IT sounds like something out of the satirical journal Annals of Improbable Research: a team of Swedish neuroscientists scanned people's brains as they smelled a testosterone derivative found in men's sweat and an estrogen-like compound found in women's urine. In heterosexual men, a part of the hypothalamus (the seat of physical drives) responded to the female compound but not the male one; in heterosexual women and homosexual men, it was the other way around. But the discovery is more than just a shoo-in for that journal's annual Ig Nobel Prize - it raises provocative questions about the science and ethics of human sexuality.
Scientists and perfume marketers who believe that humans, like other mammals, respond sexually to chemical signals called pheromones were cheered by the news. But we are a long way from dogs in heat. The role of pheromones in our sexuality must be small at best. When people want to be titillated or to check out a prospective partner, most seek words or pictures, not dirty laundry.
The difference in the brain responses of gay and straight men does not, by itself, prove that homosexuality is innate; after all, learned inclinations, like innate ones, must reside somewhere in the brain. But in this case nature probably does trump nurture. Gay men generally report that their homosexual attractions began as soon as they felt sexual stirrings before adolescence. And homosexuality is more concordant in identical than in fraternal twins, suggesting that their shared genes play a role. Homosexuality is a puzzle for biology, not because homosexuality itself is evolutionarily maladaptive (though no more so than any other sexual act that does not result in conception), but because any genetic tendency to avoid heterosexual opportunities should have been selected out long ago. Perhaps "gay genes" have some other compensating advantage, like enhancing fertility, when they are carried by women. Perhaps the environments that set off homosexuality today didn't exist while our genes were being selected. Or perhaps the main cause is biological yet not directly genetic, like differences in hormones or antibodies that affect the fetus while it is developing.
Just as puzzling is the existence of homophobia. Why didn't evolution shape straight men to react to their gay fellows by thinking: "Great! More women for me!" Probably the answer lies in a cross-wiring between our senses of morality and disgust. People often confuse their own revulsion with objective sinfulness, as when they dehumanize people living in squalor or, in the other direction, engage in religious rituals of cleanliness and purification. An impulse to avoid homosexual contact may blur into an impulse to condemn homosexuality.
Cultural conservatives like the talk-show host Dr. Laura Schlesinger ostensibly condemn homosexuality for another reason - that it is a "biological error." Actually, it is she who has made the biological error. What is evolutionarily adaptive and what is morally justifiable have little to do with each other. Many laudable activities - being faithful to one's spouse, turning the other cheek, treating every child as precious, loving thy neighbor as thyself - are "biological errors" and are rare or unknown in the natural world.
It's not just anti-gay commentators who see a moral coloring in the biology of homosexuality. Some gay groups condemn such research because it could stigmatize gay people as defective and lead to a day in which parents could selectively abort children with "gay genes." Others welcome the research because it shows that people don't "choose" to be gay and hence can't be criticized for it, nor could homosexuals convert the children in their classrooms or Scout troops even if they wanted to.
It may not be a coincidence that the new discovery came from researchers in Europe. In America, the biology of homosexuality is a politicized minefield that scares away scientists (and the universities and agencies that pay for their research). Which is a pity. Regardless of where homosexuality resides in the brain, the ethics of homosexuality is a no-brainer: what consenting adults do in private is nobody's business but their own. And the deterrents to research on homosexuality leave us in ignorance of one of the most fascinating sources of human diversity.