Conventional wisdom has long held that humans hide all signs of ovulation, even from themselves and their mates. Indeed, numerous scientific studies have been devoted to identifying what the evolutionary advantage might be to disguising fertility.
Yet the study, which publishes Oct. 10 in the online version of the scholarly journal Hormones and Behavior, found that even total strangers could detect a difference in women's grooming habits when they approached ovulation.
"The thing that's so remarkable about this effect is that it's so easily observed," said April Bleske-Rechek, the study's co-author and an assistant professor of psychology at University of Wisconsin-Eau Claire. "In our study, the approach of ovulation had a stronger impact on the way women dressed than the onset of menstruation, which is notorious for its supposedly deleterious impact."
Haselton, Bleske-Rechek and three UCLA students tracked 30 college coeds in committed relationships through an entire ovulatory cycle. Using urine tests that are nearly as accurate for determining ovulation as ultrasounds, they ascertained each woman's most fertile period -- about 10 to 15 days after menstruation -- and their least fertile period -- roughly the two weeks following ovulation. The researchers photographed the women twice: once in their fertile (follicular) phase and another time in their non-fertile (luteal) phase. To ensure that only the women's attire, grooming and accessories were taken into account, researchers masked participants' faces in the photographs with black ovals.
Researchers then arranged the photos in pairs on a kind of scientific version of the Web site "Hot or Not." Forty-two judges -- a little more than half of them women -- were asked, "In what photo is the person trying to look more attractive."
In 60 percent of the cases -- a frequency well beyond random chance -- the judges picked the high-fertility photos.
"Many things affect the clothing that women decide to put on when they leave the house, including whether they have an interview to go to or whether they're going to the library to study for an exam or what they have planned after school," Haselton said. "Just one of them is a somewhat subtle event that changes their biochemistry. And yet this change manifests itself in an observable and pretty dramatic difference in how women dress."
In one pairing, the participant wore loose-fitting jeans and clunky boots in her low-fertility photo and a skirt and cardigan in her high-fertility photo. In another, the participant appears to be wearing the same black yoga pants in both photos. She also is wearing the same sort of shirt in both photos -- a tank top. But in the low fertility photo the top is a basic, white model, while the high fertility model is a pretty color, with a slightly lower-cut neck trimmed in lace, and it's accessorized with a fancier necklace. In another pairing, the participant had donned a fringy scarf for her high-fertility photo.
The undergrads demonstrated little knowledge of the workings of ovulation and certainly weren't tracking their cycle or attempting on any conscious level to get pregnant.
Yet the women not only seemed to have paid more attention to their appearance as they approached their most fertile period, but the more fertile the women were, the more likely their pictures were to be selected.
Interestingly, the approach of menstruation did not seem to have any observable effect on how the women dressed, suggesting that, at least in this study, the onset of ovulation had a greater impact on a woman's dressing habits than so-called PMS (pre-menstrual syndrome).
"There's a popular notion that when women approach menstrual onset, they get out their bloated clothes and they pull out their sweats," Haselton explained. "So if what we were measuring was a PMS effect, you'd expect that if a woman has her photo taken one or two days before menstrual onset, then she's going to dress frumpier than someone who had her photo taken 10 days before menstrual onset. But we didn't find that to be the case."
The research builds on a new body of research showing subtle and surprising shifts in women's behavior each month as they approach their most fertile period, including a propensity to flirt with men other than their mates and an inclination to stray from their routine in ways that are suggestive of mate-shopping. Meanwhile, the findings conflict with conventional wisdom among social scientists, who have long maintained that humans are rare among primates in showing no outward signs of entering fertile phases. Our closest living relative, the Chimpanzee, famously displays swellings of the genital area when fertile.
"Something in women's minds is tracking the ovulation cycle," Haselton said. "At some level, women 'know' when they are most fertile. And we have seen some evidence that men may at some level 'know' too � although with less certainty."
Haselton and Bleske-Rechek, however, stop short of ascribing the changes they have detected to the kinds of displays of fertility that are common in the animal kingdom.
The changes may well be the byproduct of the "stew of changes" to which, mounting evidence suggests, women are subject as they approach the most fertile point of every cycle. In addition to a desire to cheat on long-term mates who are less than virile, researchers have found some evidence that women's facial features may be more attractive and they may even feel sexier as they approach ovulation, which comes midway through every cycle.
"Women may decide, 'Hey, I'm looking good!' And this affects the items they pick to wear." Haselton said.
Whatever the woman's motivation, the behavior may explain a change that has recently been quantified in men's behavior around ovulation. Men appear to respond with increase mate- guarding, Haselton has found.
"When women are in their high fertility phase, their partners are more attentive and loving toward them," Haselton said. "But we don't know exactly what it is that men are picking up on. Quite possibly, it could be something about women's behavior, including their style of dress."