定比例；即

T = t y

，其中

0 < t < 1

。消费函数因此变为

C = c_{0} + c_{1} (1 - t) y . (Keynesian consumption function)

This is a Keynesian consumption function. It consists of a constant term,

c_{0}

, which is referred to as autonomous consumption because it does not depend on current income, and the other component of consumption,

c_{1} (1 - t) y

, which fluctuates with the level of disposable income. Since we assume a proportional tax rate,

t, c_{1} (1 - t) y

is a fixed proportion of disposable income.

c_{1}

is referred to as the marginal propensity to consume and lies between zero and one (

0 < c_{1} < 1

).
这是一个凯恩斯消费函数。它由一个常数项

c_{0}

组成，这被称为自主消费，因为它不依赖于当前收入，另一个消费组成部分

c_{1} (1 - t) y

则随着可支配收入的水平波动。由于我们假设一个比例税率，

t, c_{1} (1 - t) y

是可支配收入的固定比例。

c_{1}

被称为边际消费倾向，介于零和一之间（

0 < c_{1} < 1

）。
Note that the marginal propensity to consume (MPC) shows the change (where change is denoted by a triangle symbol) in consumption as the result of a change in post-tax or disposable income:
注意，边际消费倾向（MPC）显示消费的变化（变化用三角符号表示）是由于税后或可支配收入的变化所导致的：

MPC \equiv \frac{Δ C}{Δ y^{disp}} = c_{1}, (marginal propensity to consume)

where

y^{disp} \equiv (1 - t) y

. 在哪里

y^{disp} \equiv (1 - t) y

。
In this model of consumption, private saving is

S \equiv y^{disp} - C

and adds to one’s assets. Income is the amount that can be consumed without reducing one’s assets. The corresponding marginal propensity to save is

s_{1}

and

c_{1} + s_{1} = 1

(i.e. disposable income can be either saved or spent).
在这种消费模型中，私人储蓄为

S \equiv y^{disp} - C

，并增加个人资产。收入是可以消费而不减少个人资产的金额。相应的边际储蓄倾向为

s_{1}

和

c_{1} + s_{1} = 1

（即可支配收入可以被储蓄或消费）。
If disposable income is zero, the Keynesian consumption function predicts consumption equal to

c_{0}

. For this to be the case, households have savings and/or are able to borrow. A more satisfactory model of consumption must therefore include forwardlooking behaviour in order to account for savings and we build this in to the model of consumption later in the chapter.
如果可支配收入为零，凯恩斯消费函数预测消费等于

c_{0}

。要使这种情况成立，家庭必须有储蓄和/或能够借款。因此，更令人满意的消费模型必须包括前瞻性行为，以便考虑储蓄，我们将在本章后面将其纳入消费模型中。

1.2.2 The multiplier 1.2.2 乘数

We use the Keynesian consumption function to introduce in the simplest way the concept of how equilibrium in the goods market is determined and how the multiplier process works. If we substitute the consumption function into the equation for aggregate demand, then we get a relation between aggregate demand,
我们使用凯恩斯消费函数以最简单的方式引入商品市场均衡如何确定以及乘数过程如何运作的概念。如果我们将消费函数代入总需求的方程中，那么我们就得到了总需求之间的关系。

y^{D} = c_{0} + c_{1} (1 - t) y + I + G

Figure 1.5 Keynesian cross-increase in government spending. Visit www.oup.com/he/carlin-soskice to explore the Keynesian cross with Animated Analytical Diagram 1.5.
图 1.5 凯恩斯交叉——政府支出增加。访问 www.oup.com/he/carlin-soskice 探索带有动画分析图 1.5 的凯恩斯交叉。
and output,

y

, which we can draw on a diagram with output on the

x

-axis and aggregate demand on the

y

-axis (Figure 1.5). The intercept of the curve will be (

I + G + c_{0}

) and the slope

c_{1} (1 - t)

which, given the assumption that both

c_{1}

and

t

are between 0 and 1 , will itself be between zero and one. We can also draw the goods market equilibrium condition on the same graph, which will be a

45^{\circ}

line, because in equilibrium
并输出

y

，我们可以在图表上绘制输出在

x

轴上，聚合需求在

y

轴上（图 1.5）。曲线的截距将是（

I + G + c_{0}

），斜率

c_{1} (1 - t)

，根据假设

c_{1}

和

t

都在 0 和 1 之间，斜率本身也将在零和一之间。我们还可以在同一图表上绘制商品市场均衡条件，这将是一条

45^{\circ}

线，因为在均衡状态下。

y = y^{D}

The point at which the two curves intersect shows the level of output where planned real expenditure on goods (and services) by firms, households and government is exactly equal to the level of goods being supplied in the economy. Above the

45^{\circ}

line, the demand for goods is greater than the supply.
两条曲线交点所示的点表示企业、家庭和政府对商品（和服务）的计划实际支出与经济中供应的商品水平完全相等的产出水平。在

45^{\circ}

线以上，商品的需求大于供应。

To understand the model, we disturb the initial equilibrium at point A. Suppose there is an increase in government spending. From the aggregate demand equation, we can see that this shifts the aggregate demand curve upwards by the change in government spending,

Δ G

. Aggregate demand now exceeds output (Point B). As the government increases its purchases of goods (e.g. of office equipment) the stocks of these goods in warehouses decline. The inventory management software records the fall in stocks and triggers an increase in production: output rises. This is the move from B to C, where once again

y = y^{D}

. The higher output in turn raises incomes (in the form of wages of the additional workers employed and the profits of the owners of the firms making higher sales) and according to the consumption function, some proportion of
为了理解模型，我们在 A 点扰动初始均衡。假设政府支出增加。从总需求方程可以看出，这将使总需求曲线因政府支出的变化而向上移动，

Δ G

。此时，总需求超过了产出（B 点）。随着政府增加对商品（例如办公设备）的采购，这些商品在仓库中的库存减少。库存管理软件记录库存的下降，并触发生产的增加：产出上升。这是从 B 到 C 的转变，再次

y = y^{D}

。更高的产出反过来提高了收入（以额外雇佣工人的工资和销售额增加的公司的利润的形式），根据消费函数，某个比例的
the higher income is spent on goods and services in the economy, raising aggregate demand further. The process continues until the new goods market equilibrium is reached at point

Z

, where output and aggregate demand are equal.
更高的收入用于购买经济中的商品和服务，进一步提高了总需求。这个过程持续进行，直到新的商品市场均衡在点

Z

达到，此时产出和总需求相等。

Adjustment to shifts in aggregate demand via inventory accumulation and decumulation shows how a purely quantity based adjustment to a new goods market equilibrium can take place. For the goods market to be in equilibrium it must be the case that

y = y^{D}

. If we substitute

y

into the aggregate demand equation we can rearrange to define equilibrium output in terms of the exogenous variables. Exogenous variables are those that are determined outside the model. In the model of goods market equilibrium, the exogenous variables are autonomous consumption (

c_{0}

), investment (I) and government spending

(G)

, the marginal propensity to consume

(c_{1})

and the tax rate

(t)

. If we know the value of these five variables, we can work out the level of equilibrium output,
通过库存积累和去积累对总需求变化的调整显示了如何通过纯粹的数量调整实现新的商品市场均衡。为了使商品市场处于均衡状态，必须满足

y = y^{D}

。如果我们将

y

代入总需求方程，我们可以重新排列以定义外生变量的均衡产出。外生变量是指在模型外部确定的变量。在商品市场均衡模型中，外生变量包括自主消费（

c_{0}

）、投资（I）和政府支出

(G)

、边际消费倾向

(c_{1})

和税率

(t)

。如果我们知道这五个变量的值，就可以计算出均衡产出的水平。

\begin{aligned} y & = c_{0} + c_{1} (1 - t) y + I + G \\ y - c_{1} (1 - t) y & = c_{0} + I + G \\ y & = \underset{multiplier}{\underset{⏟}{\frac{1}{1 - c_{1} (1 - t)}}} \underset{autonomous demand}{\underset{⏟}{(c_{0} + I + G)}} \end{aligned}

Because

c_{1}

and

t

are between 0 and 1 , this simplified model implies that the multiplier is greater than 1 . This means that a

1 %

increase in autonomous demand would be predicted to lead to an increase in output of more than

1 %

. This is referred to as the short-run multiplier as we hold the interest rate and all other policy responses constant-i.e. output is the only variable allowed to change. Note that if consumption was not a function of current income, i.e. if the marginal propensity to consume,

c_{1}

, is equal to zero, then the aggregate demand line is horizontal and there is no multiplier process amplifying the impact on the economy of the rise in government spending. A rule of thumb is that the multiplier in ‘normal times’ is between one and 1.5; higher values are estimated for recessions.
因为

c_{1}

和

t

在 0 和 1 之间，这个简化模型意味着乘数大于 1。这意味着自主需求的

1 %

增加预计会导致产出增加超过

1 %

。这被称为短期乘数，因为我们保持利率和所有其他政策反应不变，即产出是唯一允许变化的变量。请注意，如果消费不是当前收入的函数，即边际消费倾向

c_{1}

等于零，那么总需求线是水平的，没有乘数过程放大政府支出增加对经济的影响。一个经验法则是，在“正常时期”乘数在 1 到 1.5 之间；在衰退期间，估计更高的值。

In the consumption function,

c_{0}

is an exogenous variable, meaning that it is not influenced by any other variables in the equation (i.e. it is determined outside the model). The second part of the equation,

c_{1} (1 - t) y

, is endogenous. In order to calculate the level of aggregate consumption in equilibrium, we therefore need to first work out the equilibrium value of

y

and then compute the level of consumption.
在消费函数中，

c_{0}

是一个外生变量，这意味着它不受方程中任何其他变量的影响（即它是在模型外部确定的）。方程的第二部分，

c_{1} (1 - t) y

，是内生的。因此，为了计算均衡状态下的总消费水平，我们需要首先计算出

y

的均衡值，然后再计算消费水平。
We now combine algebra and geometry to deepen our understanding of the multiplier process. We return to the previous example shown in Figure 1.5 where the government uses fiscal policy to increase activity in the economy. Since the AD curve is flatter than the market-clearing condition, equilibrium output will increase by more than the increase in government expenditure, i.e. the multiplier is larger than one. The initial boost in government spending increases aggregate demand by

Δ G

. This increases output, and income, and so aggregate consumption increases by

c_{1} (1 - t) Δ G

. The increase in aggregate consumption increases aggregate demand by the same amount, and, given that the goods market clears, increases output and income too. Aggregate consumption increases again by

{[c_{1} (1 - t)]}^{2} Δ G

, and so on to infinity. The
我们现在结合代数和几何来加深对乘数过程的理解。我们回到图 1.5 中显示的先前示例，其中政府使用财政政策来增加经济活动。由于 AD 曲线比市场清算条件更平坦，均衡产出将增加超过政府支出的增加，即乘数大于 1。政府支出的初步增加使总需求增加了

Δ G

。这增加了产出和收入，因此总消费增加了

c_{1} (1 - t) Δ G

。总消费的增加使总需求增加了相同的数量，并且由于商品市场清算，也增加了产出和收入。总消费再次增加了

{[c_{1} (1 - t)]}^{2} Δ G

，依此类推，直到无穷大。
total increase in output is given by the sum of these changes:
产出总增加量由这些变化的总和给出：

\begin{aligned} Δ y & = Δ G + [c_{1} (1 - t)] Δ G + {[c_{1} (1 - t)]}^{2} Δ G \dots \\ = (1 + [c_{1} (1 - t)] + {[c_{1} (1 - t)]}^{2} + \dots) Δ G \end{aligned}

This is a geometric progression and its sum to infinity is given by

^{3}

这是一个几何级数，它的无穷和为

^{3}

Δ y = \underset{multiplier}{\underset{⏟}{\frac{1}{1 - c_{1} (1 - t)}}} Δ G = k Δ G

where

k

is the multiplier.
其中

k

是乘数。

Application: The paradox of thrift
应用：节俭的悖论

Imagine the economy is in a recession and the following question is put to an economist: should a policy be introduced to encourage or to discourage private saving in order to help the economy recover? On the one hand, the idea of encouraging more saving sounds helpful-if more is saved and invested in new capital stock, that would seem a good recipe for recovery. On the other hand, if more saving just means less consumption and there is no increase in investment, won’t aggregate demand fall and the recession get worse?
想象一下经济处于衰退中，经济学家面临以下问题：应该引入政策来鼓励或抑制私人储蓄，以帮助经济复苏吗？一方面，鼓励更多储蓄的想法听起来是有帮助的——如果储蓄和投资于新的资本存量增加，这似乎是复苏的好方法。另一方面，如果更多的储蓄只是意味着消费减少而投资没有增加，整体需求不会下降，衰退会变得更糟吗？
To answer this question requires spelling out how agents are assumed to behave. In an economic model, these decisions are summarized by behavioural equations. In macroeconomics, it is aggregate behaviour that is captured by these equations. In the very simple model we have set out, the behavioural equation for consumption is the Keynesian consumption function. Investment and government spending are assumed to remain constant: in the model so far, they are not affected by other variables (i.e. they are exogenous). The behavioural equations are as follows:
要回答这个问题，需要明确假设代理人的行为。在经济模型中，这些决策通过行为方程进行总结。在宏观经济学中，这些方程捕捉的是总体行为。在我们设定的非常简单的模型中，消费的行为方程是凯恩斯消费函数。投资和政府支出被假设为保持不变：在目前的模型中，它们不受其他变量的影响（即它们是外生的）。行为方程如下：

C = c_{0} + c_{1} (1 - t) y; G = \overset{―}{G}; and I = \overset{―}{I}, (behavioural equations)

where we put a bar over the

G

and the I to emphasize that these are both exogenous. From the behavioural equations, we can write planned real expenditure (aggregate demand) as
我们在

G

和 I 上加一个横杠，以强调这两者都是外生的。从行为方程中，我们可以将计划的实际支出（总需求）写为

x = 1 + [c_{1} (1 - t)] + {[c_{1} (1 - t)]}^{2} + {[c_{1} (1 - t)]}^{3} + \dots

and multiply both sides by the common factor,

[c_{1} (1 - t)]

, then
并将两边都乘以公因子

[c_{1} (1 - t)]

，然后

[c_{1} (1 - t)] \cdot x = [c_{1} (1 - t)] + {[c_{1} (1 - t)]}^{2} + {[c_{1} (1 - t)]}^{3} + \dots

and if we subtract the bottom equation from the one above, we have
如果我们从上面的方程中减去下面的方程，我们就得到了
[

\begin{aligned} x (1 - [c_{1} (1 - t)]) & = 1 \\ \Rightarrow x = \frac{1}{1 - [c_{1} (1 - t)]} . \end{aligned}

]

y^{D} = c_{0} + c_{1} (1 - t) y + \bar{I} + \bar{G}

As usual, the goods market equilibrium condition is

y^{D} = y

, and if we substitute equation 1.8 into this, then in equilibrium, planned private savings are equal to planned investment plus government spending:
如往常一样，商品市场的均衡条件是

y^{D} = y

，如果我们将方程 1.8 代入其中，那么在均衡状态下，计划的私人储蓄等于计划的投资加上政府支出：

\underset{Planned private saving}{\underset{⏟}{(1 - c_{1} (1 - t)) y - c_{0}}} = \underset{Planned I and G}{\underset{⏟}{\overset{―}{I + \bar{G}}}} (equilibrium condition)

We model the proposal to encourage savings by a fall in autonomous consumption from

c_{0}

c_{0}^{'}

. Aggregate demand falls initially by (

c_{0} - c_{0}^{'}

) and the multiplier process works in the downwards direction. Using the same logic as illustrated in Figure 1.5 in reverse, this process continues until we get to the new equilibrium at lower output. At the new lower output level,

y^{'}

, planned savings on the left-hand side of equation 1.9 is equal to planned investment plus government spending on the right-hand side, which have not changed:
我们通过自主消费从

c_{0}

下降到

c_{0}^{'}

来建模该提案以鼓励储蓄。总需求最初下降（

c_{0} - c_{0}^{'}

），乘数过程向下进行。使用与图 1.5 中所示的相同逻辑反向进行，该过程持续进行，直到我们达到较低产出的新均衡。在新的较低产出水平

y^{'}

，方程 1.9 左侧的计划储蓄等于右侧的计划投资加政府支出，这些都没有变化：

(1 - c_{1} (1 - t)) y^{'} - c_{0}^{'} = \bar{I} + \bar{G}

An important insight emerges from this example. The initial equilibrium was disturbed by a fall in autonomous consumption as households sought to increase their savings. However, the intention to save more did not lead to higher aggregate savings because income fell

(Δ y = k (c_{0} - c_{0}^{'}))

. This must be the case for equation 1.9 to hold.

^{4}

This is called the paradox of thrift because if greater thriftiness is not matched by higher investment in fixed capital or some other increase in spending, income will fall and there will be no overall increase in savings in the economy.
从这个例子中出现了一个重要的见解。最初的均衡被家庭寻求增加储蓄而导致的自主消费下降所打破。然而，增加储蓄的意图并没有导致总储蓄的增加，因为收入下降了

(Δ y = k (c_{0} - c_{0}^{'}))

。这必须是方程 1.9 成立的情况。

^{4}

这被称为节俭悖论，因为如果更大的节俭没有通过对固定资本的更高投资或其他支出的增加来匹配，收入将会下降，经济中的总储蓄也不会增加。
To summarize, the answer to the question of whether saving should be encouraged in a recession depends on the model of the economy the economist is using-and of course, on how well that model matches the real economy under study. Using the model we have developed so far in this chapter, the answer is clear: encouragement to save more will not help the economy to exit recession. The reason is that there is no mechanism in this model through which higher saving is translated into higher investment. Investment remains at

\bar{I}

throughout. Hence the result is that aggregate demand falls, output falls and the recession is deepened.
总而言之，关于在经济衰退中是否应该鼓励储蓄的问题，答案取决于经济学家所使用的经济模型——当然，也取决于该模型与所研究的真实经济的匹配程度。根据我们在本章中所开发的模型，答案是明确的：鼓励更多的储蓄不会帮助经济走出衰退。原因在于该模型中没有机制将更高的储蓄转化为更高的投资。投资始终保持在

\bar{I}

。因此，结果是总需求下降，产出下降，衰退加深。
By contrast, if the model included a central bank, then the recession could be averted by the central bank cutting the interest rate and boosting investment to offset the fall in consumption. In Section 1.2.3, we remove the simplifying assumption of exogenous investment to show the impact on the demand side when investment is responsive to changes in the interest rate and in expected post-tax profits.
相反，如果模型中包含中央银行，那么中央银行通过降低利率和增加投资来抵消消费下降，从而可以避免衰退。在 1.2.3 节中，我们去除了外生投资的简化假设，以展示当投资对利率变化和预期税后利润变化敏感时对需求侧的影响。

1.2.3 The IS curve 1.2.3 IS 曲线

Deriving the IS equation 推导 IS 方程

Throughout this book, we use the IS curve to represent the demand side of the economy. In this subsection, we derive the IS curve and use it as a starting point to think about how monetary and fiscal policy work. As we discussed in Section 1.1.2, the IS curve shows the combinations of output and the real interest rate at which the goods market is in equilibrium.
在本书中，我们使用 IS 曲线来表示经济的需求侧。在本小节中，我们推导 IS 曲线，并将其作为思考货币政策和财政政策如何运作的起点。正如我们在第 1.1.2 节中讨论的，IS 曲线显示了商品市场处于均衡时产出与实际利率的组合。

We first set out the relationship between the real interest rate,

r

, and the nominal interest rate,

i

. This relationship is shown by the Fisher equation:
我们首先阐明实际利率

r

与名义利率

i

之间的关系。这个关系由费雪方程表示：

r = i - π^{E} . (Fisher equation)

This equation says that the real interest rate is simply the nominal interest adjusted for expected inflation. Inflation is denoted by

π

and

E

is used to represent expectations. It is the real interest rate that is most important for investment and saving decisions, as it represents the true cost of borrowing (and the true return on saving). This interest rate is therefore the one used in the investment equation and in the IS curve. When the central bank sets the nominal interest rate, it does this with the intention of achieving a particular real interest rate since it aims to affect interest-sensitive spending. In Section 1.5.1 of the Appendix, we show how the Fisher equation is derived.
这个方程表明，实际利率只是名义利率经过预期通胀调整后的结果。通胀用

π

表示，而

E

用来表示预期。实际利率对于投资和储蓄决策最为重要，因为它代表了借款的真实成本（以及储蓄的真实回报）。因此，这个利率是投资方程和 IS 曲线中使用的。当中央银行设定名义利率时，它的目的是为了实现特定的实际利率，因为它旨在影响对利率敏感的支出。在附录的 1.5.1 节中，我们展示了费雪方程是如何推导出来的。
At this stage, we assume there is just one interest rate in the economy that applies to all borrowing and saving. We make the assumption to keep the maths as simple as possible. In reality, there is a spectrum of interest rates. For example, the interest rate on bank lending will typically be higher than the interest rate set by the central bank (see Chapter 5).
在这个阶段，我们假设经济中只有一个适用于所有借贷和储蓄的利率。我们这样假设是为了使数学尽可能简单。实际上，利率是一个范围。例如，银行贷款的利率通常会高于中央银行设定的利率（见第 5 章）。
The transmission of monetary policy will work through its effect on investment spending, including on new houses and on other durable goods. Up to this point, we have assumed that investment is determined by expected future profits, which we have assumed to be exogenous. We now incorporate the interest rate into the investment function as follows:
货币政策的传导将通过其对投资支出的影响来发挥作用，包括对新房和其他耐用消费品的影响。到目前为止，我们假设投资是由预期的未来利润决定的，而我们假设这些利润是外生的。我们现在将利率纳入投资函数，如下所示：

I = a_{0} - a_{1} r, (investment function)

where

r

is the real interest rate,

a_{0}

and

a_{1}

are constants and

a_{1} > 0

. The main determinant of investment is expected future post-tax profits, which is captured by the term,

a_{0}

.
其中

r

是实际利率，

a_{0}

和

a_{1}

是常数，

a_{1} > 0

。投资的主要决定因素是预期的未来税后利润，这由术语

a_{0}

表示。
We derive the IS curve as follows. Substituting the investment function into equation 1.4 gives us a relationship between the real interest rate and output, which is the IS curve. We can then use

k

to denote the multiplier and simplify to achieve a simple equation for the IS curve:
我们如下推导 IS 曲线。将投资函数代入方程 1.4，得到实际利率与产出之间的关系，这就是 IS 曲线。然后我们可以用

k

表示乘数，并简化以获得 IS 曲线的简单方程：

\begin{aligned} y & = \underset{multiplier}{\underset{⏟}{\frac{1}{1 - c_{1} (1 - t)}}} [c_{0} + (a_{0} - a_{1} r) + G] \\ = k [c_{0} + (a_{0} - a_{1} r) + G] \\ = k (c_{0} + a_{0} + G) - k a_{1} r \\ = A - a r, \end{aligned}

where

A \equiv k (c_{0} + a_{0} + G)

and

a \equiv k a_{1}

.
在

A \equiv k (c_{0} + a_{0} + G)

和

a \equiv k a_{1}

。

Figure 1.6 Deriving the IS curve. Visit www.oup.com/he/carlin-soskice to explore deriving the IS curve with Animated Analytical Diagram 1.6.
图 1.6 推导 IS 曲线。访问 www.oup.com/he/carlin-soskice 探索使用动画分析图 1.6 推导 IS 曲线。

The derivation of the equation for the IS curve highlights the fact that given

r

, equilibrium output,

y

, is found by multiplying autonomous consumption and investment, and government spending by the multiplier

k = \frac{1}{1 - c_{1} (1 - t)}

. This fixes an

r - y

combination on the IS curve. It is clear from equation 1.12 that a higher multiplier,

k

, or higher interest-sensitivity of spending to the interest rate,

a_{1}

, increases the effect of a change in the interest rate on output, making the IS curve flatter.
IS 曲线方程的推导突出了这样一个事实：给定

r

，均衡产出

y

是通过将自主消费、投资和政府支出乘以乘数

k = \frac{1}{1 - c_{1} (1 - t)}

来找到的。这在 IS 曲线上固定了一个

r - y

组合。从方程 1.12 可以清楚看出，更高的乘数

k

或支出对利率的更高敏感性

a_{1}

会增加利率变化对产出的影响，使 IS 曲线变得更平坦。
The IS curve is derived graphically in Figure 1.6 using the IS curve equation shown above. Three curves are shown on the figure: the investment function, the investment function plus autonomous consumption and government spending, and the IS curve. Two real interest rates are shown in the figure: a high real interest rate,

r_{H}

, and a low real interest rate,

r_{L}

. At

r_{H}

, investment is more costly, so the level of planned investment is low, whereas the opposite is true at

r_{L}

. This leads to a downward-sloping investment function. Autonomous consumption and government spending are unaffected by the interest rate, so when added on, the curve shifts out parallel. The last step to derive the IS curve is to multiply

I_{0} + c_{0} + G

by the multiplier. This gives output equal to planned expenditure,

y

, on the IS curve. The IS curve shows all the combinations of output and real interest rate where the goods market is in equilibrium.
IS 曲线在图 1.6 中通过上述 IS 曲线方程图示得出。图中显示了三条曲线：投资函数、加上自主消费和政府支出的投资函数，以及 IS 曲线。图中显示了两个实际利率：一个高实际利率

r_{H}

和一个低实际利率

r_{L}

。在

r_{H}

时，投资成本较高，因此计划投资水平较低，而在

r_{L}

时则相反。这导致了向下倾斜的投资函数。自主消费和政府支出不受利率影响，因此当它们被加上时，曲线平行向外移动。推导 IS 曲线的最后一步是将

I_{0} + c_{0} + G

乘以乘数。这使得输出等于 IS 曲线上的计划支出

y

。IS 曲线显示了商品市场处于均衡时所有输出和实际利率的组合。

Using the IS curve equation and the diagram, we can summarize its properties.
使用 IS 曲线方程和图表，我们可以总结其特性。

The IS curve is downward sloping because a low interest rate generates high investment and consumer durables spending, which will be associated with high output. By contrast, when the interest rate is high, investment and consumer durable spending, and consequently equilibrium output, are low.
IS 曲线向下倾斜，因为低利率会产生高投资和消费耐用品支出，这将与高产出相关联。相反，当利率较高时，投资和消费耐用品支出，因此均衡产出较低。
The slope of the IS:
IS 曲线的斜率：
a. Any change in the size of the multiplier will change the slope of the IS curve. For example, a rise in the marginal propensity to consume, $c_{1}$ , or a fall in the
任何乘数大小的变化都会改变 IS 曲线的斜率。例如，边际消费倾向的上升 $c_{1}$ ，或下降的情况。
tax rate, $t$ , will increase the multiplier, making the IS flatter: it rotates counter clockwise from the intercept on the vertical axis.
税率， $t$ ，将增加乘数，使 IS 曲线变得更平坦：它从垂直轴上的截距逆时针旋转。
b. Any change in the interest sensitivity of investment $(a_{1})$ will lead to a consequential change in the slope of the IS curve: a more interest-sensitive investment function (i.e. $↑ a_{1}$ ) will be reflected in a flatter IS curve.
b. 投资 $(a_{1})$ 的利率敏感性任何变化都会导致 IS 曲线斜率的相应变化：更具利率敏感性的投资函数（即 $↑ a_{1}$ ）将反映在更平坦的 IS 曲线上。
Shifts in the IS curve: Any change in autonomous consumption, autonomous investment or government expenditure $(c_{0}, a_{0}, G)$ will cause the IS curve to shift by the change in autonomous spending times the multiplier (i.e., the intercept of the IS curve on the vertical axis shifts).
IS 曲线的变化：任何自主消费、自主投资或政府支出 $(c_{0}, a_{0}, G)$ 的变化都会导致 IS 曲线的移动，移动幅度为自主支出变化乘以乘数（即，IS 曲线在纵轴上的截距发生变化）。

1.2.4 Forward-looking behaviour
1.2.4 前瞻性行为

The spending decisions of agents in the present are influenced by their expectations of the future. This means there is an intertemporal component to both consumption and investment.
代理人在当前的消费决策受到他们对未来的预期影响。这意味着消费和投资都具有跨时间的成分。

Households adjust their current spending based on their expected income in the future. For example, a final year economics undergraduate who secures a lucrative job contract (which is set to commence after university), has low current income, but high expected future income. Having got the job, this individual may borrow in the present, consume more and pay back the money when they start their job. This behaviour is referred to as consumption smoothing.
家庭根据他们对未来收入的预期来调整当前支出。例如，一名获得丰厚工作合同的经济学本科生（该合同将在大学毕业后开始工作），当前收入较低，但未来收入预期较高。获得工作后，这个人可能会在现在借款，消费更多，并在开始工作时偿还这笔钱。这种行为被称为消费平滑。
Firms make decisions (e.g. purchasing machinery, equipment and premises) based on a business plan that includes forecasts about future demand for their products and input costs. For example, a firm selling cars to Vietnam might choose to build a new factory (i.e. undertake investment) based on forecasts that Vietnamese incomes (and therefore their demand for cars) will continue to rise rapidly over the next 20 years. Investment is intrinsically forward looking, as it incurs a cost today, but the stream of benefits occurs in the future.
企业根据商业计划做出决策（例如购买机器、设备和场所），该计划包括对未来产品需求和投入成本的预测。例如，一家向越南销售汽车的公司可能会选择建立一个新工厂（即进行投资），基于对越南收入（因此对汽车的需求）在未来 20 年将继续快速增长的预测。投资本质上是前瞻性的，因为它今天会产生成本，但收益流将在未来发生。

Present value calculation
现值计算

We can calculate the present value of a flow of income or profits received in future periods in the following way. We take the example of a firm making an investment decision.
我们可以通过以下方式计算未来期间收到的收入或利润的现值。我们以一家企业做出投资决策为例。

We assume that firms aim to maximize profits, so they undertake investment projects if these offer a return that is higher than costs. The outlay on investment typically precedes the returns, which may be lumpy and spread over a number of years. The way to deal with this is to calculate the ‘Present Value’ (V) of the expected flow of profits

Π

, where

Π_{t}, Π_{t + 1}, \dots

is the flow of profits in period

t, t + 1

etc. Suppose the interest rate is

10 %

. Then if I save

undefined

today I will have

undefined

in a year’s time ( 100

(1 + 10 %)

). Expressed the other way around, we could say that

undefined

in a year’s time has the same value as

undefined

today-its present value is

undefined

.
我们假设公司旨在最大化利润，因此如果投资项目的回报高于成本，它们就会进行投资。投资支出通常在回报之前，这些回报可能是零散的，并且分布在若干年中。处理这个问题的方法是计算预期利润流的“现值”（V）

Π

，其中

Π_{t}, Π_{t + 1}, \dots

是第

t, t + 1

期的利润流，等等。假设利率为

10 %

。那么如果我今天存

undefined

，一年后我将拥有

undefined

（100

(1 + 10 %)

）。反过来说，我们可以说一年后的

undefined

与今天的

undefined

具有相同的价值——它的现值是

undefined

。

More generally, if interest rates are constant at

r

, the value of X in

n

years’ time is the same as that of

X / (1 + r)^{n}

today. We can calculate the present value of our stream of expected profits,

Π^{E}

, from an investment project with profits over

T

years by:
更一般地说，如果利率在

r

保持不变，则

n

年后的 X 的价值与今天的

X / (1 + r)^{n}

相同。我们可以通过以下公式计算我们预期利润流的现值

Π^{E}

，这是一个在

T

年内有利润的投资项目：

V_{t}^{E} = Π_{t}^{E} + \frac{Π_{t + 1}^{E}}{(1 + r)} + \frac{Π_{t + 2}^{E}}{(1 + r)^{2}} + \dots + \frac{Π_{t + T}^{E}}{(1 + r)^{T}} = \sum_{i = 0}^{T} \frac{1}{(1 + r)^{i}} Π_{t + i}^{E}

(expected present value calculation)
（预期现值计算）
We use the superscript E to denote the expected value of a variable.
我们使用上标 E 来表示一个变量的期望值。
If the cost of the machine is greater than the present value of the flow of profits from the machine, then it would be more profitable not to buy the machine but instead put the money in the bank or in bonds (which earn interest of

r

). Similarly, if the money to purchase the machine is being borrowed, then if the cost of the machine is greater than the present value, buying the machine will be unprofitable. On the other hand, if the present value is greater than the cost, then this investment is profitable, and a profit-maximizing firm will go ahead with it.
如果机器的成本大于机器利润流的现值，那么不购买机器而是将钱存入银行或购买债券（这些可以赚取

r

的利息）将更有利。同样，如果购买机器所需的资金是借来的，那么如果机器的成本大于现值，购买机器将是不盈利的。另一方面，如果现值大于成本，那么这项投资是有利可图的，追求利润最大化的公司将会继续进行投资。
We can apply the same logic to model consumption decisions. We can account for the fact that the future will affect consumption decisions by calculating the present value of the stream of expected income over a person’s lifetime. If we assume that individuals live forever, we can use the formula for calculating present value to calculate the expected present value of lifetime wealth (

Ψ^{E}

, pronounced ‘sigh’), which is defined at time

t

as follows,
我们可以将相同的逻辑应用于模型消费决策。我们可以通过计算一个人一生中预期收入流的现值来考虑未来将如何影响消费决策。如果我们假设个体永生，我们可以使用计算现值的公式来计算一生财富的预期现值（

Ψ^{E}

，发音为“叹息”），其在时间

t

的定义如下：

Ψ_{t}^{E} = (1 + r) A_{t - 1} + \sum_{i = 0}^{\infty} \frac{1}{(1 + r)^{i}} y_{t + i}^{E}

(expected present value of lifetime wealth)
（预期终身财富的现值）
where

\sum_{i = 0}^{\infty} \frac{1}{(1 + r)^{i}} y_{t + i}^{E}

is the present value of expected post-tax lifetime labour income and

(1 + r) A_{t - 1}

are the resources available in period

t

from the assets the individual held in the previous period.

^{5}

其中

\sum_{i = 0}^{\infty} \frac{1}{(1 + r)^{i}} y_{t + i}^{E}

是预期税后终身劳动收入的现值，

(1 + r) A_{t - 1}

是在第

t

期可用的资源，这些资源来自个人在上一期持有的资产。

^{5}

The present value calculation says that a rise in the interest rate reduces the present value of future expected income flows, which is one determinant of current investment and consumption decisions.
现值计算表明，利率的上升会降低未来预期收入流的现值，这是当前投资和消费决策的一个决定因素。

1.2.5 Consumption 1.2.5 消费

People’s income fluctuates over the life cycle; it also changes when they lose their job, move to a different job or get a promotion. Given that people prefer to smooth out
人们的收入在生命周期中波动；当他们失去工作、换工作或获得晋升时，收入也会发生变化。考虑到人们倾向于平滑收入，

the fluctuations in their income in their spending behaviour, they take account of the future and would like to be able to save and borrow. The modelling of consumption should include how households form expectations about the future and how they borrow and save.
他们的收入波动影响了消费行为，他们考虑未来，并希望能够储蓄和借款。消费建模应包括家庭如何形成对未来的预期，以及他们如何借款和储蓄。
The desire to smooth consumption in the face of fluctuations in income is captured by the assumption of diminishing marginal utility of consumption. A simple way of visualizing this is to consider the choice of consumption in a world of just two periods. If we know income will be higher next period, how does that affect consumption now? Consider the choice between either low consumption equal to income in the first period and high consumption equal to income in the second period, or having the average of the two consumption levels in each period. If there is diminishing marginal utility of consumption, more consumption always increases utility, but successive increases in consumption deliver smaller and smaller benefits. Therefore, households will make the second choice, because consuming the average in both periods offers higher utility than the first choice.
在面对收入波动时，平滑消费的愿望体现在消费的边际效用递减假设中。一个简单的可视化方法是考虑一个只有两个时期的消费选择。如果我们知道下一个时期的收入会更高，这将如何影响现在的消费？考虑在第一个时期低消费等于收入和在第二时期高消费等于收入之间的选择，或者在每个时期都有两个消费水平的平均值。如果存在消费的边际效用递减，更多的消费总是会增加效用，但消费的连续增加带来的好处会越来越小。因此，家庭会做出第二个选择，因为在两个时期消费平均值提供的效用高于第一个选择。

The two-period model also provides insight about the effect of a change in the rate of interest on consumption. A rise in the interest rate increases the opportunity cost of consumption this period and incentivizes moving consumption to the future by saving more of the current period’s income. This is called the intertemporal substitution effect. As with any change in relative prices, there is also an income effect. For a creditor, the income effect is positive and could outweigh the substitution effect, raising current consumption. However, for debtor households (making interest payments), the income effect will dampen consumption, reinforcing the substitution effect. To work through the 2-period model, see Unit 9 in CORE Econ’s The Economy (https://www.coreecon.org/the-economy/book/text/09.html).
两期模型还提供了关于利率变化对消费影响的见解。利率上升增加了本期消费的机会成本，并激励将消费转移到未来，通过储蓄更多当前期的收入。这被称为跨期替代效应。与任何相对价格的变化一样，也存在收入效应。对于债权人来说，收入效应是正面的，可能会超过替代效应，从而提高当前消费。然而，对于负债家庭（支付利息的家庭）来说，收入效应会抑制消费，强化替代效应。要详细了解两期模型，请参见 CORE Econ 的《经济学》第 9 单元（https://www.coreecon.org/the-economy/book/text/09.html）。

The permanent income hypothesis (PIH)
永久收入假说 (PIH)

The permanent income hypothesis (PIH) states that individuals optimally choose how much to consume by allocating their resources across their lifetimes.

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Their resources include their assets and their current and future income. This is a forwardlooking decision and will depend on interest rates, asset values, expectations of future income and of future taxes. The PIH predicts that optimal consumption is smooth as compared to income. For example, when individuals start work, their income is low and they borrow to consume more; when income increases, they keep consumption constant and use the excess income to pay off debts and save for retirement; then at retirement, their income falls and they draw down their savings. Figure 1.7 shows how consumption and income change over the life cycle in this simplified example. The important point is that the PIH predicts that households will borrow and save in order to smooth consumption over their lifetimes. Likewise, over the business cycle, if an individual becomes unemployed, the model predicts that they borrow in order to sustain consumption during the spell of unemployment. As we shall see in Chapter 7
永久收入假说（PIH）指出，个人通过在其一生中合理分配资源来最佳选择消费金额。

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他们的资源包括资产以及当前和未来的收入。这是一个前瞻性的决策，取决于利率、资产价值、对未来收入和未来税收的预期。PIH 预测，最佳消费相较于收入是平滑的。例如，当个人开始工作时，他们的收入较低，因此借款以增加消费；当收入增加时，他们保持消费不变，并利用多余的收入偿还债务和为退休储蓄；然后在退休时，他们的收入下降，开始支取储蓄。图 1.7 展示了在这个简化示例中，消费和收入在生命周期中的变化。重要的一点是，PIH 预测家庭会借款和储蓄，以便在其一生中平滑消费。同样，在经济周期中，如果个人失业，模型预测他们会借款以维持失业期间的消费。正如我们将在第七章中看到的。

Figure 1.7 The permanent income hypothesis over the life cycle.
图 1.7 生命周期中的永久收入假说。
on fiscal policy, the government plays an important role in smoothing consumption through the provision of unemployment benefits.
在财政政策方面，政府通过提供失业救济金在平滑消费中发挥着重要作用。

The PIH model of consumption provides a stark contrast with the Keynesian consumption function where there is no explicit consideration of the future. Aggregate consumption of households is modelled there as the consumption of a fixed amount,

c_{0}

and a fixed proportion of the current period’s disposable income.
PIH 消费模型与凯恩斯消费函数形成鲜明对比，后者没有明确考虑未来。家庭的总消费在这里被建模为固定金额

c_{0}

和当前时期可支配收入的固定比例。
The PIH model is derived in more detail in Section 1.5.2 of the Appendix, but it is useful to set out the intuition and predictions of the model here, as it provides a framework for thinking about forward-looking consumption decisions. Given that income will fluctuate over a person’s lifetime, the starting point of the PIH is their desire to smooth out fluctuations in consumption and their ability to save and borrow in order to achieve this.
PIH 模型在附录的 1.5.2 节中有更详细的推导，但在这里阐明模型的直觉和预测是有用的，因为它提供了一个思考前瞻性消费决策的框架。考虑到收入在一个人的一生中会波动，PIH 的起点是他们希望平滑消费波动的愿望，以及他们为实现这一目标而进行储蓄和借款的能力。
The next question is whether an individual prefers their smooth consumption path to be one of constant consumption each period or of rising or falling consumption. This will depend on the relationship between the interest rate on saving and borrowing and the rate at which the individual trades off consumption in the future for consumption in the present. The latter is the subjective discount rate

ρ

, which is a measure of an individual’s impatience.
下一个问题是，个人是否更喜欢他们的平滑消费路径是每个时期的恒定消费，还是消费上升或下降。这将取决于储蓄和借款的利率与个人在未来消费与现在消费之间权衡的比率之间的关系。后者是主观折现率

ρ

，这是衡量个人不耐烦程度的指标。
The household chooses a path of consumption to maximize its lifetime utility subject to its lifetime budget constraint, which is
家庭选择一种消费路径，以最大化其终身效用，同时受到其终身预算约束

Ψ_{t}^{E} = (1 + r) A_{t - 1} + \sum_{i = 0}^{\infty} \frac{1}{(1 + r)^{i}} y_{t + i}^{E} . (lifetime budget constraint, expected lifetime wealth)

Note that when considering the PIH, income,

y

is defined as post-tax income. The key to solving this problem can be thought about over just two periods, since the same considerations will apply to every subsequent pair of periods.

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The answer is a simple
请注意，在考虑永久收入假说（PIH）时，收入

y

被定义为税后收入。解决这个问题的关键可以仅在两个时期内思考，因为相同的考虑将适用于每一对后续时期。

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答案是一个简单的

relationship between consumption this period and next period, which is called an Euler equation (the derivation is in Section 1.5.2 of the Appendix):
消费在本期与下期之间的关系称为欧拉方程（推导见附录第 1.5.2 节）：

C_{t} = \frac{1 + ρ}{1 + r} C_{t + 1}^{E} . (Euler equation)

The intuition about consumption smoothing is clearest if we take the case where the interest rate,

r

, and the subjective discount rate,

ρ

, are the same. This means the household gets the same (objective) return from saving,

r

, as their (subjective) willingness to trade off consumption in the present for consumption in the future,

ρ

. In this case, the agent prefers a constant level of consumption each period,

C_{t} = C_{t + 1}^{E}

.
关于消费平滑的直觉在利率

r

和主观折现率

ρ

相同时最为清晰。这意味着家庭从储蓄中获得的相同（客观）回报

r

与他们（主观）愿意在当前消费与未来消费之间进行权衡的程度

ρ

相同。在这种情况下，代理人更倾向于每个时期保持恒定的消费水平

C_{t} = C_{t + 1}^{E}

。
We can see that if the discount rate is above the interest rate, then

C_{t} > C_{t + 1}^{E}

and consumption is falling over time, reflecting the ‘impatience’ of the household. The opposite is the case if the discount rate is below the interest rate; a patient household will have a path in which consumption rises over time, i.e.

C_{t} < C_{t + 1}^{E}

. The crucial point to note is that whichever of these consumption patterns is chosen, it is independent of the period-by-period changes in income.
我们可以看到，如果折现率高于利率，那么

C_{t} > C_{t + 1}^{E}

，消费随着时间的推移而下降，反映了家庭的“急躁”。如果折现率低于利率，则情况正好相反；一个有耐心的家庭将会有一个消费随着时间上升的路径，即

C_{t} < C_{t + 1}^{E}

。需要注意的关键点是，无论选择哪种消费模式，它都与收入的逐期变化无关。
From now on we focus on the PIH where the subjective discount rate is equal to the real rate of interest, i.e.

(ρ = r)

. To implement a lifetime consumption plan like this, given each period’s income the household must do whatever saving and borrowing is called for in order to deliver the constant level of consumption each period. If current income is above permanent income, individuals save and if it is below, they borrow.
从现在开始，我们关注主观折现率等于实际利率的 PIH，即

(ρ = r)

。为了实施这样的终身消费计划，家庭必须根据每个时期的收入进行必要的储蓄和借款，以便在每个时期提供恒定的消费水平。如果当前收入高于永久收入，个人会储蓄；如果低于永久收入，他们则会借款。
The next step is to use the Euler equation to find out how much individuals consume each period (the derivation is in Section 1.5.2 of the Appendix). When

ρ = r

, consumption is
下一步是使用欧拉方程来找出个人每个时期的消费量（推导见附录 1.5.2 节）。当

ρ = r

时，消费为

C_{t} = \frac{r}{1 + r} Ψ_{t}^{E}

(PIH consumption function)
(PIH 消费函数)
The intuition is easy to understand: an individual with this consumption function will borrow and save to deliver a perfectly smooth consumption path (in expectation). The amount they consume each period is equal to the annuity value of expected lifetime wealth and is called ‘permanent income’. The individual consumes their permanent income and the formula ensures that in expectation, they will be able to do this forever.
直观上很容易理解：具有这种消费函数的个体将借款和储蓄，以实现一个完全平滑的消费路径（在预期中）。他们每个时期消费的金额等于预期终身财富的年金价值，称为“永久收入”。个体消费他们的永久收入，公式确保在预期中，他们将能够永远做到这一点。

Predictions of the PIH PIH 的预测

How does consumption react to a change in the interest rate? If the change in the interest rate is expected to persist in the long run, then a rise in the interest rate reduces the expected present value of lifetime income flows and therefore, permanent income and consumption. On the other hand, if the central bank raises the interest rate as a policy measure, this is a temporary change. Since a higher interest rate increases the opportunity cost of current consumption, it leads to a fall in

C_{t}

(the ‘intertemporal substitution effect’).
消费如何对利率变化作出反应？如果利率的变化预计会在长期内持续，那么利率的上升会降低终身收入流的预期现值，从而减少永久收入和消费。另一方面，如果中央银行将利率作为政策措施提高，这是一种临时变化。由于较高的利率增加了当前消费的机会成本，因此会导致

C_{t}

（“跨期替代效应”的下降）。
How does consumption react when a change in income occurs? The answer depends on the nature of the change in income. According to the PIH,
消费在收入变化时如何反应？答案取决于收入变化的性质。根据永久收入假说，

Anticipated or foreseen changes in income should have no effect on consumption when they occur. The reason is that anticipated changes will already have been incorporated into consumption through the recalculation of permanent income.
预期或预见的收入变化在发生时不应对消费产生影响。原因是，预期的变化已经通过对永久收入的重新计算融入了消费中。

Hence, when the change in current income is recorded, the marginal propensity to consume is predicted to be zero (the multiplier is equal to one). A finding of ‘excess sensitivity’ to anticipated income changes would contradict the full smoothing behaviour predicted by the PIH.
因此，当记录当前收入的变化时，边际消费倾向被预测为零（乘数等于一）。对预期收入变化的“过度敏感”发现将与永久收入假说所预测的完全平滑行为相矛盾。
2. News or unanticipated changes in income should affect consumption because they require the recalculation of future lifetime wealth,

Ψ_{t}^{E}

.
2. 新闻或意外的收入变化应该影响消费，因为它们需要重新计算未来的终身财富，

Ψ_{t}^{E}

。
a. News of a temporary increase in income. If current income

y_{t}

increases unexpectedly by one unit, consumption increases by the extent to which this raises permanent income. Since the increase in one unit will be spread over the entire future, the PIH consumption function tells us that permanent income and hence consumption rise very little, just by

\frac{r}{1 + r}

times the increase in lifetime wealth. The marginal propensity to consume out of temporary income is

\frac{r}{1 + r}

. (For example, if the real interest rate is 4 per cent, the MPC is 0.038 , which implies a multiplier barely above one.) A finding of ‘excess sensitivity’ of consumption to a temporary change in income would contradict the simple PIH.
a. 关于收入暂时增加的消息。如果当前收入

y_{t}

意外增加一个单位，消费将增加到这一点提高了永久收入的程度。由于这一单位的增加将分摊到未来的整个时间段，PIH 消费函数告诉我们，永久收入因此消费的增加非常有限，仅为终身财富增加的

\frac{r}{1 + r}

倍。对暂时收入的边际消费倾向为

\frac{r}{1 + r}

。（例如，如果实际利率为 4%，则 MPC 为 0.038，这意味着乘数略高于 1。）消费对收入暂时变化的“过度敏感性”发现将与简单的 PIH 相矛盾。
b. News of a permanent increase in income. If there is news that income

y_{t}

is higher from now and for every future period by one unit, then permanent income and hence consumption rise by the full one unit.

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This means the marginal propensity to consume out of post-tax permanent income is one. A finding of ‘excess smoothness’ of consumption in response to news of permanent income changes would contradict the simple PIH.
b. 关于收入永久性增加的消息。如果有消息称收入

y_{t}

从现在起在未来的每个时期都提高一个单位，那么永久收入以及消费将增加一个单位。

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这意味着税后永久收入的边际消费倾向为 1。消费对永久收入变化消息的“过度平滑”发现将与简单的永久收入假说相矛盾。

We shall see that the predictions of the simple PIH in relation to income changes are not supported by the facts.
我们将看到，简单的永久收入假说（PIH）关于收入变化的预测并没有得到事实的支持。

Excess sensitivity to anticipated changes in income
对预期收入变化的过度敏感性

The first testable hypothesis suggests that if the change in income was known in advance, there should be no change in consumption at the time income changes. This is because consumption should already have adjusted as soon as the news arrived of the change in future income. An influential study by Campbell and Mankiw (1989) tested this hypothesis econometrically using aggregate data on consumption and income from the G7 countries. The study rejected a model in which all consumers were following PIH and could not reject a model in which half of all consumers were simply following the ‘rule of thumb’ of spending their current income. In other words, they found that current consumption was overly sensitive to expected changes in income across the G7.
第一个可检验的假设表明，如果收入的变化是事先已知的，那么在收入变化时消费应该没有变化。这是因为消费应该在未来收入变化的消息到达时就已经调整。坎贝尔和曼基尔（1989）的一项有影响力的研究使用 G7 国家的消费和收入的汇总数据对这一假设进行了计量经济学测试。该研究拒绝了一个所有消费者都遵循永久收入假说（PIH）的模型，而无法拒绝一个所有消费者中有一半仅仅遵循“经验法则”支出其当前收入的模型。换句话说，他们发现当前消费对 G7 国家预期收入变化的敏感性过高。
A study used the 2001 federal income tax rebates in the US as a testing ground: according to the PIH, the one-off (temporary) tax rebate should have very little effect on spending and if there is any effect, it should occur when the announcement was made and not when the cheques arrived. Johnson et al. (2006) were able to identify
一项研究以 2001 年美国联邦所得税退税作为测试基础：根据永久收入假说，一次性（临时）税收退税对消费的影响应该非常小，如果有任何影响，应该在公告发布时发生，而不是在支票到达时。约翰逊等人（2006）能够识别出

the causal effect of the tax rebate by using household data and the fact that the timing of the sending of cheques was based on the taxpayer’s social security number, which is random. They found that the average household spent between

20 - 40 %

of the (predictable) tax rebate in the 3-month period when the cheque was received rather than when the programme was announced. They also found that it was households with low income and wealth that responded most, underlining the likely role of limits on the ability of households to borrow, i.e. credit constraints. This is referred to as the excess sensitivity of consumption and is evidence against the predictions of the PIH.

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税收退还的因果效应是通过使用家庭数据以及支票发送时间基于纳税人的社会安全号码这一随机事实来分析的。他们发现，平均家庭在收到支票的三个月期间花费了

20 - 40 %

（可预测的）税收退还，而不是在该计划宣布时。他们还发现，低收入和低财富的家庭反应最为明显，这突显了家庭借贷能力的限制，即信贷约束的可能作用。这被称为消费的过度敏感性，是对永久收入假说（PIH）预测的证据。

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A study by Ganong and Noel (2019) provides powerful evidence about consumption behaviour in forward-looking models even when they include credit constraints. This evidence suggests that the predictions of the PIH fail not only because many households are unable to borrow to smooth their consumption but also because of the cognitive trait of present-bias (also referred to as myopia or short-sightedness). A trait of present-bias means that the desire for consumption in the present outweighs the desire to smooth consumption.
Ganong 和 Noel（2019）的研究提供了关于前瞻性模型中消费行为的有力证据，即使这些模型包含信用约束。这些证据表明，永久收入假说的预测失败不仅是因为许多家庭无法借款以平滑消费，还因为存在当前偏见的认知特征（也称为近视或短视）。当前偏见的特征意味着对当前消费的渴望超过了平滑消费的渴望。

We can learn from this study due to its focus on what happens to consumption when unemployment benefits run out. The setting is the US and centres on 17 states where entitlement to unemployment insurance benefits is exhausted after six months. The authors use bank account data for households where someone had a single spell of unemployment between September 2013 and June 2016 and observe what happens to consumption when the spell of unemployment begins and when it ends.
我们可以从这项研究中学习，因为它关注失业救济金用尽时消费发生了什么。研究背景是美国，集中在 17 个州，这些州的失业保险福利在六个月后就会用尽。作者使用了 2013 年 9 月至 2016 年 6 月期间，有人经历过一次失业的家庭的银行账户数据，并观察失业开始和结束时消费发生了什么。
They find that consumption is highly sensitive to unemployment benefits. At the beginning of the spell of unemployment when wage income is replaced by unemployment benefit, consumption falls indicating an inability or unwillingness to smooth the drop in income while unemployed. For those still unemployed when eligibility to unemployment benefits ends at six months, consumption falls again. Since the termination of unemployment benefits is predictable, the sharp further drop in consumption at that time reflects a failure to save and build up a buffer stock in a run-up to the end date that would have enabled some smoothing. The drops in consumption are largest for households with the fewest liquid assets. They find that the MPC of unemployed households ( 0.8 ) is substantially higher than for employed households.
他们发现消费对失业救济金非常敏感。在失业初期，当工资收入被失业救济金替代时，消费下降，这表明在失业期间无法或不愿意平滑收入的下降。对于在失业救济金资格在六个月后结束时仍然失业的人，消费再次下降。由于失业救济金的终止是可预测的，因此在那个时候消费的急剧下降反映了在结束日期之前未能储蓄和建立缓冲库存，从而无法实现一定程度的平滑。消费的下降在流动资产最少的家庭中最大。他们发现，失业家庭的边际消费倾向（0.8）明显高于就业家庭。

Responsiveness to news 对新闻的响应

Contrary to the PIH, studies have found that consumption responds strongly to temporary income shocks. An early example of this was the large consumption response of US veterans after the Second World War to an unexpected windfall payout of the National Service Life Insurance. This evidence questions the assumption in the PIH that households make optimizing decisions over a time horizon stretching a long way into the future and use a low discount rate (equal to the interest rate) to compare consumption at near and far horizons. The fact that people respond to a windfall by raising spending suggests that discount rates are higher than assumed in the simple
与永久收入假说（PIH）相反，研究发现消费对临时收入冲击的反应非常强烈。一个早期的例子是，第二次世界大战后，美国退伍军人对国家服务人寿保险意外一次性赔付的消费反应非常大。这一证据质疑了 PIH 中假设家庭在很长的时间范围内做出优化决策，并使用低折现率（等于利率）来比较近远期的消费。人们通过增加支出来回应意外之财的事实表明，折现率高于简单模型中假设的水平。

PIH: people appear to be more impatient than the hypothesis assumes: they display ‘present-bias’.
PIH：人们似乎比假设所假定的更不耐烦：他们表现出“当前偏见”。
It is also likely that uncertainty about whether observed income changes are temporary or permanent prevents households from acting exactly as PIH would predict. For example, if a household mistakenly thought a temporary change in their current income was permanent, then they would consume more of the income change than would be consistent with PIH behaviour. Jappelli and Pistaferri (2010) review the evidence and suggest that it shows that consistent with the PIH, consumption responds more to permanent than transitory income shocks, but that households do not revise their consumption fully into line with permanent shocks.
家庭对观察到的收入变化是暂时的还是永久的不确定性，可能会阻止他们的行为完全符合永久收入假说（PIH）的预测。例如，如果一个家庭错误地认为他们当前收入的暂时变化是永久的，那么他们会消费更多的收入变化，这与 PIH 行为不一致。Jappelli 和 Pistaferri（2010）回顾了证据，并建议这些证据表明，消费对永久性收入冲击的反应大于对暂时性收入冲击的反应，但家庭并没有完全根据永久性冲击调整他们的消费。

Credit constraints, present-bias, and uncertainty
信贷约束、现时偏见和不确定性

The evidence presented suggests there are four reasons for the failure of the simple PIH to provide an adequate model of aggregate consumption:
所提供的证据表明，简单的 PIH 未能提供足够的总消费模型的原因有四个：

The presence of credit constraints, which prevent borrowing by households who lack wealth or collateral
信贷限制的存在，阻止了缺乏财富或抵押品的家庭借款
Present-bias, which explains why some households do not smooth their consumption even when they could
现时偏见解释了为什么一些家庭即使能够也不平滑消费
Uncertainty about future income, which explains precautionary saving over and above the level predicted by the PIH
对未来收入的不确定性，这解释了超出永久收入假说（PIH）预测水平的预防性储蓄
The particular role of house price increases and of house price bubbles (this is covered in Chapters 8 and 9)
房价上涨和房价泡沫的特定作用（第 8 章和第 9 章对此进行了讨论）

Credit constraints 信贷限制

If people prefer to smooth their consumption, but are prevented from doing so because they cannot borrow to bring forward consumption when their current income is below their expected permanent income, they are said to face credit constraints. Because of information problems facing banks in assessing creditworthiness, banks are not always willing to lend to households without the wealth or collateral to secure a loan in order that they can smooth their consumption (see Chapter 5). Figure 1.8 shows the consumption response for PIH and credit-constrained households to an anticipated rise in income. The PIH households borrow to raise consumption as soon as the news of the future income increase is received, whereas the credit-constrained households do not have this option, so consumption can only rises when income does.
如果人们希望平滑消费，但由于无法借款以提前消费（当他们当前收入低于预期的永久收入时），他们就会面临信贷约束。由于银行在评估信用 worthiness 时面临信息问题，银行并不总是愿意向没有财富或抵押品来担保贷款的家庭提供贷款，以便他们能够平滑消费（见第 5 章）。图 1.8 显示了 PIH 和受信贷约束家庭对预期收入上升的消费反应。PIH 家庭在收到未来收入增加的消息后立即借款以提高消费，而受信贷约束的家庭没有这个选择，因此消费只能在收入上升时增加。
Estimates from studies using household data have found that around

20 %

of the population in the United States is credit constrained.

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A much higher estimate of closer to two-thirds comes from a study of the way people respond to automatic increases in the limits on their credit cards: if spending responds to such automatic increases, it suggests the card-owner was credit constrained.

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根据使用家庭数据的研究估计，美国约有

20 %

的人口受到信用限制。

^{10}

另一项研究对人们如何应对信用卡限额自动增加的方式进行了研究，得出的估计则高得多，接近三分之二：如果消费对这种自动增加作出反应，这表明持卡人受到信用限制。

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Figure 1.8 The consumption response of PIH and credit-constrained households to an anticipated rise in income.
图 1.8 PIH 和受信贷限制家庭对预期收入增长的消费反应。

Another explanation is that some households with wealth have few liquid assets, namely assets that can be easily sold and converted into money to use for consumption. Their spending would be expected to respond to higher credit card limits. These so-called wealthy hand-to-mouth own wealth in the form of equity in their house but the wealth is illiquid. This results in behaviour akin to that of low-income households without wealth: both have a high marginal propensity to consume (see the end of this section for a hybrid consumption function incorporating this phenomenon, and Chapter 18 for further details about recent research on this behaviour).
另一种解释是，一些富裕家庭的流动资产很少，即可以轻松出售并转化为消费资金的资产。他们的消费支出预计会对更高的信用卡额度作出反应。这些所谓的富裕但入不敷出的家庭拥有以房屋股权形式存在的财富，但这些财富是非流动的。这导致他们的行为类似于没有财富的低收入家庭：两者都有较高的边际消费倾向（有关这一现象的混合消费函数，请参见本节末尾，关于这一行为的最新研究，请参见第 18 章）。

Present-bias 当前偏见

The observation that consumption of a substantial fraction of households changes one-for-one with their current income (i.e. very little borrowing or saving) reflects two features of the real world. One is the presence of credit constraints, which can prevent households from borrowing to smooth their consumption. This seems to especially affect young and low wealth households. Faced with a spell of unemployment, for example, constraints on borrowing will prevent a household from maintaining its level of consumption.
观察到大量家庭的消费与其当前收入一一对应（即几乎没有借贷或储蓄）反映了现实世界的两个特征。一是存在信贷约束，这可能阻止家庭借款以平滑其消费。这似乎特别影响年轻和低财富家庭。例如，面对失业的情况，借款的限制将阻止家庭维持其消费水平。

Whereas credit constraints can explain the inability of households to borrow to smooth consumption, it cannot explain the failure of some households to save, which would allow for some consumption smoothing without relying on access to loans.

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由于信贷限制可以解释家庭无法借款以平滑消费的原因，但它无法解释一些家庭未能储蓄的情况，而储蓄可以在不依赖贷款的情况下实现一定的消费平滑。

Figure 1.9 The consumption response of PIH and myopic households to an anticipated fall in income.
图 1.9 PIH 和近视家庭对预期收入下降的消费反应。

It seems that there are households who are not only credit constrained, but who also experience present-bias. Figure 1.9 shows the consumption response for PIH and myopic (i.e. present-bias) households to an anticipated fall in income. The PIH households start saving as soon as the news of the fall in future income is received, which allows them to smooth consumption over the two periods. In contrast, the households with present-bias fail to reduce current consumption upon receipt of the news, meaning consumption falls dramatically when income actually falls.
似乎有一些家庭不仅受到信贷限制，还存在当前偏见。图 1.9 显示了永久收入假说（PIH）和短视（即当前偏见）家庭对预期收入下降的消费反应。PIH 家庭在收到未来收入下降的消息后立即开始储蓄，这使他们能够在两个时期内平滑消费。相比之下，具有当前偏见的家庭在收到消息时未能减少当前消费，这意味着当收入实际下降时，消费会急剧下降。

Experimental evidence on present-bias
关于当前偏见的实验证据

The short-sightedness of households is represented in a model of consumption by a higher discount rate for the short run than the longer run. A simple illustration of what this means is to consider the following experiment: if you were choosing today for eating next week, would you choose fruit or chocolate? About three-quarters of experimental subjects choose fruit. When asked whether they would choose fruit or chocolate to eat today,

70 %

choose chocolate. From this, we can see that preferences are not consistent over time, because if we run time forward to next week, the three quarters who said they would prefer fruit would have shrunk to

30 %

. When next week comes, they would in fact, choose chocolate.

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家庭的短视在消费模型中表现为短期的折现率高于长期的折现率。一个简单的例子是考虑以下实验：如果你今天选择下周吃的东西，你会选择水果还是巧克力？大约四分之三的实验对象选择水果。当被问到今天选择水果还是巧克力时，

70 %

选择巧克力。从中我们可以看出，偏好在时间上并不一致，因为如果我们把时间推到下周，表示他们更喜欢水果的四分之三的人会减少到

30 %

。当下周到来时，他们实际上会选择巧克力。

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Present-bias is also documented in experiments related to consumption and saving behaviour. For example, subjects have a budget and can choose between two accounts paying the same interest rate: a ‘freedom account’ with no constraints on withdrawal or a ‘commitment account’ where there are restrictions on withdrawal before a date chosen by the participant. From a purely economic point of view the freedom account is preferable because it imposes no constraints on withdrawal. In spite of this, more than half of investment was put in the commitment account (no withdrawal before the goal date), which reveals the desire of the experimental subjects to commit themselves to saving.

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现今偏见在与消费和储蓄行为相关的实验中也得到了证明。例如，受试者有一个预算，可以在两个支付相同利率的账户之间进行选择：一个是没有提款限制的“自由账户”，另一个是有提款限制的“承诺账户”，后者在参与者选择的日期之前不能提款。从纯经济的角度来看，自由账户是更可取的，因为它没有提款限制。尽管如此，超过一半的投资被放入了承诺账户（在目标日期之前不能提款），这揭示了实验受试者希望承诺储蓄的愿望。
Very large differences between a high short-run and a low long-run discount rate help explain why people simultaneously save in illiquid assets such as housing (where the interest rate is low) and borrow on credit cards, paying a very high interest rate. Although a person can agree that their expected wellbeing is higher if they save more now, they are unable to resist consuming and blowing a hole in their saving plan when they get an unexpected increase in their income. One way of committing to saving is to take out a mortgage. Another is for the government to automatically enrol people in schemes for saving for retirement (from which they can opt out) such as pensions.
高短期和低长期折现率之间的巨大差异有助于解释为什么人们同时在流动性差的资产（如住房，利率较低）中储蓄，并在信用卡上借款，支付非常高的利率。尽管一个人可以同意，如果他们现在储蓄更多，他们的预期幸福感会更高，但当他们意外增加收入时，他们无法抵制消费，从而破坏了储蓄计划。承诺储蓄的一种方式是申请抵押贷款。另一种方式是政府自动将人们纳入退休储蓄计划（他们可以选择退出），例如养老金。

Uncertainty and precautionary saving
不确定性和预防性储蓄

If there is uncertainty about future job opportunities or about health, then the household may want to save to insure themselves against future contingencies. For example, having a ‘buffer stock of savings’ could be important to sustain consumption in the event of job loss for a credit-constrained household.

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In the face of uncertainty, households would tend to save more early in life than the PIH predicts with the result that consumption rises with income (not by more than income) early in the life cycle. If saving for a rainy day is important, then the utility provided by having assets to tide the household over if need be outweighs the utility that would have come from higher consumption in the early years.
如果对未来的工作机会或健康状况存在不确定性，那么家庭可能希望储蓄以保障自己应对未来的意外情况。例如，拥有“储备储蓄”对于在失业情况下维持消费可能对信用受限的家庭至关重要。在不确定性面前，家庭往往会在生活早期比永久收入假说（PIH）预测的储蓄更多，导致消费随着收入的增加而上升（但不会超过收入）在生命周期的早期。如果为不时之需储蓄很重要，那么拥有资产以应对家庭的需求所带来的效用超过了早期消费所带来的效用。

House price changes and house price bubbles
房价变化和房价泡沫

Changes in the price of houses affects the consumption spending of creditconstrained home owners, including those with mortgages. A rise in house prices increases the owner’s equity in the house, which means the value of their collateral is higher and banks will lend them more, enabling them to make purchases that had previously been constrained due to lack of credit. Home equity, banks and collateral are explained in Chapter 5, a model of house price bubbles and the collateral channel of consumption is developed in Chapter 8, and the role played by the impact on consumption of the house price boom and bust in the global financial crisis is covered in Chapter 9.
房价的变化会影响信用受限的房主的消费支出，包括那些有抵押贷款的房主。房价上涨会增加房主在房屋中的权益，这意味着他们的抵押品价值更高，银行会借给他们更多的钱，使他们能够进行之前因缺乏信用而受到限制的消费。房屋权益、银行和抵押品在第五章中进行了说明，房价泡沫模型和消费的抵押品渠道在第八章中进行了探讨，而房价的繁荣与萧条对消费的影响在全球金融危机中的作用则在第九章中进行了讨论。

An empirical consumption function
经验消费函数

We now consider a consumption function estimated on macroeconomic data for the UK, US and Japan.

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The empirical model allows for permanent income considerations to affect consumption; households will have a notion of sustainable consumption constrained by their budget, which includes their income forecasts. They will therefore know that they cannot spend their existing assets now without impairing future consumption and that future income affects their sustainable consumption.
我们现在考虑一个基于英国、美国和日本的宏观经济数据估计的消费函数。

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实证模型允许永久收入的考虑影响消费；家庭将有一个可持续消费的概念，这受到他们预算的限制，包括他们的收入预测。因此，他们将知道，如果现在花费现有资产，将会影响未来的消费，并且未来的收入会影响他们的可持续消费。
In addition to permanent income considerations based on forecasts, the consumption function that is estimated includes the following factors.
除了基于预测的永久收入考虑外，估计的消费函数还包括以下因素。

Consumption is predicted to respond more strongly to current income than the PIH predicts due to the presence of credit constraints and present-bias. This is modelled by a high discount rate on future income.
消费预计将对当前收入的反应比永久收入假说（PIH）预测的更强烈，这主要是由于信贷约束和现时偏见的存在。这通过对未来收入施加高折现率来建模。
Because of income uncertainty, households save for precautionary reasons. Income uncertainty is measured by the change in the unemployment rate; an increase is predicted to reduce consumption.
由于收入不确定性，家庭出于预防原因而储蓄。收入不确定性通过失业率的变化来衡量；预计失业率的上升会减少消费。
Credit conditions matter. Higher house prices interact with increased credit availability to increase consumption. The presence of this collateral effect depends on the institutional arrangements for mortgage borrowing in the country in question. In some countries, home equity loans are available: this is a loan secured against the equity in an individual’s home. (Home equity is the difference between the current market value of the house and the remaining mortgage, hence it increases as house prices rise.) When credit conditions are eased, the collateral effect operates to boost consumption. In countries where it is not possible to get a home equity loan, as in France, Germany, Japan and China, this channel does not operate. In fact, if home equity loans are unavailable and high down-payments are required to get a mortgage (as in Japan), then higher house prices can actually dampen consumer spending as young people have to save more to accumulate the required downpayment.
信贷条件很重要。更高的房价与增加的信贷可用性相互作用，促进消费。这种抵押品效应的存在取决于相关国家的抵押贷款制度安排。在一些国家，提供房屋净值贷款：这是一种以个人住房的净值作为担保的贷款。（房屋净值是房屋当前市场价值与剩余抵押贷款之间的差额，因此随着房价上涨而增加。）当信贷条件放宽时，抵押品效应会促进消费。在法国、德国、日本和中国等国家，无法获得房屋净值贷款，因此这一渠道无法发挥作用。事实上，如果无法获得房屋净值贷款，并且获得抵押贷款需要高额首付款（如在日本），那么更高的房价实际上可能抑制消费支出，因为年轻人需要存更多的钱来积累所需的首付款。
The current real interest rate. There are many interest rate channels, some of which point to a negative and some to a positive effect of a higher real interest rate on consumption.
当前的实际利率。有许多利率渠道，其中一些指向更高的实际利率对消费的负面影响，而另一些则指向正面影响。
Increases in households’ wealth to income ratio are expected to increase consumption.
家庭财富与收入比的增加预计将促进消费。

The empirical consumption function includes the housing and financial variables that are at the centre of the discussion in Chapters 5, 8, and 9, where the banking system, financial cycles and financial crises are brought into the macro model.
经验消费函数包括住房和金融变量，这些变量是第 5、8 和 9 章讨论的中心，在这些章节中，银行系统、金融周期和金融危机被纳入宏观模型。

For the UK and US, the estimated consumption functions suggest that roughly 50 per cent of household consumption is accounted for by permanent income considerations. Current income has an important positive effect on consumption and the real interest
对于英国和美国，估计的消费函数表明，大约 50%的家庭消费是由永久收入考虑所决定的。当前收入对消费和实际利率有重要的积极影响。

rate has a negative effect. The wealth to income ratio has a positive effect but different types of wealth have different effects. For example, liquid wealth in the form of cash in a bank or savings account is more spendable than pension wealth. And the evidence is that for a given degree of access to new credit, higher debt of

$ 100

reduces consumer spending more than higher pension or stock market wealth of

$ 100

increases spending.
利率有负面影响。财富与收入的比率有正面影响，但不同类型的财富有不同的影响。例如，银行或储蓄账户中的流动财富比养老金财富更容易支出。证据表明，在获得新信贷的程度相同的情况下，

$ 100

的更高债务会比

$ 100

的更高养老金或股市财富更大程度地减少消费支出。

For the US and UK, where home equity loans are important, increased credit availability and house prices interact to raise consumption through the collateral effect. A house price boom in conditions where access to credit is easier, boosts consumption.
对于美国和英国，房屋净值贷款很重要，信贷可获得性和房价的增加相互作用，通过抵押效应提高消费。在信贷获取更容易的情况下，房价的繁荣会促进消费。

Aggregate consumption and current income: A hybrid consumption function
聚合消费与当前收入：一种混合消费函数

For the macroeconomic model, we bring together insights from the PIH with the evidence on credit constraints and present-bias to derive a hybrid aggregate consumption function. This has the same form as the Keynesian consumption function,

(c_{t} =

c_{0} + c_{1} (1 - t) y_{t})

and both provides a realistic average marginal propensity to consume and associated multiplier for use in the IS curve and matches the empirical evidence that is accruing about so-called hand-to-mouth household consumption behaviour.
对于宏观经济模型，我们结合了永久收入假说的见解与关于信贷约束和当前偏见的证据，推导出一种混合的总消费函数。它与凯恩斯消费函数的形式相同，并且提供了一个现实的平均边际消费倾向和相关的乘数，以便在 IS 曲线中使用，并与关于所谓的“手到口”家庭消费行为的实证证据相匹配。

When income increases, hand-to-mouth (HTM) households consume all of this so their MPC is equal to one. This behaviour is characteristic of households without wealth and with present-bias. Moreover, there are also richer households who display the same hand-to-mouth behaviour. These are typically households with mortgages and although they have some home equity wealth, this is illiquid. They could save and borrow to smooth fluctuations in income but are observed not to do so, preferring as discussed above to put their savings into illiquid wealth and as a result to experience a less smooth consumption path (for further detail, see Chapter 18).
当收入增加时，手头拮据（HTM）家庭会将所有收入消费掉，因此他们的边际消费倾向等于 1。这种行为是没有财富且存在当前偏见的家庭的特征。此外，还有一些较富裕的家庭也表现出相同的手头拮据行为。这些家庭通常有抵押贷款，尽管他们有一些房屋净值财富，但这部分财富是流动性差的。他们可以通过储蓄和借款来平滑收入波动，但观察到他们并没有这样做，而是更倾向于将储蓄投入流动性差的财富，因此经历了较不平滑的消费路径（更多细节见第 18 章）。

To derive a hybrid consumption function, we assume that the share of PIH households is

σ

(sigma) and of HTM households is

(1 - σ)

. Then:
为了推导混合消费函数，我们假设永久收入假说（PIH）家庭的比例为

σ

（sigma），而持有到期（HTM）家庭的比例为

(1 - σ)

。然后：

\begin{aligned} c_{t} & = σ c_{t}^{PIH} + (1 - σ) c_{t}^{HTM} \\ Using c_{t}^{PIH} & = \frac{r}{1 + r} Ψ_{t}^{E} + \frac{r}{1 + r} y_{t} and \\ c_{t}^{HTM} & = y_{t}, we have \\ c_{t} & = σ \frac{r}{1 + r} Ψ_{t}^{E} + σ \frac{r}{1 + r} y_{t} + (1 - σ) y_{t} \\ = σ {\hat{c}}_{0} + \underset{c_{1}}{\underset{⏟}{[σ \frac{r}{1 + r} + (1 - σ)]}} y_{t}, where \\ {\hat{c}}_{0} & \equiv \frac{r}{1 + r} Ψ_{t}^{E} and σ is the share of PIH households \end{aligned}

Note that since the first term in the square brackets is small, the MPC is closely approximated by the share of HTM households.
请注意，由于方括号中的第一个项很小，MPC 与 HTM 家庭的份额非常接近。

From the hybrid consumption function, we see that changes in expected lifetime wealth shift the consumption function through the ‘autonomous consumption’ term, as does a change in the share of PIH households. The marginal propensity to consume increases with the proportion of hand-to-mouth households, which may rise in a recession when the risk of unemployment goes up.
从混合消费函数来看，预期终身财富的变化通过“自主消费”项移动消费函数，持久收入假说（PIH）家庭的比例变化也是如此。边际消费倾向随着手到口家庭的比例增加而增加，这在经济衰退时可能上升，因为失业风险增加。

The Keynesian consumption function is a simplified version of the hybrid consumption function; both imply the presence of a multiplier above one in the macro model. There is one point to clarify: the

y_{t}

in the Keynesian consumption function is pre-tax income whereas in the PIH,

y_{t}

, refers to post-tax income.
凯恩斯消费函数是混合消费函数的简化版本；两者都暗示宏观模型中存在一个大于一的乘数。有一点需要澄清：凯恩斯消费函数中的

y_{t}

是税前收入，而在永久收入假说（PIH）中，

y_{t}

指的是税后收入。
To make the hybrid consumption function consistent with notation in the rest of the macro model, we write it as:
为了使混合消费函数与宏观模型其余部分的符号一致，我们将其写为：

c_{t} = σ {\hat{c}}_{0} + [σ \frac{r}{1 + r} + (1 - σ)] (1 - t) y_{t},

where

c_{t} = σ {\hat{c}}_{0} + [σ \frac{r}{1 + r} + (1 - σ)] (1 - t) y_{t},

在哪里

{\hat{c}}_{0} = \frac{r}{1 + r} Ψ_{t}^{E}

and

Ψ_{t}^{E}

now refers to expected post-tax lifetime wealth

{\hat{c}}_{0} = \frac{r}{1 + r} Ψ_{t}^{E}

和

Ψ_{t}^{E}

现在指的是预期的税后终身财富
(hybrid consumption function)
混合消费函数

1.2.6 Investment 1.2.6 投资

The decision a firm makes of whether to invest or not will depend on their expectations about future after tax profits. The role of expectations helps explain the excess volatility of investment over the other components of GDP visible in Figure 1.1. In developing the IS equation, we used a simple linear relation to model investment in Section 1.2.3, where the amount of investment depended positively on expected future profits (captured by the constant term,

a_{0}

) and negatively on the real interest rate (a measure of firms’ borrowing costs). In this subsection we introduce a more sophisticated forward-looking modelling approach-the q theory of investment.
公司是否决定投资将取决于他们对未来税后利润的预期。预期的作用有助于解释投资相对于 GDP 其他组成部分的过度波动，如图 1.1 所示。在开发 IS 方程时，我们在 1.2.3 节中使用了一个简单的线性关系来建模投资，其中投资金额与预期未来利润（由常数项

a_{0}

表示）呈正相关，与实际利率（企业借款成本的衡量）呈负相关。在本小节中，我们引入了一种更复杂的前瞻性建模方法——投资的 q 理论。

Tobin's $q$ theory of investment
托宾的投资理论

This theory was developed by Nobel Prize winner James Tobin.

^{17}

Like the PIH, Tobin’s

q

theory of investment is forward looking. Firms choose the amount of investment to undertake with a view to maximizing the expected discounted profits over the lifetime of the project. The

q

theory amounts to comparing the benefits from investment in an increase in the capital stock with the costs of doing so: if the expected benefits exceed the costs, investment should take place.
该理论由诺贝尔奖得主詹姆斯·托宾提出。与永久收入假说（PIH）类似，托宾的投资理论是前瞻性的。企业选择进行投资的金额，以期在项目的生命周期内最大化预期的折现利润。该理论相当于将增加资本存量的投资收益与其成本进行比较：如果预期收益超过成本，则应进行投资。

Marginal and average q models
边际和平均 q 模型

The theory is derived explicitly in Section 1.5.3 of the Appendix, but it is useful to set out the equation for marginal

q

here, as it provides a framework for thinking about forward-looking investment decisions:
该理论在附录的第 1.5.3 节中明确推导，但在这里列出边际

q

的方程是有用的，因为它为思考前瞻性投资决策提供了框架：

q = \frac{Marginal benefit of investment}{Marginal cost of investment} = \frac{A f_{K}}{δ + r} . (marginal q model of investment)

The firm’s production function is

y_{t} = Af (N_{t}, Kt)

with the inputs of labour and capital and the level of technology, A, which will rise with technological progress. The marginal product of capital is denoted by

f_{k}, δ

is the rate at which capital depreciates and

r

is the rate of interest.
该公司的生产函数是

y_{t} = Af (N_{t}, Kt)

，其投入包括劳动和资本，以及技术水平 A，随着技术进步而提高。资本的边际产出用

f_{k}, δ

表示，

f_{k}, δ

是资本折旧的速率，

r

是利率。
The model is intrinsically forward looking since the firm is considering the future benefits from investment spending now. Before looking at the expression on the right,
该模型本质上是前瞻性的，因为公司正在考虑现在投资支出的未来收益。在查看右侧的表达式之前，

we note that the optimal amount of investment occurs when

q = 1

. This is where the expected marginal benefit of investment is equal to the expected marginal cost. Thus, if

q > 1

, the marginal benefit of investment exceeds the marginal cost, so firms should invest to increase the capital stock until

q = 1

. If

q < 1

firms should reduce their capital stock.
我们注意到，最佳投资量发生在

q = 1

。这是投资的预期边际收益等于预期边际成本的地方。因此，如果

q > 1

，投资的边际收益超过边际成本，因此公司应该投资以增加资本存量，直到

q = 1

。如果

q < 1

，公司应该减少其资本存量。

It is important to note that the

q

theory of investment optimizes firms’ investment decisions (a flow) by pinning down the optimal capital stock. Small changes in the desired capital stock can therefore translate into large changes in the flow of investment. This is one reason why investment is more volatile than GDP (see Figure 1.1).
重要的是要注意，

q

投资理论通过确定最佳资本存量来优化企业的投资决策（流量）。因此，所需资本存量的微小变化可以转化为投资流量的巨大变化。这是投资波动性大于 GDP 的原因之一（见图 1.1）。

The marginal benefits from investment are higher and a firm should undertake more investment if there is:
投资的边际收益更高，如果有以下情况，企业应该进行更多投资：

an improvement in technology, A,
技术的进步，A，
an increase in the marginal productivity of capital, $f_{K}$ which, once multiplied by $A$ , indicates the increase in output produced by the new capital equipment,
资本边际生产力的增加， $f_{K}$ 一旦乘以 $A$ ，就表明新资本设备所产生的产出增加
a reduction in the rate of interest, $r$ ; or
利率的降低， $r$ ；或者
a reduction in the rate of depreciation, $δ$ . For example, a rise in the expected rate of depreciation (e.g. as a result of uncertainty about future legislation banning fossilfuel-using cars) would lead to a reduction in the level of current investment because it would reduce the expected benefits to the firm from additional investment in the auto industry.
折旧率的降低， $δ$ 。例如，预期折旧率的上升（例如，由于对未来禁止使用化石燃料汽车的立法的不确定性）将导致当前投资水平的降低，因为这会减少公司在汽车行业额外投资的预期收益。

q

theory a good way to represent real-world investment behaviour? Marginal

q

implies firms adjust their level of investment in each period to equate the marginal benefits and marginal costs of investment. This does not fit with the real-world data on investment. When economists looked at detailed microeconomic data for very large numbers of plants, they found that contrary to the idea of a smooth investment flow, investment was in fact very bunched.

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For example, a growing manufacturing firm might build a new factory once every 10 years.

q

理论是否是代表现实世界投资行为的好方法？边际

q

意味着企业在每个时期调整其投资水平，以使投资的边际收益和边际成本相等。这与现实世界的投资数据不符。当经济学家查看大量工厂的详细微观经济数据时，他们发现与平滑投资流的想法相反，投资实际上是非常集中在一起的。

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例如，一个不断增长的制造公司可能每 10 年建造一座新工厂。

Testing

q

theory directly is difficult because

q

itself is difficult to measure. This is because it depends on the marginal product of capital and a measure of technology, which cannot be observed. In the real world, firms do not know their production functions and do not think explicitly in terms of marginal products. To operationalize the theory, the market value of the firm as reflected in its stock market valuation is compared with the replacement cost of the capital stock. If the market value is higher then this signals that the firm should increase investment. On the other hand if the market value is below the replacement cost, then the firm would not want to build a new factory because it could buy an existing one more cheaply.
直接测试

q

理论是困难的，因为

q

本身难以测量。这是因为它依赖于资本的边际产出和技术的衡量，而这些是无法观察到的。在现实世界中，企业并不知道它们的生产函数，也不会明确考虑边际产出。为了使理论具备可操作性，企业的市场价值（反映在其股票市场估值中）与资本存量的替代成本进行比较。如果市场价值更高，则这表明企业应该增加投资。另一方面，如果市场价值低于替代成本，则企业不想建造新工厂，因为它可以更便宜地购买现有工厂。

We can define average Q as follows:
我们可以将平均 Q 定义如下：

Q = \frac{Market value of firm}{Replacement cost of capital} (average Q model of investment)

This is often called average

Q

to distinguish it from marginal

q

. Whereas

q

is the ratio of the marginal benefit of a unit of investment to its marginal cost,

Q

depends on the total expected return from the firm’s capital divided by the total cost. For publicly listed companies, the stock market provides a forward-looking measure of the market value of the firm, which can be used in the numerator of the Q equation: when a firm’s market value rises relative to its replacement cost firm, as reflected in a rise in the price of its shares, the model suggests that investment should go up. A higher interest rate and depreciation rate will raise the replacement cost of capital.
这通常被称为平均

Q

，以区别于边际

q

。而

q

是单位投资的边际收益与其边际成本的比率，

Q

则依赖于公司资本的总预期回报除以总成本。对于上市公司，股票市场提供了公司市场价值的前瞻性衡量，这可以用在 Q 方程的分子中：当公司的市场价值相对于其替代成本上升时，反映在其股票价格的上涨中，该模型表明投资应该增加。更高的利率和折旧率将提高资本的替代成本。

The idea is that the market value incorporates information about how well the firm is expected to be able to implement the investment, whether new competitors will enter the market, whether there are new technological innovations likely to affect the firm’s value, the state of the macroeconomy and the labour market; and the future path of the interest rate. Since investing in a firm is a wager on this uncertain future, investors continuously evaluate these factors and, under certain conditions, the share price and hence the market value of the firm will reflect all the information available.
市场价值的概念是，它包含了关于公司预计能够多好地实施投资的信息，是否会有新竞争者进入市场，是否有可能影响公司价值的新技术创新，宏观经济和劳动力市场的状态，以及未来利率的走势。由于投资于一家公司是对这个不确定未来的赌注，投资者不断评估这些因素，并且在某些条件下，股价以及公司的市场价值将反映所有可用的信息。

The stock market is notoriously volatile. For example, in 2008 as the financial crisis gathered pace, the UK’s leading share index-the FTSE 100-fell 31% and GDP contracted by just

1.1 %

. The following year, the FTSE 100 rose

22 %

, while GDP continued to contract, but this time by

4.4 %^{19}

In times of extreme uncertainty, the share price may not reflect the fundamental value of a firm. Large movements in share prices heavily affect market capitalization, which is the measure used to proxy for the market value of the firm in the calculation of average Q . As the stock market is volatile and subject to bubbles, fads and herd behaviour, it may not be a good indicator of the firm’s prospects.
股市 notoriously 波动。例如，在 2008 年金融危机加速时，英国领先的股票指数 FTSE 100 下跌了 31%，而 GDP 仅收缩了

1.1 %

。次年，FTSE 100 上涨了

22 %

，而 GDP 继续收缩，但这次收缩了

4.4 %^{19}

。在极度不确定的时期，股价可能无法反映公司的基本价值。股价的大幅波动严重影响市场资本化，而市场资本化是用于代理公司市场价值的平均 Q 计算中的一个指标。由于股市波动且容易受到泡沫、时尚和从众行为的影响，它可能不是公司前景的良好指标。

However, a study using US micro data (over 1,000 firms from 1982 to 1999) shows that a Q model of investment can be successfully estimated when analysts’ forecasts of a firm’s profits are used instead of the stock market price to measure the numerator in the Q equation.

^{20}

然而，一项使用美国微观数据（1982 年至 1999 年超过 1000 家公司的数据）的研究表明，当使用分析师对公司利润的预测来替代股市价格作为 Q 方程中分子的测量时，投资的 Q 模型可以成功估计。

Empirical evidence on investment
投资的实证证据

The empirical evidence on the

q

theory is that Q helps to explain investment but it is not the only influence. In particular, just as was the case for consumption, credit constraints play an important role in explaining investment spending.
关于

q

理论的实证证据表明，Q 有助于解释投资，但这并不是唯一的影响因素。特别是，就像消费的情况一样，信贷约束在解释投资支出中发挥着重要作用。

Credit constraints—the role of cash flow
信贷约束——现金流的作用

A testable prediction of

q

theory is that current cash flow should have no impact on investment. The reason is that forward-looking firms should take into account

q

理论的一个可测试的预测是，当前现金流不应对投资产生影响。原因在于，前瞻性的公司应考虑到

any credit constraints that they face: these should already be incorporated into the stock market valuation, Q. However, the role of cash flow in empirical studies of investment suggests that capital market imperfections are important. The importance of cash flow variables in estimated investment equations is reminiscent of the role of current income in consumption functions. As we have seen, the presence of credit-constrained households causes excess sensitivity of consumption to predictable changes in income. Similarly, the presence of cash flow terms in estimated investment functions strongly suggests that firms are credit constrained. Although some firms are able to borrow as much or sell as much equity as they would like in order to finance their investment plans, for others, their investment will be limited by their internal funds. This is referred to as the excess sensitivity of investment to internal funds. For firms like this, cash flow is an important determinant of investment.

^{21}

他们面临的任何信贷约束：这些应该已经被纳入股票市场估值中。然而，现金流在投资的实证研究中的作用表明，资本市场的不完善是重要的。现金流变量在估计投资方程中的重要性让人想起了当前收入在消费函数中的作用。正如我们所看到的，信贷受限家庭的存在导致消费对可预测的收入变化过度敏感。同样，估计投资函数中现金流项的存在强烈表明企业受到信贷约束。虽然一些企业能够借入或出售尽可能多的股权以融资其投资计划，但对于其他企业而言，其投资将受到内部资金的限制。这被称为投资对内部资金的过度敏感性。对于这样的企业，现金流是投资的重要决定因素。

A study using UK company data for the period 1975-86 found that company investment was significantly influenced by Q and by credit constraints.

^{22}

However, the impact of Q was found to be very small: a

10 %

rise in the stock market value of a company was associated with an immediate rise in the investment rate of only

2.5 %

. Cash flow, on the other hand, was very important. The sample period was divided into two and it was found that the impact of Q was lower and the impact of cash flow higher in the first part of the sample during which the UK economy was in a deep recession. This is consistent with the idea of credit constraints biting especially hard in a recession.
一项使用 1975-86 年期间英国公司数据的研究发现，公司投资受到 Q 和信贷约束的显著影响。然而，Q 的影响被发现非常小：公司股票市场价值的上升仅与投资率的立即上升相关联。另一方面，现金流则非常重要。样本期被分为两部分，发现 Q 的影响在样本的第一部分较低，而现金流的影响较高，此时英国经济正处于深度衰退。这与信贷约束在衰退期间特别严重的观点是一致的。

Uncertainty-the option value of waiting
不确定性——等待的选择价值

The decision of firms of whether to invest or not is also influenced by uncertainty about the future. Under certain circumstances, there can be a value to waiting to undertake an investment project. With the passing of time, more information arrives, which means that the costs of delay in lost profits may be outweighed by more secure information on the balance between the costs (including those sunk in the project) and the benefits of undertaking it. The upshot of such considerations is that an expected rate of return considerably higher than the cost of capital will be required to trigger investment. Dixit (1992) gives examples to show that including the so-called ‘option value of waiting’ can double the hurdle rate (i.e. the return required to trigger investment) for an investment project to proceed.
企业是否投资的决策也受到对未来不确定性的影响。在某些情况下，等待进行投资项目可能具有价值。随着时间的推移，更多的信息会到来，这意味着延迟所造成的利润损失的成本可能会被关于成本（包括在项目中沉没的成本）与进行投资的收益之间的更可靠信息所抵消。这些考虑的结果是，触发投资所需的预期回报率必须显著高于资本成本。Dixit（1992）举例说明，包含所谓的“等待的选择价值”可以使投资项目的门槛利率（即触发投资所需的回报）翻倍。

Because there is a limited second-hand market in capital goods, net investment decisions are largely irreversible. For those decisions to be taken, management must have strong positive views about the future growth of the markets for its products. In the absence of such positive expectations, the firm will invest little so as to reduce its risk.

^{23}

In recessions, uncertainty about market growth is high and the ‘wait and see’ behaviour prevails keeping investment depressed. The Covid-19 crisis in 2020 created great uncertainty about when businesses would be able to trade again. The effect on investment extended well beyond large firms. A survey of self-employed workers in
由于资本货物的二手市场有限，净投资决策在很大程度上是不可逆的。为了做出这些决策，管理层必须对其产品市场的未来增长持有强烈的积极看法。在缺乏这种积极预期的情况下，公司将会减少投资以降低风险。在经济衰退期间，市场增长的不确定性很高，“观望”行为盛行，导致投资低迷。2020 年的新冠疫情危机造成了企业何时能够再次营业的巨大不确定性。对投资的影响远远超出了大型企业。一项针对自雇工作者的调查显示，

the UK in April 2020 found that

44 %

of respondents reported delaying or cancelling investment in work equipment.

^{24}

2020 年 4 月的英国调查发现，

44 %

的受访者报告推迟或取消了对工作设备的投资。

^{24}

Investment and animal spirits
投资与动物精神

Tobin’s q theory suggests that new, smaller firms should invest more because their marginal benefit from additional investment usually outweighs the marginal cost. However, a harsh truth is that the majority of new firms fail; according to the Bureau of Labor Statistics, approximately

20 %

of small businesses fail within the first year, and around

70 %

fail by the end of the first decade.

^{25}

Investment requires some form of ‘spontaneous optimism’ on the side of firms. John Maynard Keynes referred to such spontaneity in his General Theory as ‘animal spirits’:

^{26}

托宾的 q 理论表明，新兴的小型企业应该更多地投资，因为它们的边际收益通常超过边际成本。然而，一个残酷的事实是，大多数新企业会失败；根据劳工统计局的数据，大约

20 %

的小企业在第一年内失败，约

70 %

在第一个十年结束时失败。

^{25}

投资需要企业某种形式的“自发乐观”。约翰·梅纳德·凯恩斯在他的《通论》中将这种自发性称为“动物精神”：

^{26}

Most, probably, of our decisions to do something positive, the full consequences of which will be drawn out over many days to come, can only be taken as the result of animal spirits-a spontaneous urge to action rather than inaction, and not as the outcome of a weighted average of quantitative benefits multiplied by quantitative probabilities.
我们大多数做出积极决策的原因，可能只能归结为动物精神——一种自发的行动冲动，而非无所作为的选择，也不是量化收益与量化概率的加权平均结果，这些决策的全部后果将在未来的许多天中显现。

Therefore, we often fail to explain a firm’s decision to invest by looking at the ratio of its stock market value to replacement cost of capital. Jack Welch, former General Electric CEO, said he had little use for analytical projections and rational business plans. He said the most important business decisions come 'straight from the gut.

^{27}

因此，我们常常无法通过查看公司市值与资本替代成本的比率来解释其投资决策。前通用电气首席执行官杰克·韦尔奇表示，他对分析预测和理性商业计划不太感兴趣。他说，最重要的商业决策来自“直觉”。

Using survey data to understand aggregate investment
使用调查数据来理解整体投资

Fresh insights into the determinants of aggregate investment come from survey data in which managers in German manufacturing firms were asked about the importance of sales, expected returns, financial and technological factors in their investment decisions in a given year (Bachmann and Zorn 2020). Economists have traditionally been sceptical about the value of survey data but well-designed econometric techniques are able to make this ‘soft’ data interpretable.
关于总投资决定因素的新见解来自于一项调查数据，该调查中德国制造业公司的管理者被询问在某一年中销售、预期回报、财务和技术因素在其投资决策中的重要性（Bachmann 和 Zorn 2020）。经济学家们传统上对调查数据的价值持怀疑态度，但精心设计的计量经济学技术能够使这些“软”数据可解释。

The main findings are that financial factors such as borrowing costs are unimportant in explaining both the level and fluctuations in investment. The bulk of the explanation for what drives aggregate investment is ‘the sales situation and expectations’-expected demand for the firm’s output. This is consistent with the importance of uncertainty in investment decisions: managers look for signals about the strength of demand before committing to irreversible investment.
主要发现是，借贷成本等金融因素在解释投资水平和波动方面并不重要。推动整体投资的主要因素是“销售情况和预期”——对公司产出需求的预期。这与投资决策中不确定性的重要性是一致的：管理者在做出不可逆投资之前，会寻找关于需求强度的信号。

These findings underline the limited role in explaining the pattern of investment in Figure 1.1 played by changes in the interest rate: it is the expected returns from expanding capacity to meet additional demand for their output that lie behind the troughs and peaks of business investment.
这些发现强调了利率变化在解释图 1.1 中投资模式方面的有限作用：推动商业投资的低谷和高峰背后，是为了满足额外需求而扩大产能所预期的回报。

1.3 APPLICATIONS 1.3 应用程序

1.3.1 The demand side and inequality
1.3.1 需求方与不平等

One of the challenges for students of macroeconomics is to understand how the rising inequality that has characterized the high-income countries over the past forty years affects business cycles and growth.
学生们在学习宏观经济学时面临的挑战之一是理解过去四十年来高收入国家所特有的日益不平等如何影响商业周期和经济增长。
Research confirms that rising inequality in the distribution of income is reproduced in rising consumption inequality. This means that a proportion of households are unable to fully insure themselves so as to smooth their consumption through borrowing and by building up ‘buffer stocks’ for use in hard times.

^{28}

研究确认，收入分配不平等的加剧在消费不平等的上升中得以再现。这意味着一部分家庭无法完全为自己投保，以便通过借贷和建立“缓冲库存”来平滑消费，以应对困难时期。

Inequality matters for the business cycle because low-income workers are both credit constrained and more exposed to the risk of unemployment in a downturn. This interaction has the effect of increasing the size of the multiplier in the economy as a whole, making recessions deeper and booms stronger than would otherwise be the case. As we shall see when we introduce the policy maker, this makes the need for policy intervention to dampen a recession more important and boosts its effectiveness.
不平等对商业周期很重要，因为低收入工人既受到信贷限制，又在经济下行时更容易面临失业风险。这种互动会导致经济整体的乘数效应增大，使得衰退更加严重，繁荣更加强劲。正如我们在引入政策制定者时将看到的，这使得政策干预以减缓衰退的必要性更加重要，并提高了其有效性。
Figure 1.10 illustrates how the marginal propensity to consume varies across categories of workers in the US. It is highest among young, black and low-income workers. Research by Patterson (2020) provides evidence for the conjecture that demographic groups with higher marginal propensities to consume are more exposed to recessions. For example, black men between the ages of 25 and 35 with earnings less than $22,000 have an average MPC of above 1 and their earnings are very sensitive to fluctuations in the economy as a whole because of their high risk of unemployment (a fall in GDP by 1 percent sees their earnings fall by 2 percent). By contrast, non-black women with earnings of

$ 48, 000

$ 65, 000

have a much lower MPC and their earnings hardly change when GDP does because they have a low risk of losing their job in a recession.
图 1.10 说明了美国不同类别工人的边际消费倾向如何变化。年轻、黑人和低收入工人的边际消费倾向最高。Patterson（2020）的研究提供了证据，支持这样的推测：边际消费倾向较高的人口群体在经济衰退中受到的影响更大。例如，年龄在 25 到 35 岁之间、年收入低于 22,000 美元的黑人男性，其平均边际消费倾向超过 1，他们的收入对整体经济波动非常敏感，因为他们面临较高的失业风险（GDP 下降 1%时，他们的收入下降 2%）。相比之下，年收入在

$ 48, 000

到

$ 65, 000

之间的非黑人女性，其边际消费倾向要低得多，GDP 变化时她们的收入几乎没有变化，因为她们在经济衰退中失业的风险较低。
As wealth becomes more unequally distributed, so too do marginal propensities to consume. At the same time, inequalities in the labour market measured by the probability of job loss affect those with low wealth. These two effects interact to deepen recessions.
随着财富分配变得越来越不平等，边际消费倾向也随之变化。同时，劳动市场中的不平等（以失业概率衡量）对低财富者产生影响。这两种效应相互作用，加深了经济衰退。
The empirical evidence of variations in the MPC across households is captured in a simple way in the model of consumption by the presence of hand-to-mouth households in the hybrid consumption function in Section 1.2.5.
家庭间边际消费倾向（MPC）差异的实证证据在消费模型中通过第 1.2.5 节混合消费函数中存在的手到口家庭简单地体现出来。

1.3.2 Consumption, investment, and the IS curve
1.3.2 消费、投资与 IS 曲线

In Section 1.2.4, we introduced models of consumption and investment that took account of forward-looking behaviour. Expectations of future income and of future profits play a key role in the spending decisions of households and firms. We also reported empirical evidence on aggregate consumption and investment, which highlighted the role of credit constraints for households and firms. What are the implications for the IS curve?
在 1.2.4 节中，我们介绍了考虑前瞻性行为的消费和投资模型。对未来收入和未来利润的预期在家庭和企业的支出决策中发挥了关键作用。我们还报告了关于总消费和投资的实证证据，突出了信贷约束对家庭和企业的影响。这对 IS 曲线有什么影响？

Figure 1.10 Marginal Propensity to Consume (MPC) for different groups in the United States.
图 1.10 美国不同群体的边际消费倾向（MPC）。
Note: Uses data from the PSID. Each estimate is derived from a separate regression including only the stated demographic group. The author used unemployment to instrument for income changes. The squares denote estimated MPC, while the lines show the

5 %

95 %

confidence interval.
注意：使用来自 PSID 的数据。每个估计值是基于仅包含所述人口统计组的单独回归得出的。作者使用失业率作为收入变化的工具变量。方框表示估计的边际消费倾向，而线条显示

5 %

到

95 %

的置信区间。
Source: Patterson, C. (2020) The Matching Multiplier and the Amplification of Recessions. The Washington
来源：帕特森，C.（2020）《匹配乘数与经济衰退的放大》。华盛顿
Center for Equitable Growth 040820-WP.
公平增长中心 040820-WP
The basic form of the IS curve was presented in Section 1.2.3 and we shall see that we can bring together the additional insights about consumption and investment by focusing on the factors that affect the slope of the IS curve and which shift it.
IS 曲线的基本形式在第 1.2.3 节中介绍，我们将看到通过关注影响 IS 曲线斜率和移动的因素，我们可以将关于消费和投资的额外见解结合在一起。

The IS equation is: IS 方程是：

\begin{aligned} y & = k (c_{0} + a_{0} + G) - k a_{1} r \\ = A - a r, \end{aligned}

where

A \equiv k (c_{0} + a_{0} + G)

and

a \equiv k a_{1}; k

is the multiplier, which in the hybrid consumption function is increased by the proportion of hand-to-mouth households. Remember that the MPC in the hybrid consumption function is approximately equal to the proportion of HTM households.
在这里，

A \equiv k (c_{0} + a_{0} + G)

和

a \equiv k a_{1}; k

是乘数，在混合消费函数中，由于手到口家庭的比例而增加。请记住，混合消费函数中的边际消费倾向（MPC）大致等于手到口家庭的比例。
A larger multiplier makes the IS curve flatter, which increases the impact on output of a change in the interest rate; it also increases the impact on output of a shift in

c_{0}, a_{0}

or G for a given interest rate. Thus, the size of the multiplier is important for understanding the effectiveness of both fiscal and monetary policy.
更大的乘数使 IS 曲线变得更平坦，这增加了利率变化对产出的影响；它还增加了在给定利率下，

c_{0}, a_{0}

或 G 的变化对产出的影响。因此，乘数的大小对于理解财政政策和货币政策的有效性非常重要。
As we shall see in Chapter 7 on fiscal policy, there was much debate about the empirical size of the multiplier in the context of the use of fiscal stimulus to respond to global financial crisis. In the context of policy making, the answers to questions
正如我们将在第七章关于财政政策中看到的，关于在应对全球金融危机时使用财政刺激的背景下乘数的经验大小存在很多争论。在政策制定的背景下，问题的答案
about the size of the government spending multiplier depend on characteristics of the economy and on the context in which a fiscal stimulus is being applied.
关于政府支出乘数的大小取决于经济的特征以及财政刺激实施的背景。
The slope of the IS curve is affected by the interest sensitivity of investment and of consumption. The theoretical prediction on the impact of the interest rate on consumption is ambiguous; a reduction in interest rates boosts consumption through some channels and dampens it through others. From the Euler equation (or indeed, the 2-period model of intertemporal substitution), we know that the substitution effect of a rise in the interest rate leads households to shift consumption to the future thereby cutting consumption in the current period. The income effect (for savers) goes in the opposite direction, raising current consumption, whereas for borrowers, for whom a rise in the interest rate reduces their income, the substitution effect is reinforced. The empirical evidence on the consumption function suggests that national institutional structures, particularly in relation to the housing market, are important for determining the strength and direction of the relationship. Aron et al. (2012) find a negative relationship between consumption and the real interest rate in the financially liberalized UK and US economies, but they find a positive relationship in Japan, where households’ huge liquid deposits (on which interest is paid) far outweigh household debt.
IS 曲线的斜率受到投资和消费对利率敏感性的影响。关于利率对消费影响的理论预测是模糊的；利率降低通过某些渠道促进消费，而通过其他渠道抑制消费。从欧拉方程（或实际上，从两期的跨期替代模型）中，我们知道，利率上升的替代效应使家庭将消费转移到未来，从而减少当前的消费。收入效应（对于储蓄者）则朝相反方向发展，增加当前消费，而对于借款者来说，利率上升减少了他们的收入，替代效应得到了加强。关于消费函数的实证证据表明，国家的制度结构，特别是与住房市场相关的结构，对于确定这种关系的强度和方向是重要的。Aron 等。（2012）发现，在金融自由化的英国和美国经济中，消费与实际利率之间存在负相关关系，但在日本却发现正相关关系，因为家庭的巨额流动存款（支付利息）远远超过家庭债务。
Looking first at consumption, the PIH (and hybrid consumption function) predict that the IS curve is shifted by anything that changes expected lifetime wealth,

Ψ_{t}^{E}

, such as changes in asset prices or in perceptions about future earnings arising, for example, from widespread fears about skill obsolescence. The empirical findings about consumption highlight three other factors that can shift the IS curve:
首先看消费，永久收入假说（PIH）和混合消费函数预测，任何改变预期终身财富的因素都会导致 IS 曲线的移动，例如资产价格的变化或对未来收入的看法变化，这些变化可能源于对技能过时的广泛担忧。关于消费的实证研究强调了三个其他可以移动 IS 曲线的因素：

The role of uncertainty: a rise in the rate of unemployment will raise savings for precautionary purposes, shifting the IS to the left.
不确定性的作用：失业率的上升将增加出于预防目的的储蓄，从而使 IS 曲线向左移动。
A boom in house prices will boost consumption in a country with home equity loans by loosening credit constraints, shifting the IS to the right. However, a house price boom could also shift the IS to the left in countries where home equity loans are unobtainable and large down-payments are needed to get a mortgage.
房价的上涨将通过放松信贷限制来促进一个拥有房屋净值贷款的国家的消费，从而使 IS 曲线向右移动。然而，在那些无法获得房屋净值贷款且需要大额首付款才能获得抵押贷款的国家，房价的上涨也可能使 IS 曲线向左移动。
A shift in credit market architecture that increases household access to credit, such as financial innovation or deregulation, will shift the IS to the right (at least until the effect of the accumulation of debt eventually offsets some of the shift).
信贷市场架构的变化，例如金融创新或放松管制，能够增加家庭获得信贷的机会，将使 IS 曲线向右移动（至少在债务积累的影响最终抵消部分移动之前）。

Turning our attention to investment, Tobin’s marginal

q

predicts that the following factors will shift the IS curve to the right:
将我们的注意力转向投资，托宾的边际

q

预测以下因素将使 IS 曲线向右移动：

An improvement in technology, A,
技术的进步，A，
An increase in the marginal productivity of capital, $f_{K}$ ; and
资本边际生产力的提高， $f_{K}$ ；并且
A reduction in the rate of depreciation, $δ$ .
折旧率的降低， $δ$ 。

Lastly, the average Q equation highlights the role of expectations of future profits as a shift factor for the IS curve: a rise in the stock market tends to boost fixed investment as it signals a rise in the value of companies relative to their replacement cost. Research
最后，平均 Q 方程突出了未来利润预期作为 IS 曲线的一个转移因素的作用：股市的上涨往往会促进固定投资，因为这表明公司相对于其更换成本的价值上升。研究
emphasizes the importance of expectations of future market growth for investment given that much investment is irreversible.
强调了未来市场增长预期对投资的重要性，因为许多投资是不可逆的。

1.4 CONCLUSIONS 1.4 结论

This chapter has provided the first building block in the model of the macroeconomy. We have set out the IS curve, which is used to model the demand side of the economy and is one of the three equations that underpin the 3-equation model that will form the core model of this textbook.
本章提供了宏观经济模型的第一个构建块。我们已经阐述了 IS 曲线，它用于建模经济的需求侧，并且是支撑本教科书核心模型的三方程模型中的三个方程之一。

The IS curve allows us to think systematically about how changes in the spending behaviour of firms, households and governments can influence output and drive business cycles. The IS curve shows the combinations of the real interest rate and output where the goods market is in equilibrium. It slopes downwards to account for the fact that households’ consumption and new housing decisions respond negatively to the interest rate; firms will also undertake fewer investment projects as the cost of borrowing increases.
IS 曲线使我们能够系统地思考企业、家庭和政府的消费行为变化如何影响产出并推动商业周期。IS 曲线显示了实际利率和产出之间的组合，在这些组合中，商品市场处于均衡状态。它向下倾斜，以考虑到家庭的消费和新住房决策对利率的负面反应；随着借款成本的增加，企业也会减少投资项目。

We return to the questions at the start of the modelling section (1.2) and see how the model we have developed helps us work towards an answer. This acts as an exercise to summarize what we have learnt in this chapter.
我们回到建模部分（1.2）开始时的问题，看看我们所开发的模型如何帮助我们朝着答案迈进。这是一个总结我们在本章中所学内容的练习。

If you give an individual a $$ 100$ bonus, how much of it will they spend? This depends on the model of household spending we are using. If consumer behaviour is well modelled by a Keynesian consumption function (i.e. $C_{t} = c_{0} + c_{1} (1 - t) y_{t}$ ) then households will spend a fixed proportion of any extra income they receive. In the case of a $$ 100$ bonus, they will spend everything left over once they have paid tax and put some money into savings. However, if consumer behaviour is better modelled by the permanent income hypothesis ( PIH ), then individuals’ current consumption is a function of their expected lifetime wealth (i.e. $C_{t} = (\frac{r}{1 + r}) Ψ_{t}^{E}$ ). As the bonus is a one-off, it will only increase expected lifetime wealth by a small amount, resulting in a small increase in consumption in this and all future periods. This leads the individual to save the majority of the bonus in the current period. The hybrid consumption function provides a useful basis for the consumption component of the IS curve as illustrated in Section 1.3.2, where we showed how the relative proportions HTM and PIH households in the economy affect the multiplier and the IS curve. From Section 1.3.1, we know that inequality affects the multiplier through its effect on the average marginal propensity to consume.
如果你给一个个人一个 $$ 100$ 奖金，他们会花掉多少？这取决于我们使用的家庭消费模型。如果消费者行为可以很好地用凯恩斯消费函数建模（即 $C_{t} = c_{0} + c_{1} (1 - t) y_{t}$ ），那么家庭将花费他们所收到的任何额外收入的固定比例。在 $$ 100$ 奖金的情况下，他们会在缴税和存入一些钱后花掉剩下的所有钱。然而，如果消费者行为更好地用永久收入假说（PIH）建模，那么个人的当前消费是他们预期的终身财富的函数（即 $C_{t} = (\frac{r}{1 + r}) Ψ_{t}^{E}$ ）。由于奖金是一次性的，它只会使预期的终身财富增加少量，从而导致在当前和未来所有时期消费的微小增加。这使得个人在当前时期保存大部分奖金。混合消费函数为 IS 曲线的消费部分提供了有用的基础，如第 1.3.2 节所示，我们展示了经济中 HTM 和 PIH 家庭的相对比例如何影响乘数和 IS 曲线。来自第 1.3 节。我们知道，不平等通过对平均边际消费倾向的影响来影响乘数。
How much will output increase following a rise in autonomous demand, such as higher government spending or a boost in the confidence of households and firms? The multiplier will determine the extent to which an $undefined$ increase in autonomous demand will increase output. In the simple model presented in Section 1.2.1 and the one based on the hybrid consumption function in Section 1.2.5, the multiplier is always greater than 1 , as both $c_{1}$ and $t$ are between zero and one. This means that in the short run, an injection of autonomous demand will always boost output more than one for one. The extent of the boost of output will depend on the size of the
在自主需求上升后，例如政府支出增加或家庭和企业信心提升，产出将增加多少？乘数将决定自主需求增加对产出的影响程度。在第 1.2.1 节中提出的简单模型和第 1.2.5 节基于混合消费函数的模型中，乘数始终大于 1，因为 $c_{1}$ 和 $t$ 都在零和一之间。这意味着在短期内，自主需求的注入将始终使产出增加超过一对一的比例。产出增加的程度将取决于其规模。
multiplier. The multiplier will be larger if the tax rate $(t)$ is low and there is a high average marginal propensity to consume $(c_{1})$ .
乘数。如果税率 $(t)$ 较低且平均边际消费倾向 $(c_{1})$ 较高，则乘数将更大。

The PIH draws attention to the difference between temporary and permanent changes in income and that distinction will become important in the analysis of tax changes.
PIH 强调了收入的临时变化和永久变化之间的区别，这一区别在税收变化的分析中将变得重要。
3. To what extent will a rise in the interest rate curtail investment? In the simple model in Section 1.2.3, where

I = a_{0} - a_{1} r

, a rise in the interest rate of

Δ r

will reduce investment by

a_{1} Δ r

. In the marginal

q

theory of investment (with

q = \frac{A f_{K}}{δ + r}

), an increase in the interest rate will increase the marginal cost of investment. If we assume that

q

is initially equal to one (i.e. the marginal cost and marginal benefit of carrying out the investment project are equal) then an increase in

r

will lead firms to reduce investment. The extent to which investment will fall depends on how quickly the marginal productivity of capital

(f_{K})

rises as investment falls to bring

q

back toward a value of 1 .
3. 利率上升在多大程度上会抑制投资？在第 1.2.3 节的简单模型中，当

I = a_{0} - a_{1} r

时，利率上升

Δ r

将减少投资

a_{1} Δ r

。在边际

q

投资理论中（与

q = \frac{A f_{K}}{δ + r}

相关），利率的增加将提高投资的边际成本。如果我们假设

q

最初等于 1（即进行投资项目的边际成本和边际收益相等），那么

r

的增加将导致企业减少投资。投资下降的程度取决于资本的边际生产力

(f_{K})

在投资下降时上升的速度，以使

q

回到接近 1 的值。

From the empirical evidence, business investment is not very sensitive to the interest rate. This focuses attention on shifts in the IS curve.
根据实证证据，商业投资对利率的敏感性不高。这使人们关注 IS 曲线的变化。

This chapter has provided a model of the demand side which can shed light on macroeconomic questions. We have discussed how the models of consumption and investment relate to characteristics of the real world. For example, the greater volatility of investment as compared with consumption observed in the data can be partly explained by the factors that influence investment spending decisions and by consumers borrowing and saving to smooth their consumption over the economic cycle. Households vary-some are myopic and find it difficult to save whereas others are prudent and save for precautionary reasons. Inequality is a major determinant of the variation in household consumption behaviour. Credit constraints facing both households and firms help better align the models of consumption and investment with the empirical data.
本章提供了一个需求侧模型，可以为宏观经济问题提供启示。我们讨论了消费和投资模型如何与现实世界的特征相关。例如，数据中观察到的投资相较于消费的更大波动性，部分可以通过影响投资支出决策的因素以及消费者借贷和储蓄以平滑其经济周期消费的行为来解释。家庭的情况各不相同，有些家庭目光短浅，难以储蓄，而另一些则谨慎，为了预防而储蓄。收入不平等是家庭消费行为变化的主要决定因素。家庭和企业面临的信贷约束有助于更好地将消费和投资模型与实证数据对齐。

Although the demand side is an important influence on economic activity, it is only part of the story of how the macroeconomy works. To develop our understanding of economic fluctuations and longer-run trends, we need to introduce the supply side. This will provide a framework for thinking about how wages and prices are set and what determines the unemployment rate. The supply side is the subject of Chapter 2.
尽管需求方对经济活动有重要影响，但这只是宏观经济运作故事的一部分。为了加深我们对经济波动和长期趋势的理解，我们需要引入供给方。这将为思考工资和价格如何设定以及什么决定失业率提供一个框架。供给方是第二章的主题。

1.5 APPENDIX 1.5 附录

1.5.1 Real and nominal interest rates, and the Fisher equation
1.5.1 实际利率和名义利率，以及费雪方程

To clarify why it is the real rather than the nominal interest rate that affects real expenditure decisions in the economy, think about a firm considering an investment project. A higher money or nominal rate of interest will not impose a greater real burden on the firm if it is balanced by correspondingly higher inflation because the
为了澄清为什么影响经济中实际支出决策的是实际利率而不是名义利率，可以考虑一个正在考虑投资项目的公司。如果名义利率或货币利率提高，但伴随有相应更高的通货膨胀，那么对公司来说，这并不会带来更大的实际负担，因为
expected profits from the investment project will be higher in money terms and the balance between the real cost and the real return on the project will not have changed.
预计投资项目的利润在货币方面将更高，项目的实际成本与实际回报之间的平衡不会改变。

The real interest rate is defined in terms of goods and the nominal interest rate, in terms of money. Thinking of a consumer good, the real rate of interest,

r

, is how much extra in terms of this good-namely

(1 + r)

units-would have to be paid in the future in order to borrow one unit of the goods today. The nominal rate of interest is how much extra in euros would have to be paid in the future in order to have one euro today. If goods prices remain constant then it is clear that the real and nominal interest rates are the same: if you lent one euro today, you would be able to buy

(1 + r)

goods in the future. In general,
实际利率是以商品为单位定义的，而名义利率是以货币为单位定义的。考虑一个消费品，实际利率

r

是指为了今天借入一单位商品，未来需要支付的额外数量，即

(1 + r)

单位。名义利率是指为了今天拥有一欧元，未来需要支付的额外欧元数量。如果商品价格保持不变，那么实际利率和名义利率显然是相同的：如果你今天借出一欧元，未来你将能够购买

(1 + r)

商品。一般来说，

1 + r_{t} = (1 + i_{t}) \cdot \frac{P_{t}}{P_{t + 1}^{E}}

where it is the expected price level in the future

(P_{t + 1}^{E})

that comes into play since at time

t

, we do not know what the price level will be at

t + 1

. If we use the following definition of expected inflation:
在这里，未来的预期价格水平

(P_{t + 1}^{E})

发挥作用，因为在时间

t

时，我们不知道在

t + 1

时价格水平会是多少。如果我们使用以下预期通胀的定义：

π_{t + 1}^{E} = \frac{P_{t + 1}^{E} - P_{t}}{P_{t}}

then 然后

\frac{P_{t}}{P_{t + 1}^{E}} = \frac{1}{1 + π_{t + 1}^{E}}

By rearranging the above expression, it follows that
通过重新排列上述表达式，可以得出以下结论：

(1 + r_{t}) = \frac{(1 + i_{t})}{(1 + π_{t + 1}^{E})}

and therefore that 因此那

r_{t} = \frac{i_{t} - π_{t + 1}^{E}}{1 + π_{t + 1}^{E}}

When expected inflation is low, the denominator of this expression is close to 1 and we have the standard approximation for the relationship between the real and the nominal rate of interest:
当预期通胀率较低时，该表达式的分母接近 1，我们得到了实际利率与名义利率之间关系的标准近似：

i_{t} \approx r_{t} + π_{t + 1}^{E}

(Fisher equation) 费雪方程
Inflation expectations will drive the divergence between the real and nominal interest rates. It should be noted that only one of these three terms is observable: the nominal interest rate,

i

. The real interest rate can be estimated from historical data on the nominal interest rate and the rate of inflation: this gives a measure of the socalled ex post real rate of interest. Alternatively, an ex ante measure can be derived from a model that is able to predict inflation. Finally, if bonds have been issued in the economy that are protected against inflation because the face value is indexed by the rate of inflation, then the yield on such a bond is a real rate of interest and can provide a third measure. But there are only a few countries that have issued index-linked or inflation-proof bonds (UK in 1981, the US in 1997, France in 1998).
通货膨胀预期将推动实际利率和名义利率之间的差异。需要注意的是，这三个术语中只有一个是可观察的：名义利率

i

。实际利率可以通过名义利率和通货膨胀率的历史数据进行估算：这给出了所谓的事后实际利率的衡量标准。或者，可以从能够预测通货膨胀的模型中得出事前的衡量标准。最后，如果经济中发行了因面值与通货膨胀率挂钩而受到通货膨胀保护的债券，那么这种债券的收益率就是实际利率，并可以提供第三种衡量标准。但只有少数国家发行了与指数挂钩或防通胀的债券（1981 年英国，1997 年美国，1998 年法国）。

1.5.2 Deriving the Euler equation and the PIH consumption function
1.5.2 推导欧拉方程和永久收入假说消费函数

Deriving the Euler equation
推导欧拉方程

The first step in deriving the Euler equation is to set out an expression for the present value of a consumer’s utility:
推导欧拉方程的第一步是列出消费者效用的现值表达式：

V_{t}^{E} = U (C_{t}) + \frac{U (C_{t + 1}^{E})}{1 + ρ} + \frac{U (C_{t + 2}^{E})}{(1 + ρ)^{2}} + \dots

(expected present value of consumption)
（预期消费现值）
The present value equation uses the expectations operator (i.e.

C_{t + i}^{E}

for all

i > 0

) to show that future consumption is uncertain in the present period. In addition, the utility of consumption is discounted to the present period by the consumer’s rate of time preference. Future consumption will be worth less in the current period to more impatient consumers.
现值方程使用期望算子（即

C_{t + i}^{E}

对所有

i > 0

）来表明未来消费在当前时期是不确定的。此外，消费的效用通过消费者的时间偏好率折现到当前时期。对于更不耐烦的消费者，未来消费在当前时期的价值会更低。

To derive the Euler equation we need to assume a specific functional form for the utility function. We choose

U (C_{t}) = \log C_{t}

because it exhibits diminishing marginal returns and is easy to work with. We can take the first differential of the utility function to find the marginal utility of consumption, which is

U^{'} (C_{t}) = \frac{1}{C_{t}}

.
为了推导欧拉方程，我们需要假设效用函数的特定函数形式。我们选择

U (C_{t}) = \log C_{t}

，因为它表现出递减的边际收益，并且易于处理。我们可以对效用函数进行第一次微分，以找到消费的边际效用，即

U^{'} (C_{t}) = \frac{1}{C_{t}}

。

The Euler equation shows the optimal

C_{t}

in relation to

C_{t + 1}^{E}

. If we just think about these two periods, then it is easy to derive the Euler equation. In period

t

the consumer must weigh up the gain from consuming more in this period, against the discounted loss of consuming less in the next period. The expected present value of consumption is effectively a discounted sum of within-period utilities. We may call this an ‘additive utility function’. Clearly, an optimal consumption path is one that maximizes the additive utility function subject to the individual’s budget constraint. In the case of just two periods, this is the same as saying that the marginal utility from consuming an additional unit of wealth today is equal to the marginal utility of consuming (

1 + r

) units of wealth in the following period; this is because saving one unit of wealth today means having (

1 + r

) units to spend in the following period; we can think of (

1 + r

) as being the ratio of the price of consumption in period

t

and its price in period

(t + 1)

. However, marginal utility from consumption in the next period must be discounted by the individual’s rate of impatience. Therefore, the additive utility function is maximized when:
欧拉方程显示了最优

C_{t}

与

C_{t + 1}^{E}

之间的关系。如果我们只考虑这两个时期，那么推导欧拉方程就很简单。在

t

期，消费者必须权衡在这一时期消费更多所带来的收益，与在下一时期消费更少所造成的折现损失。消费的预期现值实际上是一个时期内效用的折现总和。我们可以称之为“加性效用函数”。显然，最优消费路径是最大化加性效用函数，同时满足个人的预算约束。在只有两个时期的情况下，这等同于说今天消费额外一单位财富的边际效用等于在下一时期消费（

1 + r

）单位财富的边际效用；这是因为今天节省一单位财富意味着在下一时期有（

1 + r

）单位可供消费；我们可以将（

1 + r

）视为

t

期消费价格与

(t + 1)

期消费价格的比率。然而，下一时期消费的边际效用必须按个人的急迫性折现。因此，当加性效用函数最大化时：

U^{'} (C_{t}) = \frac{1 + r}{1 + ρ} U^{'} (C_{t + 1}^{E})

(Euler equation) (欧拉方程)
Setting

U (C) = \log C

, the Euler equation becomes:
设置

U (C) = \log C

，欧拉方程变为：

\begin{aligned} \frac{1}{C_{t}} & = \frac{1 + r}{1 + ρ} \frac{1}{C_{t + 1}^{E}} \\ \to C_{t} & = \frac{1 + ρ}{1 + r} C_{t + 1}^{E} . \end{aligned}

Deriving the permanent income hypothesis consumption function
推导永久收入假说消费函数

The simplest case of the Euler equation is when the rate of interest is equal to the consumer’s subjective discount rate

r = ρ

. This is the case we have focused on throughout this chapter. We can see from the Euler equation that this implies

C_{t} = C_{t + 1}^{E}

. In other words, it implies that consumption is constant in expectation in all future periods.
欧拉方程的最简单情况是利率等于消费者的主观折现率

r = ρ

。这是我们在本章中一直关注的情况。我们可以从欧拉方程中看到，这意味着

C_{t} = C_{t + 1}^{E}

。换句话说，这意味着在所有未来时期，消费在预期中是恒定的。

$^{1}$ See Section 16.7.1 of Chapter 16 for a detailed discussion of growth concepts, and the use and interpretations of natural logs.
请参阅第 16 章第 16.7.1 节，了解关于增长概念的详细讨论，以及自然对数的使用和解释。
$^{2}$ See Keynes (1936). The IS curve underlies the famous IS-LM model, which was introduced by John Hicks (Hicks 1937). For a concise and interesting discussion of its origins and impact, see Durlauf and Hester’s article entitled ‘IS-LM’ in the New Palgrave Dictionary of Economics (Durlauf and Hester 2008).
$^{2}$ 参见凯恩斯（1936）。IS 曲线是著名的 IS-LM 模型的基础，该模型由约翰·希克斯（Hicks 1937）提出。有关其起源和影响的简明有趣的讨论，请参见杜尔劳夫和赫斯特在《新帕尔格雷夫经济学词典》中题为“IS-LM”的文章（杜尔劳夫和赫斯特 2008）。
$^{3}$ This is the sum of a geometric series. We want to find an expression for the series $1 + [c_{1} (1 - t)] + {[c_{1} (1 - t)]}^{2} + {[c_{1} (1 - t)]}^{3} + \dots$ , which we call $x$ . If we put
这是一个几何级数的总和。我们想要找到级数 $1 + [c_{1} (1 - t)] + {[c_{1} (1 - t)]}^{2} + {[c_{1} (1 - t)]}^{3} + \dots$ 的表达式，我们称之为 $x$ 。如果我们放置
$^{4}$ We can confirm that the change in aggregate saving is equal to zero. Using equation 1.7:
我们可以确认总储蓄的变化等于零。使用方程 1.7：

$\begin{aligned} Δ y = \frac{1}{1 - c_{1} (1 - t)} Δ c_{0} \\ Δ S = [(1 - c_{1} (1 - t)) \frac{1}{1 - c_{1} (1 - t)} Δ c_{0}] - Δ c_{0} = 0 \end{aligned}$
$^{5}$ The assumption that individuals ‘live forever’ can be thought of as assuming that households have children or heirs and that they incorporate the utility of their children or heirs into their consumption function. In other words, households behave ‘as if’ they last forever. The assumption is clearly unrealistic, but it greatly simplifies the mathematical derivation of the PIH. From this point onward, however, we will refer to lifetime income and earnings in the text, as it is more intuitive to think about forward-looking behaviour in terms of a household maximizing utility over their lifetime.
假设个人“永生”可以被视为假设家庭有孩子或继承人，并且他们将孩子或继承人的效用纳入消费函数。换句话说，家庭的行为“就像”他们永远存在一样。这个假设显然不现实，但它大大简化了永久收入假说（PIH）的数学推导。然而，从这一点开始，我们将在文本中提到终身收入和收益，因为从家庭在其一生中最大化效用的角度来看，考虑前瞻性行为更为直观。
$^{6}$ This view of the consumption function was first developed by Modigliani and Brumberg (1954). See also Friedman (1957). For a review of Friedman’s theory, its influence on modern economics and the relevant empirical literature, see Meghir (2004).
$^{6}$ 消费函数的这一观点最早由莫迪利安尼和布伦伯格（1954 年）提出。另见弗里德曼（1957 年）。有关弗里德曼理论的回顾、其对现代经济学的影响以及相关的实证文献，请参见梅吉尔（2004 年）。
$^{7}$ See Hall (1978) for a full derivation of the PIH and an empirical test of the main implications of the theory.
$^{7}$ 请参见霍尔（1978）以获取 PIH 的完整推导及该理论主要含义的实证检验。
8 This can be derived using the PIH consumption function: $Δ C_{t} = \frac{r}{1 + r} \sum_{i = 0}^{\infty} (\frac{Δ y}{(1 + r)^{i}}) = \frac{r}{1 + r} (\frac{Δ y}{1 - \frac{1}{1 + r}}) =$ $\frac{r}{1 + r} \frac{1 + r}{r} Δ y = Δ y$
8 这可以通过 PIH 消费函数推导得出： $Δ C_{t} = \frac{r}{1 + r} \sum_{i = 0}^{\infty} (\frac{Δ y}{(1 + r)^{i}}) = \frac{r}{1 + r} (\frac{Δ y}{1 - \frac{1}{1 + r}}) =$ $\frac{r}{1 + r} \frac{1 + r}{r} Δ y = Δ y$
$^{9}$ For a detailed summary of the early literature on excess sensitivity see Deaton (1992). An excellent overview of the empirical testing of the excess sensitivity and excess smoothness hypotheses is provided by Jappelli and Pistaferri (2010).
$^{9}$ 有关过度敏感性早期文献的详细总结，请参见 Deaton（1992）。Jappelli 和 Pistaferri（2010）提供了对过度敏感性和过度平滑假设的实证检验的优秀概述。
$^{10}$ See Jappelli (1990), Mariger (1986) and Hall and Mishkin (1982).
$^{10}$ 请参见 Jappelli (1990)、Mariger (1986) 和 Hall 和 Mishkin (1982)。
$^{11}$ See Gross and Souleles (2002).
$^{11}$ 参见 Gross 和 Souleles（2002）。
$^{12}$ For instance see Ganong and Noel (2019), which was described earlier in this chapter.
例如，参见 Ganong 和 Noel（2019），在本章前面已经描述过。
$^{13}$ See Read and van Leeuwen (1998).
$^{13}$ 请参见 Read 和 van Leeuwen (1998)。
$^{14}$ See Ashraf et al. (2006).
$^{14}$ 参见 Ashraf 等人 (2006)。
$^{15}$ See Carroll (1997) for an exposition of the buffer-stock savings model.
请参见卡罗尔（1997）关于缓冲库存储蓄模型的阐述。
$^{16}$ See Aron et al. (2012). Muellbauer (2022) provides a guide to how the ECB and European central banks model the consumption function to include the role of housing.
$^{16}$ 参见 Aron 等人 (2012)。Muellbauer (2022) 提供了关于欧洲央行和欧洲中央银行如何将住房的作用纳入消费函数模型的指南。
$^{17}$ See Tobin (1969) and Tobin and Brainard (1977).
$^{17}$ 参见托宾（1969）和托宾与布雷纳德（1977）。
$^{18}$ See the survey chapter on ‘Investment’ by Caballero (1999).
$^{18}$ 请参阅 Caballero（1999）关于“投资”的调查章节。
$^{19}$ GDP data from IMF World Economic Outlook database, April 2012. Stock market data from Bloomberg News articles.
$^{19}$ 国际货币基金组织世界经济展望数据库的 GDP 数据，2012 年 4 月。股票市场数据来自彭博新闻文章。
$^{20}$ See Bond and Cummins (2001).
$^{20}$ 参见 Bond 和 Cummins (2001)。
$^{21}$ See, for example, Chirinko (1993).
$^{21}$ 参见，例如，Chirinko（1993）。
$^{22}$ See Blundell et al. (1992).
$^{22}$ 参见 Blundell 等人 (1992)。
$^{23}$ See Bloom (2014) and Bloom et al. (2007).
$^{23}$ 请参见 Bloom（2014）和 Bloom 等人（2007）。
$^{24}$ See Blundell and Machin (2020).
$^{24}$ 参见 Blundell 和 Machin (2020)。
$^{25}$ Statistics extracted from the BLS-Establishment Age and Survival Data, updated 2020.
$^{25}$ 从 BLS-机构年龄和生存数据中提取的统计数据，更新于 2020 年。
$^{26}$ Excerpt from page 161-162 of Keynes (1936).
$^{26}$ 选自凯恩斯（1936 年）第 161-162 页。
$^{27}$ Quote from Welch and Byrne (2003).
$^{27}$ 威尔奇和伯恩（2003）的引用。
$^{28}$ See Attanasio and Pistaferri (2016) for an overview of the relationship between income inequality and consumption inequality.
请参阅 Attanasio 和 Pistaferri（2016）关于收入不平等与消费不平等之间关系的概述。

THE DEMAND SIDE 需求侧

1.1 OVERVIEW 1.1 概述

Aggregate demand and spending decisions总需求和支出决策

Aggregate demand and government policy总需求与政府政策

1.1.1 Facts about the demand side and business cycles1.1.1 关于需求侧和商业周期的事实

Shares of GDP 国内生产总值的份额

National accounts 国民账户

Relative volatility 相对挥发性

Growth and cycles 增长与周期

1.1.2 Introducing the IS curve1.1.2 介绍 IS 曲线

Aggregate demand in the private sector私营部门的总需求

The IS curve IS 曲线

1.2 MODELLING 1.2 建模

1.2.1 Goods market equilibrium1.2.1 商品市场均衡

The model of goods market equilibrium商品市场均衡模型

1.2.2 The multiplier 1.2.2 乘数

Application: The paradox of thrift应用：节俭的悖论

1.2.3 The IS curve 1.2.3 IS 曲线

Deriving the IS equation 推导 IS 方程

1.2.4 Forward-looking behaviour1.2.4 前瞻性行为

Present value calculation现值计算

1.2.5 Consumption 1.2.5 消费

The permanent income hypothesis (PIH)永久收入假说 (PIH)

Predictions of the PIH PIH 的预测

Excess sensitivity to anticipated changes in income对预期收入变化的过度敏感性

Responsiveness to news 对新闻的响应

Credit constraints, present-bias, and uncertainty信贷约束、现时偏见和不确定性

Credit constraints 信贷限制

Present-bias 当前偏见

Experimental evidence on present-bias关于当前偏见的实验证据

Uncertainty and precautionary saving不确定性和预防性储蓄

House price changes and house price bubbles房价变化和房价泡沫

An empirical consumption function经验消费函数

Aggregate consumption and current income: A hybrid consumption function聚合消费与当前收入：一种混合消费函数

1.2.6 Investment 1.2.6 投资

Tobin's q q qq theory of investment托宾的投资理论

Marginal and average q models边际和平均 q 模型

The share price as indicator of expected profits股价作为预期利润的指标

Empirical evidence on investment投资的实证证据

Credit constraints—the role of cash flow信贷约束——现金流的作用

Uncertainty-the option value of waiting不确定性——等待的选择价值

Investment and animal spirits投资与动物精神

Using survey data to understand aggregate investment使用调查数据来理解整体投资

1.3 APPLICATIONS 1.3 应用程序

1.3.1 The demand side and inequality1.3.1 需求方与不平等

1.3.2 Consumption, investment, and the IS curve1.3.2 消费、投资与 IS 曲线

1.4 CONCLUSIONS 1.4 结论

1.5 APPENDIX 1.5 附录

1.5.1 Real and nominal interest rates, and the Fisher equation1.5.1 实际利率和名义利率，以及费雪方程

1.5.2 Deriving the Euler equation and the PIH consumption function1.5.2 推导欧拉方程和永久收入假说消费函数

Deriving the Euler equation推导欧拉方程

Deriving the permanent income hypothesis consumption function推导永久收入假说消费函数

Aggregate demand and spending decisions
总需求和支出决策

Aggregate demand and government policy
总需求与政府政策

1.1.1 Facts about the demand side and business cycles
1.1.1 关于需求侧和商业周期的事实

1.1.2 Introducing the IS curve
1.1.2 介绍 IS 曲线

Aggregate demand in the private sector
私营部门的总需求

1.2.1 Goods market equilibrium
1.2.1 商品市场均衡

The model of goods market equilibrium
商品市场均衡模型

Application: The paradox of thrift
应用：节俭的悖论

1.2.4 Forward-looking behaviour
1.2.4 前瞻性行为

Present value calculation
现值计算

The permanent income hypothesis (PIH)
永久收入假说 (PIH)

Excess sensitivity to anticipated changes in income
对预期收入变化的过度敏感性

Credit constraints, present-bias, and uncertainty
信贷约束、现时偏见和不确定性

Experimental evidence on present-bias
关于当前偏见的实验证据

Uncertainty and precautionary saving
不确定性和预防性储蓄

House price changes and house price bubbles
房价变化和房价泡沫

An empirical consumption function
经验消费函数

Aggregate consumption and current income: A hybrid consumption function
聚合消费与当前收入：一种混合消费函数

Tobin's $q$ theory of investment
托宾的投资理论

Marginal and average q models
边际和平均 q 模型

The share price as indicator of expected profits
股价作为预期利润的指标

Empirical evidence on investment
投资的实证证据

Credit constraints—the role of cash flow
信贷约束——现金流的作用

Uncertainty-the option value of waiting
不确定性——等待的选择价值

Investment and animal spirits
投资与动物精神

Using survey data to understand aggregate investment
使用调查数据来理解整体投资

1.3.1 The demand side and inequality
1.3.1 需求方与不平等

1.3.2 Consumption, investment, and the IS curve
1.3.2 消费、投资与 IS 曲线

1.5.1 Real and nominal interest rates, and the Fisher equation
1.5.1 实际利率和名义利率，以及费雪方程

1.5.2 Deriving the Euler equation and the PIH consumption function
1.5.2 推导欧拉方程和永久收入假说消费函数

Deriving the Euler equation
推导欧拉方程

Deriving the permanent income hypothesis consumption function
推导永久收入假说消费函数