If you believe recent studies, both may be pushing biological buttons that evolved to shape women’s behavior towards men. In this entry, Part I of a two-part blog, we’ll look at how oral contraceptive pills may be changing the way we look at each other.

Here’s some background: several studies have found that women have subtle shifts in what they find attractive in men’s faces across their menstrual cycles. Given a choice of potential mates, women tend to choose a more rugged, masculine face around the time they’re ovulating (the fertile phase of their cycle) and a more feminized face when they’re no longer likely to become pregnant (after ovulating). In particular, women in the fertile phase of their cycle prefer more masculine faces when they’re asked to pick a partner for a short-term relationship—call it the “quickie effect.”

Why? Evolutionary psychologists have claimed that it's a residue of our ancestral past. A more masculine face is a sign of higher testosterone levels, which can be one marker of having “good genes”. Healthy men who are loaded with testosterone may be stronger, more dominant, and have better immune systems (if they’re healthy, they must be more resistant to the immune-suppressing effects of testosterone). Less masculine features (presumably a sign of lower testosterone) suggest a more cooperative (better parent) kind of guy. Women are more likely to cheat on their long-term partners during the fertile phase of their cycles. So, the theory goes, since a man can never be sure that a child is his, an ancestral woman might have been trading up for “good genes” by hooking up with the hunky guy just before she ovulates while holding on to Mr. Mom for the long-term. It’s the best of both worlds—she gets genetic benefits from her lover and material benefits from her partner.

That’s one explanation for why women seem to shift their views of who’s hot and who’s not as their hormones cycle each month. But the effect may not only be in the eye of the beholder—there’s some evidence that women’s own sex appeal shifts across the menstrual cycle. This is controversial stuff, but as I describe in my recent book, some studies report that women are more flirtatious and more likely to fantasize about sex with men other than their partner at the peak of their fertile phase. Their faces, voices, and odor change in subtle ways that make them more attractive to men.

In a unique test of that hypothesis, researchers studied lap dancers at strip clubs in New Mexico and measured how much they made in tips for a two-month period. The women made an astonishing $20/hour more in tips around the time they were ovulating compared to the last 10 days of their menstrual cycle.

But here’s the kicker: women taking oral contraceptive pills didn’t get that earnings boost. In fact, “the Pill” seems to dampen all of the menstrual cycle changes in women’s sexual behavior I mentioned above (including their shifting preference for more or less masculine men). The Pill works by suppressing estrogen and keeping progesterone levels high enough to block ovulation. The usual cycling of these hormones is flattened out. And the result may be that women’s sexual behaviors and desires get flattened out along with them.

More than 80% of women have used oral contraceptive pills. If the Pill can subvert women’s evolved sexual attraction mechanisms, could it be affecting our relationships and mate choices? To answer that question, researchers in the UK surveyed more than 2500 women about their relationships with the biological father of their first child. Women who met their male partners while taking the Pill reported being less sexually attracted to and aroused by their mates compared to those who hadn’t been on the Pill. On the other hand, the Pill-taking women were more satisfied with non-sexual aspects of the relationship including how good the men were as financial providers. And, overall, women who had been using the Pill were more likely to stay with their partners over the long-term. In other words, their relationships were less sexually satisfying but more durable.

So the Pill may be muting women’s typical surge of sexuality that occurs around the time they ovulate. But other recent studies paint a more complicated picture.

Women report greater sexual jealousy during the fertile phase of their menstrual cycle (that is, around the time they are ovulating and when estrogen levels are peaking). So we might expect that women on the Pill (which blocks the normal surge in estrogen) would report less sexual jealousy. But researchers from the Netherlands recently reported that women in relationships who were on the Pill were just as sexually jealous about their partners as women in the fertile phase of their cycles who were not taking the Pill. And the effect was correlated with how much estrogen was in their contraceptive pills. The more estrogen, the more sexually jealousy women. And another new study found that women on the Pill engaged in more “mate retention behaviors” with their partners—things like checking up on their partner when he wasn’t around, monopolizing his time, using sexual favors to keep him around, and getting angry if he seemed to be flirting. Once again, estrogen levels in the oral contraceptive pills correlated with how much the women said they used these tactics. So it may be that the Pill changes women’s relationship behavior in two ways: by flattening hormone cycles and by a direct effect of estrogen.

But there are some puzzling things about these studies: why would the Pill make women less sexually adventurous but also more sexually jealous? It’s not at all clear. Still, it does seem that 21st century hormones may be fooling with our relationships.

Our brains were shaped by our evolutionary past to have mental mechanisms for solving life’s key challenges—like choosing a mate. The Pill’s effect on our sexual behavior is just one example of how our modern world may be tweaking our brains’ ancestral software. In Part II, we’ll see how the Recession may be good news for companies that promise to make women more sexually attractive. Stay tuned.

 1. Roberts SC, Klapilova K, Little AC, et al. Relationship satisfaction and outcome in women who meet their partner while using oral contraception. Proceedings Biological sciences / The Royal Society 2011.

2. Welling LL, Puts DA, Roberts SC, Little AC, Burriss RP. Hormonal contraceptive use and mate retention behavior in women and their male partners. Hormones and behavior 2012;61:114-20.

3. Cobey K, Buunk A, Roberts S, et al. Reported jealousy differs as function of menstrual cycle stage and contraceptive pill se: a within-subjects investigation. Evolution and Human Behavior 2012;33:395-401.

4. Alvergne A, Lummaa V. Does the contraceptive pill alter mate choice in humans? Trends Ecol Evol 2009;25:171-9.

5. Miller GA, Tybur J, Jordan B. Ovulatory cycle effects on tip earnings by lap dancers: economic evidence for human estrus? Evolution and Human Behavior 2007;28:375-81.

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