Don’t Ask, Don’t Smell

Researchers at the Karolinska Institute in Sweden showed in 2001 that pheromones stimulate the human hypothalamus–a part of the brain that governs sexual arousal. In 2005, they added a fascinating new facet by introducing gay men into the study. “Gay men are a great control group for this kind of study,” says Dean Hamer, a geneticist at the National Institutes of Health, “because they’re pretty much the same as straight men except for that one factor.”

As in the earlier tests, the 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 straight men, the results were exactly the opposite.

And the gay men? Sure enough, when exposed to male pheromones, their hypothalamuses lit up just like the women’s. Female hormones did nothing for them.

While Hamer acknowledges that the gay men’s different brain activity could be either a cause of their sexual orientation or an effect of it, he says, “it certainly seems unlikely that somehow being interested in men would cause the brain to rewire itself in such a dramatic way.” This might be tested, he suggests, by studying the responses of people at different ages to monitor any change from early childhood through adulthood.

Similar testing on lesbian women has proven less conclusive. In 2006, researchers led by Ivanka Savic of the Stockholm Brain Institute and Hans Berglund of the Karolinska University Hospital reported that, while lesbians processed neural responses more like heterosexual men than heterosexual women, the similarity between homosexual men and heterosexual women seemed stronger than the similarity between lesbians and heterosexual men.

Sandra Witelson, a neuroscientist at the Michael G. DeGroote School of Medicine at McMaster University in Ontario, Canada, finds these differences fascinating, suggesting that sexual orientation may well have a different basis in women than in men. “It’s not as simple as gay men being more like women, and lesbians being more like men. There are two sexes, which we know are biologically different in some respects, and variation in sexual behavior within each sex is not necessarily the same as in the other sex.”