The new research adds to growing evidence that homosexuality is determined by biology. The results were particularly marked for homosexual men, who were found to have a strong preference for the natural scent of other gay men, which heterosexuals found unattractive.

The studies by teams in the United States and Sweden suggest that gender preference has some physiological basis. This follows other evidence that genetic inheritance and hormonal conditions in the womb may play a part in creating predispositions to homosexuality, which are then triggered — or not — by later environmental factors and experience.

In one of the studies, which is published today in the journal Psychological Science, researchers at Monell Chemical Senses Centre in Philadelphia used samples of under-arm sweat to compare the odour preferences of heterosexual and homosexual men and women. The researchers found that people of different orientations have different preferences, and that the sweat they produce does not appear to smell the same.

“Our findings support the contention that gender preference has a biological component that is reflected in both the production of different body odours and in the perception of and response to the body,” said Charles Wysocki, who led the research.

In the study, six heterosexual men, six heterosexual women, six gay men and six gay women were each asked to wear cotton gauze pads in their armpits throughout the day. The pads were then cut into sections and placed in plastic squeeze bottles. In the next phase of the experiment, 80 homosexual and heterosexual men and women — 20 of each — were asked to sniff two bottles and choose the odour they preferred. Dr Wysocki’s team made four comparisons: sweat from gay men versus straight men, gay men versus straight women, straight women versus lesbian women, and gay men versus lesbian women.

Homosexual men and women had different preferences from heterosexuals of both genders. The effects were particularly marked for homosexual men, who had a pronounced preference for body odour of other gay men. Sweat from homosexual men was judged least favourably by all other groups.

In the second study, published today in the journal Proceedings of the National Academy of Sciences, a team from the Karolinska Institute in Stockholm used positron emission tomography scanning to examine the brain’s response to two hormone derivatives, AND and EST, which have been proposed as human pheromones — powerful scents known to convey sexual signals in many species but which have an uncertain role in human beings.

The scans showed a different pattern of activity in response in heterosexual and homosexual men, particularly in a brain region known as the hypothalamus that is involved in sexual arousal. The brain activity of gay men turned out to be much more similar to that of straight women, suggesting that sexual orientation rather than gender was the determinant.

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