Science

The Wax and Wane of Ovulating-Woman Science

Evolutionary psychology has long been mocked for its weird obsession with the menstrual cycle, but at least a sliver of that work has held up to scrutiny.

A woman holding an analog clock in front of her lower abdomen.
Photo illustration by Slate. Photo by nensuria/iStock/Getty Images Plus.

Two decades ago, in late summer 1998, the journal Nature came out with an outrageous claim: Both women and men, a research paper argued, prefer faces with more “girlish” features. The authors of the study, based in Scotland and Japan, had expected the opposite result—that square-jawed, hunky faces, more Harrison Ford than Leonardo DiCaprio, would be deemed more attractive. “Our team has been working on this study for four years,” one of the scientists, Ian Penton-Voak, told the New York Times in advance of publication. “When it was found early on that there was a preference for feminized male faces, nobody believed it, so we did it again, and again. The preference for a feminized face keeps coming up.”

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Could it really be the case that everyone prefers a man with a gentle nose and a low-T brow? If so, then why are (or were) Harrison Ford and Leonardo DiCaprio both considered highly sexy? And what about the other sexy ’90s dyads of George Clooney and Jude Law, and Johnny Depp and Nick Nolte? The Nature data were no less perplexing for evolutionary psychologists like Penton-Voak. From that field’s perspective, manly features are indicative of a male’s reproductive fitness. Given this assumption, one might guess that women have evolved to find those traits the most appealing, since they help identify the sort of men with whom you could make the strongest, most immunocompetent children. What would women get from delicate men?

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A year later, in the summer of ’99, Penton-Voak and colleagues offered the beginnings of an explanation. For a second study, also out in Nature, and also drawn from research done in Scotland and Japan, they once again asked young women to evaluate male faces that had been digitally feminized to varying degrees—only now they had the women do so twice, at different points during their menstrual cycles. They found that a woman’s predilection for men with girlish features waxed and waned throughout the lunar month: When she looked at faces in the days leading up to ovulation, her tastes would tend a bit more masculine; later on she’d flip back the other way.

This led the psychologists to propose a big idea, and one that helped to spawn a vibrant field of research: What if women had evolved to make specific shifts in sexual preference across their cycles? A woman might be drawn to macho men while in her fertile phase, since that’s when their fitter genes would be the most useful. Meanwhile, the same person might have eyes for other sorts of men—those whose “low masculine appearance suggests cooperation in parental care,” as the authors put it—at other times, when long-term bonding would help ensure a safe environment for her family.

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This hypothesis—that, for evolutionary reasons, a woman’s desires and behaviors oscillate throughout her menstrual cycle—would lead to many other odd and interesting results. One early line of research, using sweaty T-shirts as a stimulus, found that fertile women were more attracted to (or less disgusted by) the undergarment odors of men with symmetrical faces. Another found that women’s tastes for manly body types, manly voices, manly competition, and manly height all have a tendency to peak during the days leading up to ovulation. Yet another said that women at mid-cycle are somehow sexier—better-looking, better-smelling—to men.

In 2008, a pair of researchers at the University of New Mexico—evolutionary psychologist Steve Gangestad and biologist Randy Thornhill—reviewed the decade’s worth of research that had followed on those early Nature papers, and they concluded there was reason to believe that women experience a period of “oestrus”—i.e., once per month, human females enter into heat. “Women possess ‘dual sexuality,’ ” Gangestad and Thornhill wrote, “consisting of a phase of oestrus and a phase of extended (non-fertile) sexuality in their cycles.” Later on that year, Gangestad and Thornhill wrote up a book-length disquisition on this theory called The Evolutionary Biology of Human Female Sexuality.

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With this established, the psychological study of ovulating women launched into its own highly fertile phase—and another decade of obsessive focus on the signs and symptoms of “dual sexuality” followed. A host of bewildering, bizarre results made their way into the journals. Fertile women are better at sussing out which men are gay, one study claimed. Their breasts become more symmetrical, said another. They wear skirts instead of pants. They have a better sense of smell. They’re more assertive. They seek out more variety in mini candy bars, but strive to lose more weight. They turn their backs on God, at least when they aren’t married, and they say they’ll vote for Barack Obama.

As these data points sprayed across the pages of academic journals, it got harder and harder for anyone to connect them in a coherent way. The simple picture painted in the field’s early days—that menstrual shifts had women variously pursuing “cads” when fertile and hooking up with “dads” otherwise—was stretched and embellished in order to fit an increasingly disparate set of findings. Evolutionary psychologists guessed that hormonal changes around ovulation would have different effects on women with attractive partners compared with those with ugly partners, or that a single woman’s preferences when fertile would be different from a married woman’s. Some effects of oestrus could even be negated, at least according to one study, when a woman just imagined that she had a handsome boyfriend.

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Eventually, the underlying theory of this field of research grew so encrusted with garish findings, so brittle and baroque that it finally collapsed into a string of nonresults. The most recent came on Aug. 28, when a German researcher named Ruben Arslan and his colleagues published data from a massive survey study of a thousand female subjects, who answered questions about their menstrual status and sexual feelings and behaviors repeatedly across a six-week stretch. The study found no evidence to support the claims—drawn from the smaller, less careful studies of the past—that women put on sexy clothing when they’re ovulating, or that they become particularly attracted to people other than their boyfriends.*

There have been lots of findings of this type in 2018. Julia Jünger, a postdoctoral researcher at the same lab at the University of Göttingen where Arslan started his research, has also posted work this year that picks apart prevailing notions in the field of ovulation research. One of her studies, out in March, found that fertile-phase women have no special preference for images of torsos showing greater upper-body strength. Another showed that ovulating women are not especially attracted to flirtatious men, and a third suggests they aren’t more attuned to men with deeper and more manly voices.

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Lastly, at the University of Glasgow, Benedict Jones has just finished up a large-scale study of these issues that he started half a dozen years ago, with a European research grant of 1 million pounds. Jones has brought hundreds of women into his lab for repeated visits, tracking their hormonal changes over time along with their behaviors on a broad array of tasks. In two papers published this year, he and colleagues showed that a woman’s hormonal status doesn’t change her preference for facial masculinity, nor does it make her more desirous of short-term, uncommitted trysts.

The implications of this newer research have not escaped the founding fathers of the field. “In terms of overall effects, I don’t think there is anything,” admitted Gangestad in a recent interview, referring to his theory of women’s so-called dual sexuality. There may still be some difference in fertile-phase effects for women who are single and those with long-term partners, he said, “but the probability I would put on that is pretty low.” In other words, the man who literally co-wrote the book on human oestrus now believes that he and others in the field were the victims of pervasive problems in the way that the psychology was done—a bum steer that affected not just their research, but many others’ too. “When we wrote the book, we were drawing on a broad literature,” Gangestad told me, “but some of what we wrote was just garbage because we trusted all that work, including our own.”

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This may sound familiar at this point. The open science movement—and the corresponding focus on replicating prior work with larger samples and more rigorous analyses—has burned its way through many bookshelves of results in university libraries and the popular ideas they sustained. In particular, several monumental theories of psychology—involving concepts such as ego depletion, social priming, and stereotype threat—have now been cast in doubt, as certain classic findings get reclassified as false-positive results.

Yet the story of the cads-and-dads research—and the broader rise and fall of ovulational psychology—may stand apart from all the rest. That’s because this year’s incineration of its core results is in no way unexpected. Rather, it’s the coda to a longer process of critique, and one that started several years before the replication crisis went mainstream. These menstrual cycle studies have been ridiculed as sexist pseudoscience and rebutted on their merits for about as long as they’ve been touted in the press.

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Journalists at Slate, for example, took a hacksaw to their methodology as early as 2010—at a time when mainstream publications rarely seemed to hacksaw peer-reviewed research. Though we hadn’t yet been made aware of flexible analyses, “p-hacking,” and the other common means of juicing stats to make them more amenable to scientific publication, keen-eyed skeptics such as Amanda Schaffer and Emily Yoffe poked around the methods sections of these papers and found them full of holes. Sample sizes seemed too small, as a rule, while findings that were inconsistent with the evolutionary theory as we understood it were simply wished away.

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Scientists from other fields launched attacks as well, claiming that participants for this research were almost always drawn from a weird, unrepresentative set of women: undergraduate psychology majors. “In many ways evolutionary psychology resembles religious belief,” wrote the evolutionary geneticist Jerry Coyne on his blog, in agreeing with the Slate barrage, “at least in the fervor of many of its advocates and their tendency to completely ignore data that don’t support their hypothesis.”

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The following year, a stripper by the name of “Bubbles” took aim at what might have been the best-known finding from the field: a 2007 study that suggested exotic dancers make $15 more per hour when they are in the fertile window. After calling out this paper’s paltry sample size—only 18 women over two months, really?—Bubbles shared results from her own longitudinal, n-of-1 analysis spanning her last three years of menstrual cycles. There was no consistent link between her earnings and her ovulation, she said.

By 2013, more sophisticated, statistical critiques of ovulating-woman science were showing up in mainstream media. In Slate, Andrew Gelman debunked a claim that ovulating women have a thing for wearing red or pink. “There is no reason to assume the researchers were doing anything nefarious or trying to manipulate the truth,” he wrote. “Rather, like sculptors, they were chipping away the pieces of the data that did not fit their story, until they ended up with a beautiful and statistically significant structure that confirmed their views.” (Meanwhile, the initial scientific basis for that study—a prior paper claiming that men find women more attractive in the presence of the color red—has also been debunked.)

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Then came a pair of papers in 2014, each of which would try to pool results from at least 50 prior studies, published and unpublished, on whether ovulation changes women’s desire for short-term relationships with manly men. One found no effect; the other said there was a “robust relationship.” It seems like every controversial field of research gets to have a moment just like this one, where disputes break down into a pair of dueling meta-analyses. The reason for this is that the methods used to carry out these omnibus reviews—which studies to include, how to correct for publication bias, and so forth—are themselves subject to a range of vexing scientific controversies. “Although the number of meta-analyses has exploded, many don’t bring clarity,” wrote Jop de Vrieze in an excellent summary of the issue from September.

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This long history of doubts about the ovulating-woman papers, and their rank dismissal by certain members of the press, only makes it more surprising that the coda to this story also has a twist. It’s certainly true that recent replication failures have been sanding many of the sparkles from this research literature: Ovulating women do not really change the way they vote or pray or choose their candy, and being in the fertile phase does not, in fact, lead a woman to wear red or commit adultery with a bearded paramour. It no longer seems the case that women have evolved to seek out flings with manly dudes and then settle down with sweet-faced doting dads.

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Yet certain other findings from this lame, lambasted literature have turned out to be amazingly robust.

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Amidst the data and debris from this year’s grand debunkings, there are some signs of clear effects: It appears to be quite true, for example, that sexual desire has a tendency to fluctuate throughout the menstrual cycle, getting stronger on average as a woman’s hormones shift for ovulation. This may strike readers as an epic, no-duh finding. (“In summary, my mom predicted many of these results quite well,” Arslan wrote on Twitter.) But it’s now been demonstrated in a more convincing, scientific way that provides a path for more important research down the road. If it’s true, as Arslan found, that the use of oral contraceptives flattens out such changes in desire, then we still have much to learn about the nature of that medication’s psychosexual effects. The only randomized, double-blind, placebo-controlled clinical trial on this topic, published in 2016, suggested that the pill might exert a small downward push on women’s sexual desire, pleasure, and arousal. Since this could influence a woman’s choice of medication, it would be nice to know for sure that it’s for real.

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Another major theme of ovulating-woman research also seems to have withstood the closer scrutiny: Women do rate themselves as sexier when they’re in the fertile phase, according to the recent studies—and, perhaps more surprisingly, so do men who look at them in photographs.

It’s not clear yet what these changes mean, or whether they are (or aren’t) adaptive in an evolutionary sense. There’s also little data at the moment on how such effects differ across individuals, as they surely must. Still, the fact that these effects are real suggests there might be something worth pursuing after all. More insight in this field, based on rigorous results, would help us better understand the functions of hormonal shifts and what might happen when we deliberately obstruct them, even if they can’t tell people what to wear to look sexy or how to avoid cheating. But in a way, this recent run of replication failures has served to break apart the ugly, overcomplicated framework from this useful topic of research, and then to brush away the rubble from its scientific fundaments. The findings may not be as notable as we’d been led to think, but at least they’re right in front of us, as building blocks for something new.

Correction, Oct. 25, 2018: The original version of this piece incorrectly stated that the ovulating women in Arslan’s study were not more likely to fantasize about men other than their boyfriends. In fact, they expressed increased sexual attraction for both their current partners and for other men.

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