If there is one thing that has always seemed obvious about homosexuality, it''''s that it just doesn''''t make sense. Evolution favors traits that aid reproduction, and being gay clearly doesn''''t do that. The existence of homosexuality amounts to a profound evolutionary mystery, since failing to pass on your genes means that your genetic fitness is a resounding zero. "Homosexuality is effectively like sterilization," says psychobiologist Qazi Rahman of Queen Mary College in London. "You''''d think evolution would get rid of it." Yet as far as historians can tell, homosexuality has always been with us. So the question remains: If it''''s such a disadvantage in the evolutionary rat race, why was it not selected into oblivion millennia ago?

Twentieth-century psychiatry had an answer for this Darwinian paradox: Homosexuality was not a biological trait at all but a psychological defect. It was a mistake, one that was always being created anew, in each generation, by bad parenting. Freud considered homosexuality a form of arrested development stamped on a child by a distant father or an overprotective mother. Homosexuality was even listed by the American Psychiatric Association as a mental disorder, and the idea that gays could and should be "cured" was widely accepted. But modern scientific research has not been kind to that idea. It turns out that parents of gay men are no better or worse than those of heterosexuals. And homosexual behavior is common in the animal kingdom, as well—among sheep, for instance. It arises naturally and does not seem to be a matter of aloof rams or overbearing ewes.

More is known about homosexuality in men than in women, whose sexuality appears more fluid. The consensus now is that people are "born gay," as the title of a recent book by Rahman and British psychologist Glenn Wilson puts it. But for decades, researchers have sought to identify the mechanism that makes a person gay.


Something seems to flip the sexuality switch before birth—but what? In many cases, homosexuality appears to be genetic. The best scientific surveys put the number of gays in the general population between 2 and 6 percent, with most estimates near the low end of that range—contrary to the 10 percent figure that is often reported in the popular media. But we know gayness is not entirely genetic, because in pairs of identical twins, it''''s often the case that one is gay and the other is not. Studies suggest there is a genetic basis for homosexuality in only 50 percent of gay men.