London, England, Aug 15, 2008 / 01:00 am (CNA).- A British study of the contraceptive pill suggests the drug can change the kind of men to whom a woman is drawn by unconsciously making genetically incompatible men more attractive to her. The findings suggest factors caused by the Pill could be behind some fertility problems, miscarriages, and relationship breakdowns.

A research team at the University of Liverpool found that the Pill can alter the type of male scent that women find most attractive, possibly affecting the kind of men they choose as mates, the Times reports. The drug is believed to disrupt an instinctive mechanism that helps bring together people with complementary genes and immune systems.

Couples with a wide range of immune system genes have increased chances of having a healthy child who is more resistant to infection.

Couples with different genes are also reportedly less likely to experience fertility problems or miscarriages.

Researchers said if the Pill in fact disrupts women’s attraction to men based on their smell, women who start or stop taking the Pill could no longer find their boyfriend or husband as attractive. This could contribute to the end of such relationships.

Several previous studies have suggested that women tend to prefer the smell of men who are genetically different from them in a cluster of genes called the major histocompatibility complex (MHC), which governs the immune system. Some of these studies also claim this effect is not found among Pill users.

In the latest study, published in the journal, Proceedings of the Royal Society, a group of 97 women was tested, some of whom began taking the Pill during the course of their research. Having had their MHC genes tested, the women were asked to sniff T-shirts worn in bed by men with different patterns of MHC genes.

While the study did not find any preference for dissimilar MHC genes, the women who started taking the Pill had their preferences shift towards the scent of men with genes more similar to their own.

This finding suggests the use of the Pill has an effect on women’s perceptions of scent attractiveness.

“The results showed that the preferences of women who began using the Pill shifted towards men with genetically similar odors,” study leader Craig Roberts said, according to the Times. “Not only could MHC-similarity in couples lead to fertility problems, but it could ultimately lead to the breakdown of relationships when women stop using the Pill, as odor perception plays a significant role in maintaining attraction to partners.”

The Pill is the most popular form of contraceptive in England, being used by 25 percent of women ages 16 to 50.

The researchers said more work is needed to explain the differing results of studies examining the relationship of women’s mate preferences to MHC genetic similarity. They further cautioned that the importance of scent in human mating preferences is still uncertain.

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