The XX Factor

Study Finds That Women Aren’t Run by Their Periods. Scientists Everywhere Are Confused.

It’s not because she’s on her period. It’s because you’re looking at your iPad.

Photo by Peter Bernik/Shutterstock

What do women want? Over the past two decades, scientists have endeavored to answer this question by bringing women into their labs, asking about their sexual preferences, and then monitoring their menstrual cycles to try to extract clues from the ebb and flow of hormones in their mysterious female bodies. In recent years, these researchers have told us that the status of our monthly cycle on Election Day can influence our decision to favor Mitt Romney’s chiseled individualism or Barack Obama’s maternal health care policies, that our periods determine whether we feel like nesting with our partners tonight or heading out to proposition a stranger, and that our cycle urges us to swing with Tarzan at our most fertile and cuddle up with Clay Aiken when that month’s egg is out of the picture. Last month, psychologists at the University of Southern California published a meta-analysis of 58 research experiments that tested whether a woman’s preferences for masculinity, dominance, symmetry, health, kindness, and testosterone levels in her male romantic partners actually fluctuate across her menstrual cycle. The answer: They do not.

The analysis, published in the appropriately titled journal Emotion Review, looked at studies that used a variety of sociological tools to examine women’s preferences for a host of masculine cues, such as a man’s gait, body hair, chin length, facial symmetry, or social interactivity, all through the prism of their menstrual cycles. They looked at studies that were focused on testing women’s preferences in short-term relationships (like one-night stands) and long-term commitments (like marriages), and at studies that didn’t specify a relationship type at all. They included experiments that charted a woman’s menstrual cycle and fertility using hormonal tests and self-reports, ones that included women on hormonal contraception, and those that did not. All in all, they found that both fertile and nonfertile women preferred men who were more masculine, dominant, symmetric, and healthy; that those preferences remained relatively constant across their menstrual cycles; and that they applied to women’s feelings about both short-term and long-term relationships. Meanwhile, women who were at the nonfertile stage of their cycles—where they experience similar hormones to pregnant women—didn’t suddenly prefer kinder, gentler men.

So why do we keep hearing of how our menstrual cycles rule our love lives, year after year? The researchers found that cycle shifts in women’s preferences were more pronounced in studies that relied on an extremely long estimation of a woman’s fertility window (up to 12 days) when women are actually highly unlikely to become pregnant outside of a six-day period, meaning that those studies were more likely to introduce errors into their findings. Earlier studies in the sample—those conducted between 1998 and 2003—were more likely to report statistically significant findings, but more recent studies failed to replicate those initial conclusions. And experiments that did not go on to be published were more likely to produce nonsignificant findings—meaning that scientific journals may favor studies that appear to confirm the link between menstrual cycles and mating preferences, whether or not they’re representative of the wider literature on the subject.

The researchers suspect that the drive to chart women’s choices on their fertility calendars reflects our desire to understand human behavior via rudimentary evolutionary explanations: “More modern evolutionary approaches,” they write, “recognize that social learning and innovation are central human adaptations that are enabled by biological processes” and that ”the evolution of the human brain did not stop with these ancient sensory, perceptual, and motivational systems.” For one thing, women just don’t menstruate like they used to—while our ancestors spent the bulk of their adulthoods either pregnant or lactating, modern women in industrialized societies menstruate regularly throughout their lives, taking just a couple short breaks to have some kids. The rise of birth control has also radically disrupted any evolutionary influences. Other studies have found that women desire greater masculinity in their partners if they live in economies with low GDPs, “in which men’s work may involve manual labor jobs and male brawn,” while women in wealthier countries that “rely more on knowledge workers” are freer to prefer “better-looking men.”

In other words, a woman’s cultural conditioning is even more powerful than progesterone. Women’s endocrine processes have officially taken a back seat to our own mental and physical capacities to regulate our preferences and our cycles to better contribute to our societies. But that doesn’t mean that evolutionary scientists will soon evolve beyond this menstruation explanation. The study’s authors are careful to note that while the existing body of literature doesn’t demonstrate any connection between menstrual cycles and mating choices, more refined experiments could always establish a link.