What makes people gay? Biologists may never get a complete answer to that question, but researchers in Sweden have found one more sign that the answer lies in the structure of the brain.
Scientists at the Karolinska Institute studied brain scans of 90 gay and straight men and women, and found that the size of the two symmetrical halves of the brains of gay men more closely resembled those of straight women than they did straight men. In heterosexual women, the two halves of the brain are more or less the same size. In heterosexual men, the right hemisphere is slightly larger. Scans of the brains of gay men in the study, however, showed that their hemispheres were relatively symmetrical, like those of straight women, while the brains of homosexual women were asymmetrical like those of straight men.
The number of nerves connecting the two sides of the brains of gay men were also more like the number in heterosexual women than in straight men.
Just what these brain differences mean is still not clear. Ever since 1991, when Simon LeVay first documented differences in the hypothalamus of gay and straight men, researchers have been struggling to understand what causes these differences to occur. Until now, the brain regions that scientists have come to believe play a role in sexual orientation have been related to either reproduction or sexuality. The Swedish study, however, is the first to find differences in parts of the brain not normally involved in reproduction — the denser network of nerve connections, for example, was found in the amygdala, known as the emotional center of the brain. "The big question has always been, if the brains of gay men are different, or feminized, as earlier research suggests," says Dr. Eric Vilain, professor of human genetics at University of California Los Angeles, "then is it just limited to sexual preference or are there other regions that are gender atypical in gay males? For the first time, in this study it looks like there are regions of the brain not directly involved in sexuality that seem to be feminized in gay males."
Vilain, who studies the genetic factors behind sexuality and sexual orientation, notes that it may turn out that the brains of gay men possess only some 'feminized' structures, while retaining some masculine ones, and this is reflected in how they act on their sexuality. "We know from studies that men, regardless of their sexual orientation, retain masculine characteristics when it comes to their sexual behavior," he says. Both gay and straight men, for example, tend to prefer younger partners, in contrast to women, who gravitate toward older partners. Most men are also more likely than women to engage in casual sex, and to be aroused by visual stimuli. "So I expect that some regions of the brain will remain masculine even in gay men," says Vilain. For something as complex as sexual orientation, it's no surprise that everything from genes to gender to environment may play a role in ultimately determining your perfect partner.