Your birth control pill is designed to prevent pregnancy, but it may also influence who you are attracted to, according to a small new study that found women on the pill preferred men with less masculine faces.


In the two-part investigation, published recently online in the journal Psychoneuroendocrinology, 18 heterosexual college-aged women in the U.K. were shown composite images of men's and women's faces. They were told to adjust the images until they'd zeroed in on the face they'd be most attracted to for a short-term or long-term relationship.


The women were tested twice: Once when they were not on the pill, and three months later when all of them were. The study also included a larger control group of women who never took the pill.


The results revealed that the women were less partial to masculine facial features when they were on the birth control pill than when they were not on it. Masculine facial features included larger jaws, more prominent brow ridges and more angular features. However, being on the pill did not influence the women's preferences when they were looking at photos of other women, suggesting that the pill somehow affected what they found attractive in potential mates, but not in faces in general.


In a second experiment, the researchers recruited 170 couples. In half of them, the woman was on birth control when the couple met. The researchers showed photographs of the men to additional groups of volunteers -- both men and women -- who rated how masculine the men's faces were. In keeping with the findings of the first experiment, the women who were on the pill when they met their partners tended to be in a relationship with men whose faces were less manly.


So how exactly does the birth control pill sway women's preferences toward less masculine faces?


"I think it is still an open question about the actual mechanism," said study researcher Anthony Little, a research fellow in the school of psychology at Scotland's University of Stirling. "The pill changes the hormonal profile of women, and this hormonal profile influences bodily processes concerned with conception and reproduction." Hormones also appear to affect psychological processes, such as determining preferences, though the process behind this is not well understood, he added.


But the current study seems to raise as many questions as it answers. The researchers did not look at the type or dose of birth control that women took, and focused only on heterosexual women. Still, Little said the findings suggest there are potential ramifications to taking birth control pills that are not fully understood.


"Our effects demonstrate an effect of pill use on both preferences and partner choice," he said. "This may have implications for relationship stability because women may switch between using or not-using the pill during the course of a relationship."


A handful of prior studies have also suggested that birth control affects women's partner choices. A 2012 study (also led by a University of Stirling researcher) found that women who took birth control pills when they met their partner were later less attracted to them and less sexually satisfied than non-pill users, but were happier with their partners in other areas, such as their finances.


A 2009 journal review of a small group of studies looking at the issue concluded that the pill can significantly impact women's mating choices, such as making them less likely to pick partners who are different from them genetically. But some experts disagreed with the findings: In a letter to the journal's editor, two independent researchers wrote that while "pill-induced behavior effects" warrant further investigation, the review was "based more on speculation than actual scientific facts."


The Centers for Disease Control estimates that 82 percent of 15- to 44-year-old women in the U.S. have taken the birth control pill during their lifetimes.