BRAIN scans have provided the most compelling evidence yet that being gay or straight is down to biology rather than choice. Tantalisingly, the scans reveal that in gay people, key structures of the brain governing emotion, mood, anxiety and aggression resemble those in straight people of the opposite sex.
鈥淭his is the most robust measure so far of cerebral differences between homosexual and heterosexual subjects,鈥� says Ivanka Savic, who conducted the study at the Karolinska Institute in Stockholm, Sweden.
Previous studies have also shown differences in brain architecture and activity between gay and straight people, but most were based on people鈥檚 responses to sexually driven cues that could have been learned, such as rating the attractiveness of male or female faces.
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To get round this, Savic and her colleague, Per Lindstr枚m, chose to measure brain features that are probably fixed at birth. 鈥淭hat was the whole point of the study, to show parameters that differ, but which couldn鈥檛 be altered by learning or cognitive processes,鈥� says Savic, whose results appear in Proceedings of the National Academy of Sciences ().
Firstly, they used MRI scans to measure the overall volume and shapes of brains in a group of 90 volunteers consisting of 25 heterosexuals and 20 homosexuals of each gender. Most notably, they found that lesbian women and straight men had asymmetric brains, with the right hemisphere slightly larger than the left. Gay men, meanwhile, had symmetrical brains like those of straight women.
Secondly, they used scans based on positron emission tomography (PET) to measure blood flow to the amygdala, an almond-shaped region found in both lobes of the brain that plays a key role in emotional reactions. The images revealed how the amygdalas are connected to other parts of the brain, giving clues to how this might influence behaviour.
They found that the patterns of connectivity in gay men matched those of straight women, and vice versa (see Diagram). In straight women and gay men, the signals from the amygdala ran mainly into the regions of the brain that mediate mood and anxiety.
This finding is significant, says Savic, as it might explain why women are three times as likely as men to suffer from mood disorders or depression. Gay men have higher rates of depression too, she says, but it鈥檚 difficult to know whether this is down to biology, or having to deal with homophobia.
In straight men and lesbians, the amygdala fed their signals mainly into the sensorimotor cortex and the striatum, regions of the brain that trigger 鈥渇right or flight鈥� in response to fear. 鈥淚t鈥檚 a more action-related response than in straight women,鈥� says Savic.
鈥淭his study demonstrates that homosexuals of both sexes show strong cross-sex shifts in brain symmetry,鈥� says Qazi Rahman, a leading researcher on sexual orientation at Queen Mary University of London. 鈥淭he connectivity differences reported in the amygdala are striking.鈥�
鈥淧aradoxically, it鈥檚 more informative to look at things that have no direct connection with sexual orientation, and that鈥檚 where this study scores,鈥� says Simon LeVay, a prominent US author who in 1991 reported finding differences between straight and gay men in a part of the brain called the hypothalamus.
But as Savic herself acknowledges, the study can鈥檛 say whether the brain differences are genetic, or result from unusually high or low exposure in the womb to sex hormones such as testosterone.
The Human Brain 鈥� With one hundred billion nerve cells, the complexity is mind-boggling. Learn more in our cutting edge special report.
