The researchers found that the testosterone compound activated the hypothalamus in homosexual men and heterosexual women, but not heterosexual men. Conversely, the estrogen compound activated the hypothalamus only in heterosexual men.
"It shows a different physiological response to the same external stimulus," said Ivanka Savic, a neuroscientist at the Karolinska Institute and the study's lead researcher. "This response [occurred] in the brain region involved in reproductive behavior."
When the study subjects sniffed scents such as cedar or lavender, all of their brains reacted only in the region that handles smellsnot sexual behavior.
The results show that the human brain reacts differently to potential pheromones compared with common odors.
"It directly shows a link between brain activity and sexual orientation," said Hamer, the NIH geneticist.
Hamer cautions that the gay men's different brain activity could be either a cause of their sexual orientation or an effect of it. But, he said, "it certainly seems unlikely that somehow being interested in men would cause the brain to rewire itself in such a dramatic way."
Other studies have also found that gay and straight men respond differently to the body odors of others.
Scientists at the Monell Chemical Senses Center in Philadelphia, Pennsylvania, found that gay men preferred odors from other gay men, while odors from gay men were the least preferred by straight men and women.
The Monell Center's results were released yesterday and are to be published in the journal Psychological Science in September.
"There are many ongoing studies in the field, and I think that we soon will have better clarification," said Savic, the Karolinska Institute neuroscientist. "At the moment, there are no definite proofs."
However, the new studies boost the hypothesis that homosexuality has a genetic basis and is not simply the result of learned behavior.
"This, incidentally, is not in any way controversial for biologists," Hamer said. "It's completely expected from the basic tenets of biology. It's only controversial because of the social and political controversy over homosexuality."
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