PET and MRI show differences in cerebral asymmetry and functional connectivity between homo- and heterosexual subjects – Brief Review and a cautionary note

This is the abstract of a study which shows that the brains of gay men are similar to the brains of straight women in certain ways while the brains of lesbians are similar to the brains of straight men.

The study looked at hemispheric asymmetry and functional connectivity, two areas where scientists have found differences between men and women.

They scanned the brains of 25 straight men (HeM), 25 straight women (HeW), 20 gay men (HoM), and 20 lesbians (HoW). Fifty of the subjects also participated in a test of blood flow that is used to analyze connections between the right and left amygdalae.

Results (from the abstract):

“HeM and HoW showed a rightward cerebral asymmetry, whereas volumes of the cerebral hemispheres were symmetrical in HoM and HeW. No cerebellar asymmetries were found. Homosexual subjects also showed sex-atypical amygdala connections. In HoM, as in HeW, the connections were more widespread from the left amygdala; in HoW and HeM, on the other hand, from the right amygdala. Furthermore, in HoM and HeW the connections were primarily displayed with the contralateral amygdala and the anterior cingulate, in HeM and HoW with the caudate, putamen, and the prefrontal cortex.”

They conclude:

“The present study shows sex-atypical cerebral asymmetry and functional connections in homosexual subjects.”

This study shows the critical importance of controlling for sexual orientation when studying the question of gender identity and the brain.

An extremely high proportion of people with gender dysphoria are primarily attracted to people of their birth sex. In the general population, only about 5% of people are primarily gay or lesbian.

Studies of gender identity and the brain should include both gay and straight control groups.

There is also a need for studies comparing the brains of transgender people who are attracted to people of their birth sex to gay men and lesbian women who are cis. This might be the most fruitful avenue of research into what causes gender dysphoria.

Original Abstract:

PET and MRI show differences in cerebral asymmetry and functional connectivity between homo- and heterosexual subjects by Ivanka Savic and Per Lindström in Proceedings of the National Academy of Sciences of the United States of America, July 8, 2008, vol. 105 no. 27.

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