The Double Life of Women

The invisible turns of the reproductive cycle shape the everyday behavior of women and men. A woman’s cycle influences not just her preference in a partner, but her personality as well.

Step into any bar or party and it won’t take you long to spot her. She’s the woman with the ringing laugh, the daring clothes, the magnetic appeal that has drawn a circle of admirers around her. If the room were a solar system, she would be the sun—and at the outer reaches, you notice, are several other women seated quietly in her shadow.

Why does this woman command all the attention? Psychologists, image experts, and dating advisers propose a host of explanations: It’s her extraverted personality, her come-hither look, her approachable persona. But an evolutionary biologist observing the scene would offer a more surprising interpretation, one that may help explain barroom dynamics and much more: It’s her “real” time of the month. The belle of the bar is likely reaching peak fertility, while her drabber companions are slogging through a non-fertile phase.

Not long ago, such an explanation would have been intellectual heresy. Sure, biologists could tell when chimpanzees were ready to mate: Once every 28 days, the genitalia of female chimps swell and turn a dramatic shade of pink. And estrus, as the state of sexual receptivity is known, is also readily apparent in less exotic animals, as anyone who’s seen a house cat in heat can attest. Every female mammal on earth, it was believed, advertises her period of greatest fertility—except the female human. In woman, estrus was “lost” somewhere in the long meander of evolution. “That’s the conventional, traditional view of human estrus,” says Randy Thornhill, professor of biology at the University of New Mexico. “But it turns out to be wrong.”

Over the past decade, evolutionary biologists and psychologists have uncovered abundant evidence that women do, in fact, provide clues to the timing of ovulation, the moment when an egg is released and ready to be fertilized. Though these changes are far subtler than those in other species, they have a powerful effect on women’s perceptions, preferences, and behavior—and the reaction of others to her. Monthly shifts even affect men’s feelings and actions. Indeed, the invisible but influential turns of the reproductive cycle shape the everyday behavior of us all. “Human ovulation is not an observable event, and men and women have no explicit awareness of it,” says Martie Haselton, associate professor of communication studies and psychology at UCLA. “But the effects of the menstrual cycle on human behavior are surprisingly strong.”

Take, for example, women’s preferences in male partners. We may think that each woman has an unchanging “type”—but it turns out that women prefer quite different kinds of men depending on whether or not they are fertile. In the two days or so of the ovulatory phase—the time when women are most likely to become pregnant—they gravitate toward men with more “masculine” traits. That means a man who sports a leaner, V-shaped body, and a face with a squarer chin, straighter, heavier eyebrows, and thinner lips; one who speaks in a lower-pitched voice, and displays more aggressive, dominant behavior. When a woman is in the follicular or luteal phases—during which the uterus sheds its lining and then builds it up again, and in which she generally cannot become pregnant—she prefers men with softer features, less-defined bodies, higher voices, and a gentler manner.

So pronounced are these preferences that Thornhill and his University of New Mexico colleague Steven Gangestad have proposed that women actually have two sexualities: one when they’re ovulating, and another during the rest of the month. These distinct modes emerge out of two competing reproductive goals. “Women want to get the highest-quality genes for their children,” says Thornhill, and high genetic quality in a man is indicated by his degree of testosteronization—the extent to which the male hormone testosterone has affected his brain, his face, and the rest of his body.

Once she is pregnant or in the non-fertile part of her cycle, however, a woman’s aims do an abrupt about-face: She wants to secure the most generous and stable source of goods for herself and her offspring. Now the nice-guy provider starts to look appealing. “When women are in what we call the extended-sexuality phase, their preferences shift towards men who appear to have a willingness to share resources like food and protection with her and her children,” says Thornhill.

The influence of the menstrual cycle on women is apparent not only in whom they desire but in how they act. Women who are in the ovulatory phase show more interest in erotic materials than women in the luteal or follicular phases; given a choice of movies to watch, they select ones with more romantic or sexual themes. They take more care with their appearance, and they choose more revealing clothes to wear. In 2004, a group of researchers from the University of Vienna digitally analyzed pictures of 351 women going out to Austrian nightclubs and collected a saliva sample from each. Women whose clothes were tight or showed a lot of skin had higher levels of estradiol, a female hormone that is elevated around the time of ovulation.

By Annie Murphy Paul, published on November 01, 2010

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