Sexual Orientation and Neuroanatomy: An MRI Study of Gray Matter Differences in Homosexual, Bisexual, and Heterosexual Women and Men

In this study, we attempted to replicate past work focusing on differences in neuroanatomical structures between heterosexual and homosexual men and women. We also performed the first analyses of sexual orientation and neuroanatomy to include bisexual men and women. Sex differences in raw subcortical volumes were consistent with past work and a broader literature on sex differences, showing larger raw subcortical volumes in male groups than female groups. However, we did not confirm past findings showing larger raw volumes in heterosexual than in homosexual men in the left thalamus or right thalamus. Additionally, we did not confirm past findings showing thicker cortices in heterosexual men than in homosexual men in visual/occipital areas (right cuneus, right lingual gyrus, right pericalcarine cortex) or a frontal area (right pars triangularis). Exploratory whole-brain analyses revealed several areas of difference between women that may be of interest for future confirmatory research. Bisexual women had smaller volumes in a region of the olfactory tubercule than heterosexual women as well as a thicker right anterior insula region than homosexual women. Homosexual women had smaller volumes in regions of the inferior parietal lobule (IPL) than both heterosexual women and bisexual women. The functional relevance of these brain areas in terms of understanding female sexual orientation is unclear. However, based on these areas, future work may wish to consider the potential social, emotional, attentional, interoceptive, or general reward-related characteristics that may differentiate women with different attraction patterns. In contrast to previous work, no differences were found between groups of men (heterosexual, bisexual, or homosexual) in any of our analyses. Finally, in terms of whole-brain analyses of sex differences, heterosexual women had both thicker cortices and larger (relative to the whole brain) gray matter volume than heterosexual men in the superior frontal gyrus, in contrast to large-scale studies of sex difference. Although statistically significant at a stringent threshold (FWE-corrected), our whole-brain findings should be interpreted and generalized with caution. The heterogeneity of patterns across analyses of sexual orientation and brain structure (and even across studies of sex/gender and brain structure) suggests that findings may potentially depend upon particular sample characteristics, and potentially Type 1 error due to the testing of many different brain areas.