Brain Differences Linked to Sexual Orientation

For at least 20 years, neuroscientists have been trying to discover whether there are specific anatomical differences between heterosexual and homosexual men and women. An early post-mortem study found that a small region of the anterior hypothalamus was smaller in homosexual men than in heterosexual men, and no different from heterosexual women. More recent brain imaging studies reported sexual orientation‐related differences in cortical regions devoted to vision, some asymmetries between the two hemispheres and differences in the thickness of the cortex at the front of the brain.

Overall, these specific brain regions in homosexual males tended to be similar to heterosexual women (more female‐typical), while these same brain regions homosexual women tended to be similar to heterosexual men (more male‐typical). These initial discoveries led scientists to think that some behavioral and cognitive traits related to sexual orientation may be reflected in, subtle but consistent, differences in brain anatomy.

Many of these early imaging studies were limited by small sample sizes and did not include female, both heterosexual and homosexual, comparison groups. A recent study addressed this question, and also introduced the role of specific genes, by utilizing the largest neuroimaging‐genetics dataset available on same‐sex sexual behavior.

MRI data were obtained from 18,645 males and females, aged 40-69 years, in order to extract brain patterns related to same‐sex sexual behavior. Self‐reported same-sex sexual behavior was assessed via questionnaire. Participants were asked: “Have you ever had sexual intercourse with someone of the same sex?” with sexual intercourse defined as vaginal, oral, or anal intercourse, making the measure unambiguous. Answering options were: “Yes.” “No.” “Prefer not to answer.” Those answering “Prefer not to answer” were excluded from the study.

The authors reported that their methods of data analysis were effective for classifying heterosexual males and females (a critical baseline of reliability) and that these unique patterns of classification were less pronounced in non‐heterosexual individuals. Essentially, the differences in brain anatomy that distinguish heterosexual males and females were present, however they were less pronounced when comparing male and female homosexuals. Most interestingly, when the authors adjusted for potential confounding variables, and there were many, the same-sex sexual behavior-related shifts in brain patterns were most predominant in females.

The brain region that showed the most consistent sexual orientation‐related differences in both male and female homosexuals was the calcarine sulcus. This sulcus is the region of cortex at the back of the brain that is primarily responsible for processing visual information. The origin of sexual orientation‐related differences in brain anatomy is unknown, however, recent studies suggest that numerous genes might be involved, as well as many non‐genetic factors.

The investigators also discovered an interesting trend with regard to the linkage between genetic patterns and changes in brain anatomy. Genetic factors that are associated with variations in some cortical structures showed opposite effects on cortical volume in male versus female homosexuals who reported same-sex sexual behaviors. In other words, the brain regions of interest in this study became larger in homosexual females and smaller in homosexual males, as compared to their heterosexual counterparts. In spite of these interesting findings, the authors emphasized that genetic or neuroimaging data cannot be used to predict an individual's sexual orientation.

© Gary L. Wenk, Ph.D.