for National Geographic News
A new study shows that gay men respond differently from straight men when exposed to a suspected sexual stimulus found in male sweat.
When homosexual men smelled the odor of male sweatmore specifically, a chemical in the male hormone testosteronetheir brains responded similarly to those of women.
The findings suggest that brain activity and sexual orientation are linked. It also supports an opinion held by most scientists, that people are bornnot bredgay.
"This is one more line of evidence that there's a biological substring for sexual orientation," said Dean Hamer, a geneticist at the National Institutes of Health in Bethesda, Maryland.
Hamer is the author of The Science of Desire: The Gay Gene and the Biology of Behavior. He was not involved in the research, which was conducted by scientists at Karolinska Institutet in Stockholm, Sweden.
The study was published today in the research journal Proceedings of the National Academy of Sciences.
The scientists exposed heterosexual men and women and homosexual men to chemicals found in male and female sex hormones. One chemical is a testosterone derivative produced in men's sweat. The other chemical is an estrogen-like compound in women's urine.
These chemicals have long been suspected of being pheromones, molecules emitted by one individual that evoke some behavior in another of the same species. Pheromones trigger basic responses, such as sexual attraction, in many animals.
But scientists have long debated if humans respond to pheromones. The new study suggests that pheromones indeed play a part in making humans sexually attractive to one another.
In a previous study a few years ago, the Swedish researchers showed that the brain's hypothalamus region, which is involved in sexual behavior, becomes activated when men smell EST (the estrogen derivative) and women smell AND (the testosterone compound), but not vice versa.
For their new study, the scientists added a sexual-orientation element, which revealed a difference in the brain activity of gay and straight men.
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