Birth control pills affect sexiness of man

Birth control pills may affect how appealing a woman finds a man's scent -- potentially steering her toward a mate who is genetically similar to her, according to British researchers.

The sense of smell is thought to be important to mate-seeking animals and humans. Genes of the major histocompatability complex (MHC) play a role in a person's odor, and people tend to be attracted to those with an MHC makeup that is dissimilar to their own.

This could have evolutionary significance, since genetic diversity in a couple increases the chances of having healthy children.

But in the new study, researchers found that after women began using birth control pills, their smell preferences tended to shift -- making them more likely to find the scent of a genetically similar man "sexy."

The findings appear in the journal Proceedings of the Royal Society B: Biological Sciences.

For the study, researchers led by Dr. S. Craig Roberts, of the University of Liverpool, recruited 110 women between the ages of 18 and 35. They had each volunteer rate the odors of six men, using t-shirts that the men had worn overnight.

The women were asked to rate odor "pleasantness" -- considered a correlate of "sexiness" -- and odor intensity. Odor "desirability" was tested with the question, "Based on this smell, how much would you like this man as a long-term partner?"

For each woman, the investigators pre-selected three MHC-similar and three MHC-dissimilar men.

At the time of the first sniffing session, none of the women was using oral contraception. A second session took place three months later, after 40 women had started using birth control pills.

The researchers found that, in general, the more pleasant a woman found a man's odor, the more desirable he was to her. But among women who began using birth control pills, the definition of a sexy odor changed over time.

"The results showed that the preferences of women who began using the contraceptive pill shifted towards men with genetically similar odors," Roberts said in a written statement.

If the sense of smell is in fact a strong player in humans' mate choices, the researchers write, birth control pills just might nudge a woman toward a less-than-ideal man.

"Not only could MHC-similarity in couples lead to fertility problems," Roberts said, "but it could ultimately lead to the breakdown of relationships when women stop using the contraceptive pill, as odor perception plays a significant role in maintaining attraction to partners."

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