Camperio did not quite know at first what to make of these results—or how they might help him understand the Darwinian paradox of homosexuality. Then one day, he was driving through the forest with his daughter, on the way to their country house. Their tradition was to play mathematical games to keep themselves entertained. This time, he began talking about a different puzzle. "I began explaining my research," Camperio recalls. "I explained to her that we found out that homosexuals come from large families. I told her that there is an inheritance from the mother—she's giving the homosexual genes to her son. I said, 'This is impossible—how can they be surviving?'"
His daughter, 15, replied, "But Dad, did you check if this factor that makes sons homosexual is not the same factor that makes the mother produce more children and have big families?"
Camperio stopped the car, looked her in the eyes, and said, "Shit! What is this? It's a great suggestion!"
The next day he left his daughter in the country and went back to the lab to investigate the idea. Sure enough, the mothers of homosexuals in the study did indeed have between a quarter and a third more children than the mothers of heterosexuals. Camperio also uncovered another dramatic finding: In families with gay sons, the aunts from the mother's side had many more children than the aunts on the father's side—the large families, in other words, were on the maternal side. Camperio realized his daughter was right. "There was something in the genes that, in the male, changed his sexual orientation, and in the female, increased her probability of having children," he says.
What could it be? Camperio spent the next few years going to gay men and begging them to let him interview their mothers and aunts—a daunting task in deeply Catholic Italy. In the end, it took him three years to get 30 subjects. When he interviewed the women, though, he found they had fewer miscarriages, fewer infections, and used fewer contraceptives than the mothers and aunts of heterosexuals, though the differences were only slight. One difference, though, was not slight at all: The homosexuals' mothers and aunts had had between three and four times as many sexual partners. They seemed to really like having sex with men.
Camperio's explanation for all this relies, like Rahman and Wilson's hypothesis, on sexually antagonistic selection. Perhaps, he suggests, the mothers of some homosexuals have a "man-loving" gene. In women, it would be adaptive, causing them to have more sex and more children. But in men, the "man-loving" gene would be expressed differently, causing homosexuality. To the gay sons, that would be an evolutionary disadvantage—but one outweighed by the advantage to the mothers, who would have more than enough other children to compensate. And so gayness in men would persist in these families—as a side effect of a trait that is beneficial to the women.
But even Camperio says his results can explain no more than 20 percent of the incidence of homosexuality. "The more we study, the more we find there will be other mechanisms," he says. His research confirms that there are many ways to become gay—including, perhaps, one way that is much stranger than the rest.
The gay men in Camperio's study didn't just have larger families than the straight men. They also had more older brothers—and not just because they came from larger families. It's true across the board: The more older brothers a man has, the more likely he is to be gay. The "fraternal birth order effect" was first uncovered by Ray Blanchard and Anthony Bogaert of the Center for Addiction and Mental Health in Toronto, and has since been replicated by a dozen other studies.
For every older brother a man has, his chances of being gay go up by around a third. In other words, if you have two older brothers, you're nearly twice as likely to be gay—regardless of whether the older brothers are themselves gay. It is not possible to explain that as an effect of genetics.
Some researchers have tried to explain it as an effect the older brothers have on their sibling's environment. Perhaps a boy grows up homosexual, one argument goes, because the presence of older brothers means more incestuous sex play early in life. Or perhaps their presence makes his parents treat him differently.
But in another study, Bogaert found that it was only biological older brothers that contributed to the effect. Men who grew up with older stepbrothers or adopted brothers—brothers born of different wombs—were no more likely to become gay. Meanwhile, men with biological older brothers who died in infancy or who were raised separately—including brothers they had never even met and sometimes didn't even know about—did manifest the effect. In other words, the effect could not be explained through upbringing.
If it wasn't genetic and it wasn't upbringing, then what could it possibly be? The answer is the prenatal environment—the result of something that occurs as the fetus develops in the womb.
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