File this one under "most unexpected side effect": birth-control pills both lower a woman's attractiveness and inhibit her ability to choose a good mate. That's the claim put forward by a study in this month's Trends in Ecology and Evolution. The review examines the surprisingly large body of research previously conducted on the relationship between birth control and female attractiveness. Taken as a whole, the studies suggest "oral contraceptives could interfere...with the ability to attract the preferred man."
Why, exactly, would the pill stand in the way of a good date? Dr. Alexandra Alvergne and her colleague Dr. Virpi Lumma reviewed a decade of research on the behavioral side effects of hormonal contraceptives to figure out the answer. They found that, when the pill inhibits ovulation, it also eliminates a monthly period when a woman's attractiveness rises. The theory goes like this: Over the course of a menstrual cycle, hormonal fluctuation slightly alters woman's facial appearance, her vocal pitch, even body odor. And during ovulation, those changes increase a woman's attractiveness because they indicate fertility. While such cues are admittedly subtle, they do get noticed: a study investigating tip earnings by lap dancers showed that earnings varied across menstrual cycles, with ovulating women as the highest earners. Less subtle cues may also be at play, since studies have found women tend to dress more provocatively during ovulation.
Pill users also did worse at selecting compatible mates, at least in genetic terms. There's a basic tenant in biology that genetically dissimilar partners produce the most robust offspring. Subtle cues—things like facial symmetry and odor—tip us off to degrees of genetic compatibility, ideally attracting us to a genetically dissimilar partner. Turns out, pill users and other non-ovulating women don't do so well at reading those signs.
One study, conducted at the University of Liverpool last year, found pill users more likely to prefer the odor of genetically incompatible men. Women who were more fertile, on the other hand, made better mate choices. In evolutionary terms, that makes sense: we need our wits about us if childbearing is a strong possibility. "Taking the pill [might] make you choose a partner you would not have otherwise chosen," says Alvergne. "It affects your natural preferences. And this could have consequences for your relationship."
The review study paints a pretty bleak picture for women on the pill (which, incidentally, is already having a rough month): both less attractive and less able to choose a decent boyfriend? As a dating woman on the pill, I can admit to a momentary panic: Is my birth control sabotaging my dating life? How much less attractive, exactly, are we talking? Even Alvergne was surprised by the strong relationships that her review found. "I knew about the stuff about cyclicity but was surprised that it affected men as well," she admits.
To be sure, a lot of questions remain unanswered. No studies have investigated the long-term relationship between the pill and mate choice, whether pill users tend to have worse relationships or, even further out, have less healthy children. And Alvergne is careful to caution against women reading her study as a birth-control critique. "The pill has been fantastic for women in terms of social mobility, choosing whether they want to reproduce," she says. "That's really good; we just don't want to be blind. We want to know of the possible side effects, so then women can make the choice."
Perhaps the most interesting part of the study is the authors' rethinking of side effects, looking beyond immediate physical ailments to more distant, less tangible consequences. "We're better off knowing that smokers are more at risk with birth control," says Alvergne. Likewise, she argues, "we should know about the behavioral and psychological impacts."
But should we? I can see it both ways: both the benefit of increased knowledge and the drawback of deterring women from a reliable form of birth control. Personally, I don't plan on ditching the pill any time soon—even if it does mean a few Friday nights alone.