But 21st-century Western society, and the homosexuals therein, could be something of an anomaly in human history, according to Paul Vasey, an associate professor of psychology at the University of Lethbridge in Alberta. “Do I think the social contexts are less representative of our evolutionary past? I’d say yes,” Vasey says. “The kind of gay men and lesbian women we are familiar with in Western culture express their homosexuality as egalitarian—they’re not differentiated as far as gender.” In other times and other places, one partner typically adopts a masculine role, while the other adopts the feminine. Another problem with kinship selection studies that look only in England and, in particular, the United States, is that kinship ties for homosexuals might not be as strong as they would be elsewhere. “The United States is profoundly homophobic,” says Vasey, “so you can’t be directing altruism at your family members if they’ve kicked you out and you’ve moved to the other side of the country.”
Vasey has therefore been studying a group of Samoans called fa’afafine, whom he describes as more of a third sex than homosexual as commonly construed. These men, who grow up to dress and act like women and are extremely integrated into their society, would be offended to find themselves described as homosexual. They don’t have sex with each other, and because their appearance isn’t masculine, they don’t consider themselves men who sleep with other men. In an initial study, Vasey found that the fa’afafine do exhibit heightened avuncular tendencies with their nieces and nephews. “They babysit, they teach them about the culture, they give them money,” he says. To make sure that these traits do not merely reflect a more general fondness for all children, Vasey will soon head back to Samoa to refine and, with luck, replicate the study.
To put the kibosh on the idea that the evolution of a gay gene presents an unsolvable conundrum, Sergey Gavrilets, a theoretical evolutionary biologist at the University of Tennessee, developed a mathematical model of how a set of what he calls “sexually antagonistic” genes might evolve. “Sexuality can be explained as one example of sexual conflict—there may be genes that are beneficial for one sex but detrimental for the other,” he says. He didn’t put kin selection into the equation. (“I don’t think it works, because if you have zero children and compensate for the loss by helping someone else, that help would have to be extremely efficient.”) But he did include Camperio-Ciani’s results and also assumed that gay genes would be passed down on the mother’s side, on the X chromosome, as indicated by Hamer.
The model shows that over centuries an effect you might call the homophobe’s paradox has been at work on the human genome: The more intolerant the society, the more likely it is to maintain gay genes. If a society’s conventions keep homosexuals in the closet, then they will be more likely to conform, get married, and have children. This is especially true if gay genes are also responsible for making women more fecund. Imagine, for instance, that for every extra child that such a gay gene–carrying woman has, a gay man can have one fewer and the balance necessary for the survival of the gene is still maintained. The more children he has, thanks to what his contemporaries demand of him, the less evolutionary pressure there is for his female counterpart to have more. “As a society becomes more intolerant, there’s more pressure to have offspring,” says Gavrilets. “The real [evolutionary] cost of being homosexual isn’t too big if you’re forced to have kids.” On the other hand, the more tolerant the society, the more gay men can be free to be who they are, so the more likely they will be childless—and the more difficult it will be for any female in the family to make up for the loss.
“Bullshit,” says Bocklandt. “A mathematical model is a nice exercise, a mental masturbation about how these things could work, but it makes better sense to do that once we know a bit more. One of the problems that none of the mathematical models take into account is that we have no idea what it meant to be gay 10,000 years ago. We have some idea what it meant 200 years ago but not 10,000.”
It is also becoming increasingly clear that gay genes are not the only biological factor that influences homosexuality. Some homosexual men appear to have their sexuality oriented not by their DNA but by the environment they experienced in the womb. Ray Blanchard, a psychiatric researcher at the University of Toronto, found in 1996 that men with older brothers were more likely to be gay than those without. His study showed that for every older brother a man has, his odds of being gay go up by 33 percent. If the likelihood that a couple’s first son will be gay is 2 percent (a reasonable guess), Blanchard says, the probability that the 5th son will be gay is only 6 percent. But if some poor woman has 14 sons, the 15th would have a 50 percent chance of being gay.
The more intolerant the society, the more likely it is to maintain gay genes. The real evolutionary cost of being homosexual isn’t too big if you’re forced to have kids.
Blanchard’s discovery has been replicated more than 20 times. Most recently, in 2006, psychologist Tony Bogaert of Brock University in Ontario quashed the possibility that the older-brother effect results from boys having been teased, beaten, or otherwise affected by their older brothers. Bogaert collected data from gay and heterosexual men who grew up in nonbiological families—in most cases, either adopted or raised in “blended” families. “This allowed me to really pit biological versus nonbiological explanations against each other,” Bogaert says. The statistics came back the same as they had in previous studies. “It’s not the brother you lived with; it’s the environment within the same womb—sharing the same mom.”
The older-brother effect accounts for about 15 to 30 percent of gay men, Blanchard estimates. How sharing a biological mother could induce homosexuality in men remains pure guesswork, however. Bogaert and Blanchard hypothesize that with each male child the mother develops an immunity to certain male-specific proteins, like molecules relating to the Y chromosome. Perhaps her body sees them as foreign and mounts an immune attack, which might alter certain structures of the male brain.
Whatever the cause of homosexuality, be it genetic, hormonal, or even sociological, the result is a change somewhere in the brain. Simon LeVay, once a neurobiologist at the Salk Institute, thought he found that part of the brain in 1991. Comparing the gray matter in the cadavers of gay and straight men, he found that an area in the anterior hypothalamus known as INAH-3 was smaller in gay men, about the size it is in women. A study by William Byne of the Mount Sinai School of Medicine in New York City turned up similar results.