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Sweat odor affects women and homosexual men alike

University Of Chicago : 05 July, 2007  (Technical Article)
Strong airborne chemicals emitted in male perspiration and associated with sexual reproduction trigger a heightened response in the brains of homosexual men similar to that seen in heterosexual women.
Heterosexual men did not share the brain response to the chemicals in male sweat, according to a team of brain imaging specialists from the Karolinska Institute in Stockholm.

The current climate of debate over whether homosexuality is a matter of choice or is inborn makes such research extremely controversial, noted team leader and neuroscientist Dr. Ivanka Savic of the institute's Center for Gender Related Medicine.

'I want to be extremely cautious, this study does not tell us anything about whether sexual orientation is hardwired in the brain. It doesn't say anything about that,' Savic said.

'We're saying that we've detected measurable differences in the brains of homosexual men in regions that are known to mediate reproductive and sexual behavior.'

The added issue of human pheromones, the existence of which has been demonstrated only indirectly, according to critics, and any role they may play in sexual preference should add to the fray.

A leading pioneer in pheromone research, University of Chicago psychologist Martha McClintock, commended the team for its work.

'I found it really intriguing,' she said. 'This clearly shows that humans are processing these steroids and odors in a sexually specific way. It's another piece of evidence demonstrating that chemical signals, or chemosignals, I've dubbed them, play a significant role in human behavior.'

The subjects of the study smelled various compounds, among them two odorless substances closely related to the hormones testosterone and estrogen, as the researchers conducted PET scans that measure blood flow in different regions of the brain.

All three groups, 12 homosexual males, 12 heterosexual men and 12 heterosexual women, responded to common odors such as lavender in a similar fashion, engaging only the regions of the brain that process smell.

But the brains reacted differently to other chemicals.

A compound known as EST, derived from the female sex hormone estrogen, increased blood flow in part of the hypothalamus in heterosexual men but not in heterosexual women. Conversely, a testosterone-related substance known as AND lit up the brains of women and gay men, but not heterosexual men.

The hypothalamus is a region of the brain lying just above the pituitary gland, which is the master control for hormones, the body's chemical messengers.

'We've measured a physiological response that we think may be related to sexual orientation, though it could be due to something else,' Savic said.

For example, the response could be due to differences in previous exposure to male perspiration, she said. Or, it's possible that the brains of homosexual men undergo change as a consequence of being gay.

Demonstrating cause and effect has been a continual challenge to research on sexual orientation. Other researchers have presented evidence suggesting physical variations in the brains of homosexuals, but it is unclear why those differences may exist. They might be due to the organizing effects of sex hormones as fetuses grow in utero, for example, or they could be the result of environmental factors experienced since birth.

The chemicals AND and EST are considered to be potential pheromones, undetectable chemical signals produced by one individual that can influence the biology of another. Animals, fish and insects give off and respond to pheromones, but their existence in humans remains debatable despite compelling evidence from McClintock and others, Savic said.

Savic said the results of the team's research on homosexual women are still being compiled for publication.

In another study also released Monday, researchers at the Monell Chemical Sciences Center in Philadelphia found that a person's preference for another person's body scent depends, in part, on the sexual orientation of both parties.

In the study, 82 heterosexual and homosexual men and women were asked to indicate their preference among samples of underarm sweat collected from 24 men and women of varied sexual orientation.
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