Striking similarities between the brains of gay men and heterosexual women have been discovered by neuroÂÂscientists, offering fresh evidence that sexual orientation is hardwired into neural circuitry.
Scans reveal that homosexual men and heterosexual women have symmetrical brains, with the right and left hemispheres almost exactly the same size. Conversely, lesbians and heterosexual men have asymmetrical brains, with the right hemisphere significantly larger than the left.
Scientists at the Stockholm Brain Institute in Sweden also found that certain brain circuits linked to emotional responses were the same in gay men and heterosexual women.
The findings, published this week in the American journal Proceedings of the National Academy of Sciences, suggest the biological factors that influence sexual orientation, such as exposure to testosterone in the womb, may also shape the brain’s anatomy. The study, led by neurobiologist Ivanka Savic, builds on previous research that has identified differences in spatial and verbal abilities related to sex and sexual orientation. Tests have found gay men and heterosexual women fare better at certain language tasks, while heterosexual men and lesbians tend to have better spatial awareness.
Savic and her colleague Per Linstrom took MRI brain scans of 90 volunteers, who were divided into four age-matched groups according to whether they were male or female, heterosexual or homosexual. The scans showed that the right-hand side of the brain in heterosexual men was typically 2% larger than the left. Lesbians showed a similar asymmetry, with the right-hand side of the brain 1% larger than the left. Scans on homosexual men and heterosexual women revealed both sides of the brain were the same size.
The results could explain a study earlier this year led by Qazi Rahman at Queen Mary, University of London, which found gay men and heterosexual women share a poor sense of direction compared with heterosexual men, and are more likely to navigate using landmarks alone. The right-hand side of the brain dominates spatial capabilities, so may be slightly more developed in heterosexual men and lesbians. An earlier study by the same team found gay men and heterosexual women outperformed lesbians and heterosexual men at tasks designed to test verbal fluency.
Savic’s team has yet to confirm whether the differences in brain shape are responsible for sexual orientation, or are a consequence of it. To find out, they have begun another study to investigate brain symmetry in newborn babies, to see if it can be used to predict their future sexual orientation. ”These differences might be laid down during brain development in the womb, or they could happen after birth, though it could very likely be a combination of the two,” said Savic.
In another series of tests, Savic and Lindstrom used a technique called positron emission tomography to look at brain wiring in a smaller group of volunteers. They found heterosexual women and gay men shared brain circuitry that links a region called the amygdala, which plays a key role in emotional responses, to other parts of the brain.
In heterosexual men and lesbians, the amygdala feeds signals directly into parts of the brain that trigger the ”fight or flight” response, which Savic says is more physically active than the wiring seen in gay men and heterosexual women.
The research is part of a larger effort to identify differences between the male and female brain, in the hope they will shed light on why some mental disorders affect men and women differently. Depressive disorders are far more common and persistent in women, while autism is around four times more common in boys than girls.
”There’s a well-known uneven sex distribution in the number of psychiatric disorders and trying to understand sex differences, and differences in orientation, may give you a hint of the mechanism underlying these diseases,” said Savic. —