A new Swedish study has found that gay men and women share some of the brain characteristics of heterosexual people of the opposite sex.
The research compared the size of the brain’s halves in 90 adults, and found that gay men and heterosexual women had halves of a similar size, while the right side was bigger in both lesbian women and heterosexual men.
The scientists also discovered that gay men and straight women show similarities in the area of the brain responsible for emotion, anxiety and mood, researchers from Sweden’s Karolinska Institute said on Monday.
The study highlights the potential biological foundation of sexuality. One British scientist told the BBC News the research provided evidence that sexual orientation was set in the womb.
Scientists have long observed that homosexual males and females have differences in certain cognitive abilities, something that might imply differences in their brain structure. But the Swedish study is the first time scientists have used brain scans to attempt to identify the source of those differences.
The research involved a group of 90 healthy gay and heterosexual men and women, each of whom received brain scans measuring the volume of both sides, or hemispheres, of their brain. The researchers found that lesbians and heterosexual men shared a particular “asymmetry” in their hemisphere size, whereas gay men and heterosexual women had no difference in the size of the different halves of their brain. In other words, at least structurally, the brains of gay men were more like heterosexual women, and gay women more like heterosexual men.
Further research revealed other significant differences, particularly in an area of the brain known as he amygdala. In gay women and heterosexual men, there were more nerve connections in the right side of the amygdala. The opposite was true in homosexual men and heterosexual women, in which more neural connections were seen in the left side of the amygdala.
“The observations cannot be easily attributed to perception or behavior,” the researchers wrote in a report about the study.
“Whether they may relate to processes laid down during the fetal or postnatal development is an open question.”
Dr Qazi Rahman, a lecturer in cognitive biology at Queen Mary, University of London, told BBC News he believed the foundations for these differences were established during fetal development.
“As far as I’m concerned there is no argument any more – if you are gay, you are born gay,” he said.
The amygdala, he said, was critical because of its role in directing the rest of the brain in response to various emotional stimuli, such as the presence of a possible mate or the “fight or flight” response.
“In other words, the brain network which determines what sexual orientation actually ‘orients’ towards is similar between gay men and straight women, and between gay women and straight men,” he said.
“This makes sense given that gay men have a sexual preference which is like that of women in general, that is, preferring men, and vice versa for lesbian women.”
The Karolinska team said their study does not address whether the differences in brain shapes are inherited or perhaps due to exposure to hormones such as testosterone in the womb, and whether they determine sexual orientation.
“These observations motivate more extensive investigations of larger study groups and prompt for a better understanding of the neurobiology of homosexuality,” the researchers wrote.
The study was published online June 16 in the journal Proceedings of the National Academy of Sciences.
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