Do Birth Control Pills Influence Sexual Desire?

A study shows progesterone is associated with more sex — in committed relationships

Photo Illustration: R. A. Di Ieso
Dec 09, 2016 at 4:47 PM ET

Why do women choose to have sex even when they won’t get pregnant? It’s a question scientists have long been asking, much to the bemusement of any woman who has ever had an orgasm or taken birth control pills. Now, researchers behind a new study say they are a step closer to solving the mystery.

The reason researchers regard this as such a riddle is because it’s hard to explain in evolutionary terms. Most female mammals only show interest in sex during estrus, when it is possible for them to get pregnant, while female humans have sex all the damn time. In fact, at least for women in committed relationships, there’s no difference in how often they have sex over their course of their cycle, according to research. They are just as likely to have sex when they’re ovulating as when they are not.

To dig into this question, researchers from the Norwegian University of Science and Technology and the University of New Mexico looked at how women’s birth control use interacted with their sex lives. That’s because different types of contraception can hormonally mimic different phases in a woman’s cycle. Typically, for a woman not on hormonal birth control, estrogen peaks before ovulation, when a woman can conceive. Progesterone peaks when a woman is not ovulating. Hormonal contraceptives typically contain synthetic progesterone, called progestin, and/or estrogen.

As a press release puts it, “some contraceptives mimic hormones that are more characteristic of ovulation, whereas others mimic hormones when women can’t conceive.”

The researchers surveyed nearly 400 women, all of whom were in committed relationships and using birth control, about their sex lives. They also had them report the degrees to which they were committed to their relationship. Overall, women who were more committed to their relationships had more sex. But this correlation was even stronger when the women were on contraception that had more progestin, which is associated with a woman’s non-fertile phase. This falls in line with the theory that sex during the non-ovulatory phase is all about fostering commitment — or, in scientist terms, “pair bonding.”

Women who used contraception with more estrogen, which is associated with ovulation, were most sexually active when they were in a less committed relationship. The researchers surmise that this might be because “low-commitment relationships may be more novel, and novelty may contribute to ‘estrus-like’ sexual interests.”

This all jibes with a previous study of women who were not taking birth control. Researchers found that in the non-fertile phase, when progesterone levels peak, women’s initiation of sex was positively associated with their degree of interest in the relationship. But that wasn’t the case during the ovulatory phase, when estrogen peaks. It’s also consistent with a study finding that a woman’s level of progesterone predicted her interest in her primary partner, while estrogen levels did the opposite.

As co-author Leif Edward Ottesen Kennair explained, “The function of sex in humans outside ovulation is an evolutionary mystery. But we believe that it has to do with binding the parties in the relationship together.”