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Is it SO hard to click a link?

Study: Gay men's brains react differently to scent

Monday, May 9, 2005 Posted: 8:37 PM EDT (0037 GMT)

WASHINGTON (Reuters) -- A compound taken from male sweat stimulates the brains of gay men and straight women but not heterosexual men, raising the possibility that homosexual brains are different, researchers in Sweden reported on Monday.

It also strengthens the evidence that humans respond to pheromones -- compounds known to affect animal behavior, especially mating behavior, but whose role in human activity has been questioned.

The pheromone in question is a derivative of testosterone called 4,16-androstadien-3-one, or AND.

"AND is detected primarily in male sweat," the researchers write in this week's issue of the Proceedings of the National Academy of Sciences.

In a previous study, Ivanka Savic of Karolinska University Hospital in Stockholm and colleagues found that the hypothalamus region of the brain became activated when women smelled AND and when men smelled a corresponding compound in female urine called EST.

This time they compared the reactions of 12 women, 12 heterosexual men and 12 homosexual men.

They let them smell EST, AND, and ordinary odors such as lavender, and used positron emission tomography to watch their brain responses.

"In contrast to heterosexual men, and in congruence with heterosexual women, homosexual men displayed hypothalamic activation in response to AND," Savic's team wrote.

And a region of the brain called the anterior hypothalamus responded most strongly -- an area that in animals "is highly involved in sexual behavior".

But other smells were processed the same in all three groups.

"These findings show that our brain reacts differently to the two putative pheromones compared with common odors, and suggest a link between sexual orientation and hypothalamic neuronal processes," Savic's team wrote.

In most animals, pheromone signals go to the hypothalamus region of the brain via a pit-like structure in or near the nose called the vomeronasal organ.

People have a vomeronasal pit but there are no nerves connecting it to the brain, leading biologists to question whether humans respond to pheromones.