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Mood changer? Birth control pills may affect how women perceive the smells of men.

Nancy R. Cohen/Photodisc

Sniffing for Mr. Right

Weight gain and moodiness top the list of the unpleasant side effects of birth control pills. But could the pill also desensitize a woman's sniffer? New research suggests that oral contraceptives can reduce a woman's ability to smell the best mate. Although birth control can't be blamed for every bad relationship, the findings could help explain how people find their ideal love.

Most guys splash on a little cologne before a first date, but past research shows that their natural scent may be the better attractant. Natural odor reflects a person's immune system composition, a collection of genes called the major histocompatibility complex (MHC). In a 1995 study, women sniffed men's shirts and rated the shirts of men with dissimilar MHCs as more appealing than clothes worn by men who had similar MHCs to their own. Having a different MHC--and thus a different genetic profile--from your mate ensures hardier children, the thinking goes, as your offspring inherit the ability to resist a wider range of diseases. The study did find an exception to this rule, however: Women on birth control tended to prefer the shirts of men with MHC profiles similar to their own.

To further explore the effect of birth control, Craig Roberts, a biologist at the University of Liverpool in the U.K., and colleagues repeated the T-shirt test with 37 women. Before the women began taking oral contraceptives, they showed no preference between shirts--a finding that conflicted with the 1995 study, the team reports today in the Proceedings of the Royal Society B Biology. After women started taking the pill, however, they marked shirts from men with similar MHCs as more desirable than shirts from men with different MHCs. A control group of women who did not go on birth control showed no significant MHC preference in two separate tests. The researchers also interviewed the women about their relationship status and found that, regardless of their use of birth control pills, women in relationships preferred shirts from men with dissimilar MHCs more than did single women.

Overall, the findings show that oral contraceptives can influence a woman's MHC preference, says Roberts. But because the control women showed no MHC preference, it's not clear that the hormones in birth control pills directly affect a women's sense of smell. "It could be correlated with how much they like men generally," Roberts says, explaining that the birth control users gave the shirts higher ratings than did the controls in both tests.

Steven Gangestad, a psychologist at the University of New Mexico in Albuquerque, calls the data "intriguing." More research is needed to untangle these results, he says, but they are worthy of future study.