The human nose can not only sniff out suitable sexual partners, but it works especially well for gay and lesbians, according to the first study of how body odours are linked to sexual orientation.
Gay men showed a strong preference for the body odor of other gay men in the scientific test of how the natural scent of someone's body can contribute to the choice of a partner.
Although previous studies have shown that body odour plays a role in making heterosexual men or women attractive to members of the opposite sex, this is the first study that has investigated its role in sexual orientation. Charles Wysocki of the Monell Chemical Senses Centre in Philadelphia, a non-profit research institute, said the findings underline the importance of natural odors in determining a sexual partner whatever the sexual orientation of the person involved.
"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 body odours," Dr Wysocki said.
The study involved 24 heterosexual and homosexual men and women who for around nine days were subjected to a "wash-out" period when they used scent-free soap and shampoo and did not eat food with garlic, cumin or curry.
After this, they wore sterile cotton pads under their armpits for a day. These were collected and stored to use as a bottled source of their body odour. A panel of 82 heterosexual and homosexual men and women, not including the donors of the armpit pads, were asked to sniff each bottled body odour and evaluate its pleasantness according to a set of criteria. In a study in the journal Psychological Science, the scientists found: "Heterosexual males and females preferred odours from heterosexual males relative to gay males; gay males preferred odours from other gay males.
"Heterosexual males and females and lesbians over the age of 25 preferred odours from lesbians, relative to the odours from gay males; gay males preferred the odours of other gay males relative to lesbians," they say.
Dr Wysocki said the strongest finding was that gay men prefer the smell of other gay men and that lesbians responded differently to body odour compared to heterosexual women. "The overall conclusions are that the body odour you most prefer or least prefer does not depend on where it comes from but it also depends on who you are, in other words, your sexual orientation," Dr Wysocki said.
Gay men preferred the odors from gay men and heterosexual women, but odors from gay man were the least preferred by heterosexual men and women and by lesbians, he said. Other studies showed that body odor is linked with a set of genes involved in controlling the immune system - called the major histocompatability complex - and heterosexual men and women preferred the odour of those with a different set of these genes to their own. One theory is that this could be an evolutionary mechanism to avoid inbreeding. The latest study suggests the genes involved in body odour may also play a role in sexual orientation and that gay men and lesbians can recognize and identify the odor of others who share their sexual preference. This kind of scent-based gaydar enables gays to pinpoint potential partners instantly.
That result could still be explained by learning and culture - perhaps the brains of the subjects were responding to the way the chemicals reminded them of people with whom that had been intimate. Harder to explain is the Monell finding that gays and lesbians give off different smells, as well.
"There might be some environmental cause, but it's more likely it reflects some biological difference between straight and gay," said Simon LeVay, a neuroscientist and author of The Sexual Brain.
"This helps people understand that being gay or lesbian is a deep-seated aspect of our personhood - it's part of who we are," said LeVay, who is gay.
"This is another brick in the wall that indicates it's not a matter of choice," said Ritch Savin-Williams, a developmental psychologist from Cornell University. Homosexuality appears related to genetics and environmental factors such as exposure to hormones in utero. The key distinction is not nature versus nurture, he said, but choice versus non-choice.
LeVay said some gay people worry that a biological explanation will further "pathologize" homosexuality. "That's a legitimate concern," he said, but he says he believes science will advance gay rights by showing that sexual orientation is like race - a category you're born into and can smell the difference.