Why Women Have Sex (5 page)

So when a woman says that she had sex with a man because he smelled nice, her sexual motivation has hidden roots in an evolutionary adaptation. At an unconscious level, women are drawn to men with whom they are genetically compatible.

Another reason why a man’s scent is so important comes from the unusual discovery that body symmetry has sexual allure. Most human bodies are bilaterally symmetrical: The left wrist generally has the same circumference as the right wrist; the left ear is generally as long as the right ear; from the eyes to the toes, the left and right halves of people’s bodies roughly mirror each other. Each individual, however, carries small deviations from perfect symmetry. Two forces can cause faces and bodies to become more asymmetrical. One is genetic—the number of mutations an individual has, which geneticists call
mutation load.
Although everyone carries some genetic mutations (estimates are that the average person has a few hundred), some people have a higher mutation
load than others, and those with more mutations tend to be more asymmetrical. The second force is environmental. During development, some individuals sustain more illnesses, diseases, parasites, and bodily injuries than others, and these environmental insults create asymmetries in the body and face. Symmetry, in short, is a sign of good health—an indication that a person carries a low mutation load and has experienced few environmental injuries, or at least possesses the capacity to sustain environmental injuries without their leaving much of a mark.

If body symmetry is attractive because of how we evolved, so is the fact that women are able to detect the scent signature for symmetry, a useful skill when you consider that some asymmetries may not be immediately visible.

But could a woman possibly smell body symmetry? In one study, men wore white cotton T-shirts for two nights. The T-shirts were then sealed in plastic bags. In the laboratory, scientists used calipers to measure the various physical components of the men’s bodies, including their wrists, ankles, and earlobes, in order to evaluate their degree of symmetry. Then women smelled each T-shirt and provided a rating of how pleasant or unpleasant it smelled. Women judged the T-shirt odors of symmetrical men to be the most attractive and deemed the odors of asymmetrical men to be repulsive. Four independent studies have replicated the finding.

Women find the scent of symmetry particularly attractive when they are in the fertile phase of their ovulation cycle—precisely the time in which they are most likely to conceive. This apparently reflects an evolutionary adaptation in women to reproduce with men possessing honest signals of good health, including high-quality genes. When women have extramarital affairs, they tend to choose symmetrical men as partners—yet another indication of the importance of symmetry in sexual attraction.

A person’s scent can influence not only a woman’s mate choice, but also when and how frequently she chooses to have sex and possibly the chance she will become pregnant.

Researchers have shown that exposure to male pheromones can increase a woman’s fertility. Pheromones are substances secreted from the glands at the anus, underarms, urinary outlet, breasts, and mouth. In nonhuman mammals, a specialized olfactory structure, the vomeronasal organ, acts as the locus for receiving pheromonal signals, which control most animals’ and insects’ mating rituals. One study found that frequent sexual exposure to men (at least once a week) regularized women’s menstrual cycles, increased fertile basal body temperature, and increased estrogen in the phase of the menstrual cycle following ovulation, called the luteal phase. Another study showed that women who slept with a man two or more times during a forty-day period had a significantly higher incidence of ovulation than those who had slept with a man less often.

Once again, sexual attraction plays a role. Dr. Winnifred Cutler, the director of the Athena Institute, found that exposure to male pheromones influences a woman’s sexual attraction to a man. In her study, thirty-eight heterosexual men aged twenty-six to forty-two recorded their baseline levels of sexual behavior and dating experiences for a two-week period. Then for a month they wore either their regular aftershave, or the same aftershave but with an added synthetic version of a pheromone naturally secreted by men. The men did not know which aftershave they were wearing. During the test month, the men continued to record their sexual and dating experiences. The results showed that compared to their baseline levels of sexual activity, the men who wore the “pheromone-charged” aftershave engaged in higher rates of sexual petting and intercourse, had more frequent informal dates, and spent more time sleeping next to a partner. Over the same period, they reported no change in their frequency of masturbation—so the increase in the rest of their sexual activity could not simply have resulted from men having a higher sex drive due to their own exposure to the extra pheromone.

Sensitivity to scent does not just provide a means for identifying good hygiene or emotionally resonant perfumes. Scent also gives women cues about a partner’s immune system and body symmetry, and pheromones can unconsciously shape how women become sexually attracted and aroused.

We’ve seen how body symmetry, because it indicates good health, is attractive to women. Body symmetry is also linked with men’s muscularity, and studies conducted both in the United States and on the Caribbean island of Dominica have found that symmetrical men have a larger number of sex partners than asymmetrical men. When women identify the specific qualities that attract them to a sexual partner, they frequently mention “the person had a desirable body”—the sixteenth most frequent reason cited for having sex in our original study. But what sorts of bodies do women find sexually desirable?

Perhaps the most obvious characteristic is
height
. Studies consistently find that women consider tall men to be attractive, although only to an extent—taller than average, but not too tall. In analyses of personal ads, 80 percent of women state a desire for a man six feet tall or taller. Men who indicated in their personal ads that they were tall received far more responses from women. Women prefer tall men as marriage partners, and place an even greater emphasis on height in shorter-term sex partners. Women even take height into consideration when selecting sperm donors!

A study of British men found that taller than average men have had a greater number of live-in girlfriends than their shorter peers. Two studies found that taller than average men tend to have more children, and hence are more reproductively successful. Women seem to find tall men better candidates for romance and reproduction.

Could there be a logic underlying women’s desires for tall men? In traditional cultures, tall men tend to have higher status. “Big men” in hunter-gatherer societies—high-status men who command respect—are
literally
big men, physically. In Western cultures, tall men tend to have higher socioeconomic status than short men. Another study found that recruiters choose the taller of two applicants for a sales job 72 percent of the time. Each added inch of height adds several thousand dollars to a man’s annual salary. One study estimated that men who are six feet tall earn, on average, $166,000 more across a thirty-year career than men seven inches shorter. Taller policemen are assaulted less often than shorter policemen, indicating that their stature either commands more respect from criminals or causes them to think twice before attacking. Height deters
aggression from other men. In the jargon of evolutionary biology, height is an “honest signal” of a man’s ability to protect. Women report simply feeling safer with tall mates.

Another answer comes from recently discovered correlates of male height. Tall men, on average, tend to be healthier than short men, although men at the extreme high and low end of the distribution have more health problems. So tall men tend to have better job prospects, to have more economic resources, to enjoy elevated social status, to afford physical protection, and to be healthy—a bounty of adaptive benefits.

(We will see how sizes in other arenas matter in chapters 2 and 7.)

Height, of course, is not the only aspect of men’s bodies that sexually excites women.

Studies of mate preferences reveal that women desire strong, muscular, athletic men for long-term mating as well as for sexual liaisons. Most women show a distinct preference for a particular body morphology—namely, a V-shaped torso that reveals a high
shoulder-to-hip ratio
(broad shoulders relative to hips). They are attracted to a lean stomach combined with a muscular (but not muscle-bound) upper torso.

In fact, both sexes judge men with a high shoulder-to-hip ratio to be more physically and socially dominant—which may give a clue to its appeal, since women generally are not attracted to men who appear as though they could be easily dominated by other men. Men exhibiting a high shoulder-to-hip ratio begin having sexual intercourse at an early age—sixteen or younger. They report having more sex partners than their slim-shouldered peers. They have more sexual affairs with outside partners while in a relationship. And they report more instances of being chosen by already-mated women for sexual affairs on the side. Shoulder-to-hip ratio also arouses the green-eyed monster: Potential rivals with a high shoulder-to-hip ratio trigger jealousy in men.

Men with strong, athletic, V-shaped bodies tend to succeed in competitions with other men compared to their frailer peers. Across cultures, physical contests such as wrestling, racing, and throwing allow women to gauge men’s physical abilities, including speed, endurance, and strength.

Scientific research, though, has discovered that men overestimate the degree of muscularity that women actually find attractive, assuming that they need to pump up more, or puff up more, to be attractive. One study compared the muscularity of men’s bodies in
Cosmopolitan
(whose readership is 89 percent women) with
Men’s Health
(whose readership is 85 percent men). Researchers rated the muscularity of men’s bodies depicted in each magazine. The level of muscularity depicted in
Cosmopolitan
(4.26) was nearly identical to the level of muscularity women rate as ideal in a sexual partner (4.49). Men, in contrast, mistakenly believe that women desire a more muscular sex partner (5.04), which corresponds more closely with the muscularity of men shown in
Men’s Health
(5.77).

Images of muscle-bound men have almost certainly fostered men’s misperception of what women find most sexually attractive—just as photo spreads of impossibly thin women have led women to overestimate the degree of thinness that men find most attractive. After viewing repeated images of V-shaped bodies, men become more dissatisfied with their own bodies, just as women become more unhappy with their bodies after seeing images of size zero models. Fully 90 percent of American men report that they want to be more muscular. The figure among the less media-saturated Ghana is 49 percent. Ukrainian men lie in between, with
69
percent reporting a desire to be more muscular. As one researcher summed it up, the average man “feels like Clark Kent but longs to be like Superman.”

He could have been a model. When he acted interested in me, I couldn’t believe it. We had sex once. Strangely enough, he kept calling me afterward. I didn’t continue with the relationship for several reasons. One, he was just a pretty face, but I think he was really crazy about me. Two, never date a guy prettier than you are. It’s terrible for your self-esteem and your sanity.

—heterosexual woman, age 26

Masculine facial features are heavily influenced by the production of testosterone during adolescence, when the bones in the face take their
adult form. From an evolutionary perspective, puberty marks the time when men and women enter the arena of mate competition. They begin to allocate time, energy, and effort to the tasks of mate selection and mate attraction. In men, the amount of muscle mass, as we have seen, contributes to success in competition with other men and sexual attractiveness to women. And testosterone turns out to be the magical hormone that promotes men’s muscle mass and masculine facial features.

So why don’t all men have masculine faces and ripped bodies? The answer strangely hinges on a negative side effect of testosterone. High testosterone production compromises the body’s immune functioning, leaving men less able to fight off diseases and parasites. Now here is the paradox: Only men who are above average in healthiness during adolescence can “afford” to produce the high levels of testosterone that masculinize the face. Less healthy adolescents cannot afford to compromise their already precarious immune systems, and so produce lower levels of testosterone at precisely the time when facial bones take their adult form. A masculine-looking face signals a man’s health, his ability to succeed in competing with other men, and his ability to protect. And that is the best explanation for why most women find somewhat more masculine faces (but not the most masculine faces) to be the most attractive.

But when we consider a woman’s fertility status and whether she is evaluating a man as a casual sex partner or a husband, the dynamics shift. In a series of scientific studies, women were asked to judge the attractiveness of a variety of men’s faces at different points during their ovulation cycle—during the most fertile phase (the five days leading up to ovulation) and during the least fertile, post-ovulation luteal phase. The subjects evaluated the faces for sexiness, their attractiveness as a casual sex partner, and their attractiveness as a long-term mate. Women found above-average masculine faces to be the sexiest and the most attractive for a casual sexual encounter. In contrast, women judged somewhat less masculine faces to be more attractive for a long-term relationship. Women’s sexual desires for testosterone-fueled facial cues of masculinity were especially strong during the fertile window of their cycle.