What the brain and the nose tell scientists about sexual desire ­ and orientation ­ in the human male.
By MICHAEL D. LEMONICK

TIME MAGAZINE May 23, 2005

If you want to stir up trouble at a party--or better still, a bar--try bringing up the question of whether homosexuality is something people are born with or something they choose. The issue has always been controversial, and it's currently at the center of a national political debate as well, thanks to the question of gay marriage. As a result, whenever science has something to say about the biology of sexual preference, it's bound to make headlines.

That's exactly what happened last week. Researchers at the Karolinska Institute in Sweden who had earlier shown that hormonelike pheromones stimulate the human hypothalamus--a part of the brain that governs sexual arousal--took the experiment one provocative step further. Writing in the Proceedings of the National Academy of Sciences, they reported that gay men don't respond to the chemicals the same way that straight men do. "It clearly substantiates the idea that there's a biological substrate for sexual orientation," says Dean Hamer, a geneticist at the National Institutes of Health and the author of Science of Desire: The Gay Gene and the Biology of Behavior (Simon & Schuster; 272 pages). "This is a highly significant result."

The experiment was elegantly simple. Just as they had in a series of tests in 2001, the Swedish scientists isolated two substances suspected of being human pheromones--an estrogen- like chemical distilled from women's urine and a testosterone-related chemical derived from male sweat. Using both MRI and PET scans, the researchers found that women registered the female pheromone in the smell-processing part of the brain. But when women sniffed male pheromones, their hypothalamuses lit up as well. In men, the results were exactly the opposite.

All that had been shown before. What was new in the recent experiments was the inclusion of gay men. "Gay men are a great control group for this kind of study," says Hamer, "because they're pretty much the same as straight men except for that one factor." Sure enough, when the Swedish scientists ran the experiment this time, the results were striking: when gay men were exposed to male pheromones, their hypothalamuses lit up just like a woman's. Female hormones did nothing for them.

What the study doesn't show, however--despite what some scientists claimed--is that sexual preference is biologically hardwired and thus present from birth. That idea is pretty much accepted by most gays and by many biologists as well. But it is refuted by those--generally on the religious right--who have a stake in believing that homosexuality is a personal choice rather than an inborn trait.

Even though last week's study strengthens the argument that desire may be triggered in part by chemical signals, it doesn't necessarily prove that gay men are preordained to pick up on male pheromones. It could also be that their brains learn to respond to them over time and with experience.

You might be able to test the proposition, says Hamer, by doing the experiment on people at different ages, to see if the response changes after early childhood. Nobody has tried that yet. The Swedish team is currently working on a related study to test how lesbians respond to female pheromones. Last week's paper also can't answer the question of how important a role pheromones play in desire. Conventional wisdom used to be that people could not detect them at all.

That's because the vomeronasal organ, a pheromone-sensitive structure in the nose that's very active in mice, for example, is largely vestigial in humans. Although it now seems that pheromones are somehow involved in arousal, their role could still be minimal. Says Hamer: "They're certainly not as important as they are in the mouse, who can't rely on gawking at cheerleaders to get turned on." Still, there's no harm in taking a sniff next time you meet someone attractive--as long you do it discreetly.

© 2005 Time, Inc.