Using scanning technology, researchers in Sweden found that the brains of gay men and women were wired differently to the brains of heterosexual people of the same sex, but were similar to the brains of heterosexual people that were of the opposite sex to them. Thus a gay man’s brain was in some ways more like the brain of a heterosexual woman than a heterosexual man, and the brain of a gay or lesbian woman was more like that of a heterosexual man than a heterosexual woman.

The study was the work of Drs Ivanka Savic and Per Lindström from the Stockholm Brain Institute at the Department of Clinical Neuroscience, Karolinska Institute and is published online in the June 16 issue of the Proceedings of the National Academy of Sciences (PNAS).

Scientists had already discovered that the brains of heterosexual and gay people of the same sex responded differently to stimuli, such as objects of sexual attraction and smells. The question that still remained was did these differences arise because of inherent brain differences or because of differences in the way gay and heterosexual people had “learned” to perceive these stimuli?

Savic and Lindström chose to investigate this further by examining the structure of the brain of gay and heterosexual men and women using PET and MRI scanners. They focused on volume differences in the two brain hemispheres (hemispheric asymmetry), and also on how the brains were “wired” (functional connectivity) because previous research had shown these to vary between men and women.

The researchers enrolled 90 adult volunteers: 25 heterosexual men (HeM), 25 heterosexual women (HeW), 20 homosexual men (HoM), and 20 homosexual women (HoW). All 90 underwent scans to measure overall and hemispherical brain volume using magnetic resonance technology, and 50 of them also had PET scans to measure blood flow in the brain, which can be analyzed to see the connections to and from the amygdalae in the two halves of the brain.

The amygdalae are clusters of neurons involved in a wide range of brain functions from emotional expression, processing and “storage” of memories to smell interpretation.

The results showed that:

    • The brains of heterosexual men (HeM) and homosexual women (HoW) were similar in that the volumes of their two brain hemispheres were not symmetrical (rightward cerebral asymmetry).

    • The brains of homosexual men (HoM) and heterosexual women (HeW) were similar in that the volumes of their two brain hemispheres were symmetrical.

    • There were also opposite sex similarities between the gay and heterosexual participants in the way their amygdalae connected.

    • For example, in homosexual men (HoM) and heterosexual women (HeW), there were more connections from the left amygdala.

    • But in homosexual women (HoW) and heterosexual men (HeM), there were more connections from the right amygdala.

    • Also, there were significant differences in the main connections from the amygdala to other parts of the brain.

  • In homosexual men (HoM) and heterosexual women (HeW), these were primarily between the contralateral amygdala and the anterior cingulate, while in heterosexual men (HeM) and homosexual women (HoW), the amygdala connections were mostly with the caudate, putamen, and the prefrontal cortex.

The authors concluded that the brains of homosexual subjects demonstrated “sex-atypical cerebral asymmetry and functional connections”. These differences could not be explained simply by “learned effects”, and they suggested a “linkage to neurobiological entities”.

A cognitive biology expert told BBC News that he believed these brain differences were decided early in the development of the fetus. There was no longer any argument, “if you are gay, you are born gay,” he said.

“PET and MRI show differences in cerebral asymmetry and functional connectivity between homo- and heterosexual subjects.”
Ivanka Savic and Per Lindström.
PNAS, Published online June 16, 2008.

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Source: PNAS, BBC.

Written by: Catharine Paddock, PhD