Gay men and straight women share brain characteristics that suggest sexual preferences may be innate rather than learned, researchers said. Lesbians and heterosexual men also had similar brain tendencies.

Scientists at the prestigious Stockholm Brain Institute in Sweden, who conducted the research, say MRI and PET reveal homosexual men and heterosexual women have symmetrical brains, with the right and left hemispheres almost exactly the same size. Conversely, lesbians and straight men have asymmetrical brains, with the right hemisphere — the side associated with spatial ability — significantly larger than the left.

The findings, published the current issue of the US 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, according to the researchers, the Guardian reports.

Similar trends emerged when scientists tracked brain circuits in a region of the brain involved in emotional learning and in activating the fight-or-flight response. The researchers noted strong similarities between gay men and straight women, and lesbians and straight men.

The study, led by the 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 straight women fare better at certain language tasks, while heterosexual men and lesbians tend to have better spatial awareness.

“This is a very interesting study demonstrating a possible neurobiological relationship in brain size between gay men and straight women,” Paul Sanberg told HealthDay. He is distinguished professor of neurosurgery and director of the University of South Florida Center for Aging and Brain Repair in Tampa.

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.

“I do think this is pointing to some type of neurobiological underpinning [to sexual orientation],” Keith A. Young, associate professor of psychiatry and behavioral science at the Texas A&M Health Science Center College of Medicine in Waco, told HealthDay. He was not involved in the study.

“It’s hard to know if that’s related to genes, or what might happen in the womb — I think those are the two primary options,” said Young, who is also co-director of the Central Texas Veterans’ Health Care System Neuropsychiatry Research Program in Temple. “How do those affect early brain development, and how might either genes or exposure to hormones in the womb change the trajectory of the development of emotional processing centers?”

Savic and her colleague Per Linström took MRI brain scans of 90 volunteers who were divided into four groups of similar ages according to whether they were male, female, heterosexual or homosexual. The scans showed the right 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.

“It’s likely that these differences are not influenced by learning or socialization,” Qazi Rahman, a lecturer in cognitive biology at Queen Mary University of London, told Bloomberg. “We’ve known for some time that homosexuals of both sexes may show differences in certain abilities, which are known to reside on one or the other of two sides of the brain.”

HealthDay points out that the neurobiology of sexual orientation remains a controversial topic. Some other research suggests that the brain activity of homosexual individuals in areas unrelated to sex mirror brain activity in straight individuals of the opposite sex. And certain psychological studies have revealed differences in how men and women use the brain’s different hemispheres for verbal tasks, for example.

The research by the Swedish institute 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, the Guardian reports. For example, major 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.

Source: Gay men have similar brains to straight women, scientists claim | Guardian

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