June 16, 2008 -- Imaging studies reveal key differences in the brains of gay and straight men and women that could add to the understanding of the biology of homosexuality.
Using magnetic resonance (MR) imaging, researchers from Stockholm's Karolinska Institute found that the brains of straight men and gay women tended to be slightly asymmetric, with the right hemisphere somewhat larger than the left.
The asymmetry was not seen in straight women and gay men.
And building on earlier imaging research showing different patterns of brain activation between the sexes in response to emotional stimuli, the Swedish researchers showed that certain activation patterns for gay men and women tended to more closely resemble those of the opposite sex than their own.
The Brain and Sex
While earlier brain imaging studies have shown differences in brain activation between gay and straight men and women when measuring sexual attraction, the study is among the first to show the differences in areas unrelated to sexuality.
"This and other research seems to be sending the same message, which is that sexual orientation is part of a package. It is not an isolated trait," says neuroscientist Simon LeVay, PhD, who has long studied sexuality and the brain.
In 1991, LeVay was among the first to identify differences in the brains of homosexual and heterosexual people when his research identified a small region of the hypothalamus that was larger in straight men than in women and gay men.
Among other things, the hypothalamus is involved with regulating sexual response.
In the newly reported study, published online Monday in the journal PNAS, researchers analyzed the brains of 90 gay or straight men and women using MR imaging.
'A Strong Case'
Fifty of the study participants also underwent three-dimensional imaging involving positron emission tomography (PET), designed to measure activation within the brain.
In general, the PET imaging revealed what the authors termed "remarkable similarity" between straight women and homosexual men in the connectivity of a part of the brain known as the amygdala, which plays a key role in emotional reactions.
LeVay, who is gay, says the research as a whole makes a strong case for homosexuality being a function of biology and not choice.
"Nothing about the left and right brain has anything to do with sexuality, he says. "The preponderance of research argues rather strongly for a common biological process that is contributing not only to sexual orientation, but to other things as well."