Popping a birth-control pill could significantly impact your long-term relationship choices, new research suggests.

Comparing the relationships of women who were on the pill when they met their partner with those who weren't, researchers found that pill users were less sexually satisfied, but happier overall with their relationships than women not taking birth control when they first met their mate.

"These results are in line with what we and other groups have seen in the lab," study researcher Lisa DeBruine, of the University of Aberdeen, told LiveScience in an email. "Although they're less sexually satisfied, they are MORE satisfied with nonsexual aspects of their relationship, including a partner’s financial provision and support." [6 (Other) Great Things Sex Can Do For You]

Polling the pill

The study, performed by lead author Craig Roberts, of the University of Stirling in the U.K., polled about 2,000 women from all over the world, including the U.S., U.K. and the Czech Republic. The average age of the women was around 37, and to make sure the levels of relationship commitment were comparable, they were asked about their relationships with the fathers of their first children.

The women filled out surveys about their relationships and sex lives. Oddly, these couples who met while the woman was on hormonal birth control were also more emotionally satisfied and were more likely to stay together, on average about two years longer. If those couples did separate, the break-up was more likely to be initiated by the woman.

While the nonsexual parts of their relationship were satisfying, these women showed increasing sexual dissatisfaction during their relationships, while there was no change in nonusers. This increasing sexual dissatisfaction could lead to a tipping point between emotional and financial satisfaction and a desire to be sexually satisfied, the researchers suggest.

Pill use and attraction

Though the biology behind this phenomenon is unknown, DeBruine believes it's most likely related to the hormones found in the pill used to control fertility. "The biological mechanisms are almost certainly linked to hormones," she said. "Hormones influence our bodies and behavior in many ways, so we're still researching exactly how hormones affect mating behavior."

These hormones influence what characteristics women are attracted to in a mate. Studies have shown that a main component of attraction is a desire to find a genetically dissimilar partner,specifically in genes called the major histocompatibility complex (or MHC) that are involved in the body’s immune system. Having a large selection in these genes improves the immune system and can make for healthier offspring in the long run.

When women are on the pill, however, they are in an "eternally pregnant state," meaning they aren’t ovulating and may be wired to instead to seek out genetically similar men — essentially, the genetic equivalent of a relative — because evolutionarily, family would help raise the baby. This desire for genetically similar men was found in a study published in the journal Proceedings of the Royal Society B: Biological Sciences. That same 2008 study found that women not taking hormonal birth control prefer genetically dissimilar men and those with higher testosterone levels and more masculine features.

And supporting the lower sexual satisfaction in birth-control couples, previous research has indicated that females with genetically similar partners express lower sexual satisfaction and have a higher interest in having sex with someone who isn't their partner; essentially, they think about cheating.

Though the new study didn't test genetic similarity, they found that women who had been on a progesterone birth-control pill when their relationship started expressed sexual dissatisfaction and desire to cheat.

Complex relationships

Jonathan Schaffir, a doctor at Ohio State University Medical Center who has studied the link between hormonal contraception and sexual desire but who wasn't involved in this study, suggested that trying to tease out the effect of hormonal birth control on something as complex as mate choice can be misleading.

"It's not likely that a small change in a single biological parameter will make a big difference," noting that the differences the study found, while statistically significant, were small. He said a forward-looking study, or an investigation of possible biological effects of the hormone treatment would have been more convincing. [The History and Future of Birth Control]

"This type of retrospective study cannot fully explain all of the complex processes that shape relationships," DeBruine told LiveScience. "Our findings do suggest that these processes play out slightly differently in women who were and were not using the pill when they met their partner."

Schaffir worries about how women might interpret these findings. "Sometimes people give good forms of birth control a bad name by suggesting that they might have [these kinds of] effects," Schaffir told LiveScience. "I don't want women to have the impression that they shouldn’t use reliable birth control because of dubious psychosocial effects."

The study was published today (Oct. 11) in the journal Proceedings of the Royal Society B: Biological Sciences.

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