To millions of women it has been the great liberator over the past four decades, allowing them the freedom to control their fertility and their relationships. But the contraceptive Pill could also be responsible for skewing their hormones and attracting them to the “wrong” partner.
A study by British scientists suggests that taking the Pill can change a woman’s taste in men—to those who are genetically less compatible.
The research found that the Pill can alter the type of male scent that women find most attractive, which may in turn affect the kind of men they choose as partners. It suggests that the popular form of contraception—used by a quarter of British women aged between 16 and 50—could have implications for fertility and relationship breakdowns.
The findings, from a team at the University of Liverpool, add to growing evidence that the hormones in the Pill influence the way that women assess male sexual attractiveness.
The Pill is thought to disrupt an instinctive mechanism that brings together people with complementary genes and immune systems. Such a couple, by passing on a wide-ranging set of immune system genes, increase their chances of having a healthy child that is not vulnerable to infection.
Couples with different genes are also less likely to experience fertility problems or miscarriages. Experts believe that women are naturally attracted to men with immune system genes different to their own because of their smell.
Commenting on the latest study, the researchers said that it could indicate that the Pill disrupts women’s ability to judge the genetic compatibility of men by means of their smell.
They said that this might not only impact on fertility and miscarriage risk, but could even contribute to the end of relationships as women who stop or start taking the Pill no longer find their boyfriend or husband so attractive.
Several previous studies have suggested that women tend to prefer the smell of men who are different from them in a cluster of genes called the major histocompatibility complex (MHC), which governs the immune system. Some of these studies have also found that this effect is not seen among Pill users.
The latest study, published in the journal Proceedings of the Royal Society, has now assessed the impact of Pill use in the same women, both before and after they began using oral contraception. A group of 97 women was tested, some of whom started taking the Pill during the course of the research. All had their MHC genes tested and were asked to sniff T-shirts worn in bed by men with different patterns of MHC genes.
Unlike some previous studies, the research did not find any preference for dissimilar MHC genes. However, when the women started taking the Pill their preferences shifted towards the scent of men with more similar genes to their own.
This suggests that Pill use has an effect on perceptions of scent attractiveness, even if there is no underlying female preference for similar or dissimilar MHC genes.
Craig Roberts, who led the study, said: “The results showed that the preferences of women who began using the Pill shifted towards men with genetically similar odours. 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 odour perception plays a significant role in maintaining attraction to partners.”
The research also found differences between women in relationships, who tended to prefer odours of men with different MHC genes, and single women, who tended to prefer the smell of MHC-similar men.
This could potentially indicate that if women are tempted to have an affair, they are more likely to choose a man with very different genes, to maximise the diversity of any offspring that they might have.
The scientists said that more work was needed to explain the way various studies have obtained different results on whether women naturally prefer men with different or similar MHC genes. They also cautioned that the importance of scent in human mating preferences remains uncertain.
The research backs up an earlier study of how women’s perceptions of partners can alter when taking the Pill. Psychologists from St Andrews and Stirling universities found that women on the Pill tend to prefer macho types with strong jaw lines and prominent cheekbones.
However, women who are not taking that form of contraception seem to be more likely to go for more sensitive types of men without traditionally masculine features.