Study reveals a new dilemma for women deciding whether to take the contraceptive pill at the start of a relationship

Women on the pill are less attracted to their partners and less sexually satisfied than women not taking the pill, according to a study involving more than 2,500 couples.

But relationships that start when women are using this contraceptive method tend to last longer, the researchers found, and these women are happier with the non-sexual aspects of the partnership, such as care and support for children.

In the study, led by Craig Roberts at the University of Stirling, scientists used questionnaires to gauge the quality of 2,519 women's relationships with men, with whom they had at least one child. Around half of the women were on the pill when they met their partners.

The results showed both positive and negative consequences of the woman using the pill when she met her partner. "Such women may, on average, be less satisfied with the sexual aspects of their relationship, but more so with non-sexual aspects," said Roberts, whose work is published on Wednesday in Proceedings of the Royal Society B.

"Overall, women who met their partner on the pill had longer relationships – by two years on average – and were less likely to separate. So there is both good news and bad news for women who meet [their partner] while on the pill. One effect seems to compensate for the other."

The mechanism for the finding is by no means certain, but Roberts speculated that it could have something to do with a set of genes called the major histocompatibility complex (MHC), a key component of a person's immune system. This set of genes also has an important evolutionary role in guiding people to find mates who are genetically dissimilar, because genetic diversity within their offspring's MHC will lead to an increased chance of surviving novel diseases and environments.

"Women tend to find genetically dissimilar men attractive because resulting babies will more likely be healthy," said Roberts. "It's part of the subconscious 'chemistry' of attraction between men and women."

Previous research has shown that women's mate preferences change over the course of their menstrual cycle as hormone levels fluctuate. During non-fertile stages, they are more attracted to men who appear more caring and reliable and who might be good fathers. At their most fertile phase, they prefer men with more "masculine" faces.

The hormonal levels of women using the pill, however, do not alter much across a month and most closely reflect those typical of the non-fertile phases of the menstrual cycle. "It seems that our preferences are shaped by these hormonal levels, so preferences of women on the pill don't change around ovulation in the way seen in normally cycling women," said Roberts.

Women on the pill may end up finding men with MHC genes similar to theirs more attractive. This may lead to lower sexual satisfaction but will land them with more reliable men who, ultimately, make them more generally satisfied.

Roberts said that his study controlled for factors such as individual and country-specific attitudes to sex and relationships. He said that the results might be something for women to consider when deciding whether to settle down with a partner.

"If there are hormonal influences that affect that choice in a less-than-ideal way then we need to pay attention to it." He suggessted that if a woman was worried about this, then it might be worth trying coming off the pill and using a non-hormonal form of contraception for a few months. "If that doesn't induce any kind of change in the satisfaction with her partner, then that's the best you can do."

He added: "In my view, choosing a partner is one of the most important decisions we make in our lives. If it works we stick with them for the rest of our lives and we have children with them and we marry our economic resources together. It's a pretty big step so you want to get it right."