Taking the pill doesn't just prevent pregnancy—hormonal sex.
Researchers at the Norwegian University of Science and Technology (NTNU) and the University of New Mexico surveyed 387 women who used hormonal birth control and were in heterosexual relationships. Some were followed for a 12-week period and asked how often they had sex, while others answered a one-time questionnaire about how many times they’d had sex in the past week. They were also asked how committed they were to their relationships, and what types of birth control they were using.
Women who felt the most committed to their relationships had the most sex when they were on birth control with higher levels of progestin and lower levels of a form of estrogen called estradiol. On the other hand, women who felt less loyal to their partners had the most sex when they took birth control with lower levels of progestin and higher levels of estrogen.
The hormonal differences only influenced frequency of intercourse—and not oral sex, , or sexual fantasies.
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These findings, published in Evolution & Human Behavior, help support the theory that couples are biologically driven to have sex outside of ovulation because it helps bring them closer together emotionally. (The fact that women desire sex even when they're not ovulating, unlike most animals, is an "evolutionary mystery," says Leif Edward Ottesen Kennair, PhD, professor of psychology at NTNU.)
The study results also support the idea that hormones—natural or synthetic—can affect that biological drive to get it on.
Previous research suggests women who aren’t as invested in their relationships may not have as strong a biological drive to have sex for non-reproductive reasons, such as bonding, compared to someone who is deeply committed to their partner.
These findings might seem to imply that switching birth control could help couples have more sex—but the authors say they can’t give that type of advice just yet. The study was only able to show associations, not cause and effect, says Kennair. Plus, he told Health.com, “It is not merely the effect of hormones, but an interaction between hormones and greater levels of investment in the relationship.”
If anything, says Kennair, the study suggests that women who are strongly loyal to their partners have more sex during non-fertile times “in order to increase their partner's attachment/commitment,” even if they’re not be aware of these subconscious motivations.