The Biology Of Beauty

None of this surprises evolutionary psychologists. They note that the facial features we obsess over are precisely the ones that diverge in males and females during puberty, as floods of sex hormones wash us into adulthood. And they reason that hormonal abundance would have been a good clue to mate value in the hunter-gatherer world where our preferences evolved. The tiny jaw that men favor in women is essentially a monument to estrogen -- and, obliquely, to fertility. No one claims that jaws reveal a woman's odds of getting pregnant. But like breasts, they imply that she could.

Likewise, the heavy lower face that women favor in men is a visible record of the surge in androgens (testosterone and other male sex hormones) that turns small boys into 200-pound spear-throwers. An oversized jaw is biologically expensive, for the androgens required to produce it tend to compromise the immune system. But from a female's perspective, that should make jaw size all the more revealing. Evolutionists think of androgen-based features as "honest advertisements" of disease resistance. If a male can afford them without falling sick, the thinking goes, he must have a superior immune system in the first place.

No one has tracked the immune responses of men with different jawlines to see if these predictions bear out (Thornhill has proposed a study that would involve comparing volunteers' responses to a vaccine). Nor is it clear whether penis size figures into these equations. Despite what everyone thinks he knows on the subject, scientists haven't determined that women have consistent preferences one way or the other.

OUR FACES ARE OUR SIGNAtures, but when it comes to raw sex appeal, a nice chin is no match for a perfectly sculpted torso -- especially from a man's perspective. Studies from around the world have found that while both sexes value appearance, men place more stock in it than women. And if there are social reasons for that imbalance, there are also biological ones. Just about any male over 14 can produce sperm, but a woman's ability to bear children depends on her age and hormone levels. Female fertility declines by two thirds between the ages of 20 and 44, and it's spent by 54. So while both sexes may eyeball potential partners, says Donald Symons, an anthropologist at the University of California in Santa Barbara, "a larger proportion of a woman's mate value can be detected from visual cues." Mounting evidence suggests there is no better cue than the relative contours of her waist and hips.

Before puberty and after menopause, females have essentially the same waistlines as males. But during puberty, while boys are amassing the bone and muscle of paleolithic hunters, a typical girl gains nearly 35 pounds of so-called reproductive fat around the hips and thighs. Those pounds contain roughly the 80,000 calories needed to sustain a pregnancy, and the curves they create provide a gauge of reproductive potential. "You have to get very close to see the details of a woman's face," says Devendra Singh, the University of Texas psychologist. "But you can see the shape of her body from 500 feet, and it says more about mate value."

Almost anything that interferes with fertility -- obesity, malnutrition, pregnancy, meno-pause -- changes a woman's shape. Healthy, fertile women typically have waist-hip ratios of .6 to .8, meaning their waists are 60 to 80 percent the size of their hips, whatever their actual weight. To take one familiar example, a 36-25-36 figure would have a WHR of .7. Many women outside this range are healthy and capable of having children, of course. But as researchers in the Netherlands discovered in a 1993 study, even a slight increase in waist size relative to hip size can signal reproductive problems. Among 500 women who were attempting in vitro fertilization, the odds of conceiving during any given cycle declined by 30 percent with every 10 percent increase in WHR. In other words, a woman with a WHR of .9 was nearly a third less likely to get pregnant than one with a WHR of .8, regardless of her age or weight. From an evolutionary perspective, it's hard to imagine men not respond- ing to such a revealing signal. And as Singh has shown repeatedly, they do.