The virtually visceral responsiveness to physical features in flirtation may also be as good a guarantee as one can get that a potential partner shapes up on a hidden but crucial aspect of health immunity to disease. Scientists know that the testosterone that gives men jutting jaws, prominent noses and big brows, and, to a lesser extent, the estrogen that gives women soft features and curving hips, also suppresses the ability to fight disease. But looks have their own logic, and bodies and faces that are exemplars of their gender signal that their bearer has biological power to spare; after all, he or she has survived despite the hormonal "handicap."
Take the case of such elaborate male ornamentation as peacock tails and stag antlers. In the 1980s, evolutionary biologists William Hamilton and Marlena Zuk linked such features to inborn resistance to disease parasites. Antlers and tail feathers are known to be attractive to females of their species and are major machinery of flirtation. But developing and maintaining such extravagant equipment is costly, taking huge nutritional resources and even slowing the animals down, making them more vulnerable to predators.
The only animals that can afford such ornamentation are those with tiptop constitutions. So, like big bones, big horns, big tails and big spurs in animals, jutting-jaws are honest markers for a healthy immune system. Scientists point out that such features are in fact respected by other men as well as attractive to women. Studies show that tall, square-jawed men achieve higher ranks in the military than do those with weak chins.
Whatever specific physical features men and women are primed to respond to, they all have a quality in common-symmetry. That is, attributes deemed attractive have an outward appearance of evenness and right-left balance. Unlike the color and condition of tail feathers, symmetry serves not so much as an honest marker of current health status, but as a signal of a general capacity to be healthy. Symmetry, says Gangestad, is "a footprint left by your whole developmental history." It alone explains why Elizabeth Taylor, Denzel Washington and Queen Nefertiti are universally recognized as beautiful-and full of sex appeal.
"Bilateral symmetry is a hot topic these days," beams Albert Thornhill, a biologist at the University of New Mexico and a pioneer in the study of symmetry in attraction and flirtation. He and Gangestad believe it is a marker of "developmental precision," the extent to which a genetic blueprint is realized in the flesh despite all the environmental and other perturbations that tend to throw development off course.
Recent studies conducted by the two demonstrate not only that women prefer symmetrical men, they prefer them at a very specific time-when they are most fertile. "We found that female preferences change across the menstrual cycle," Gangestad reports. "We think the finding says something about the way female mate preferences are designed. Because the preference for male symmetry is specific to the time of ovulation, when women are most likely to conceive, we think women are choosing a mate who is going to provide better genes for healthy babies. It's an indirect benefit, rather than a direct or material benefit to the female herself."
In their study, 52 women rated the attractiveness of 42 men-by their smell. Each of the men slept in one T-shirt for two nights, after which the women were given a whiff of it. Prior to the smell test, all the men had undergone careful calipered measurement of 10 features, from ear width to finger length. Those whose body features were the most symmetrical were the ones whose smells were most preferred, but only among women who were in the ovulatory phase of their menstrual cycle. At other times in their cycle, women had no preference either for symmetrical or asymmetrical males.
The preference for symmetry is not limited to humans. Thornhill first stumbled upon symmetry two decades ago, during experiments with scorpion flies in Australia, Japan and Europe. He noticed that females chose particular male flies on the basis of the level and quality of "nuptial gifts," nutrients passed to the female during courtship and mating.
"That was the first inkling I had that insects were very sophisticated about their mating strategies," Thornhill recalls. But the more time he spent recording the sexual lives of scorpion flies, the more he realized that the females were selecting partners long before they sampled any gifts, and they were reckoning by the symmetry of the males' wings. "I discovered that males and females with the most symmetrical wings had the most mating success and that by using wing symmetry, I-and presumably the fly-could predict reproductive fitness better than scent or any other factor."
Since then, Thornhill and colleagues around the world have conducted more than 20 separate tests of symmetry of everything from eyes, ears and nostrils to limbs, wrists and fingers. Even if they never speak a word or get closer than a photograph, women view symmetrical men as more dominant, powerful, richer and better sex and marriage material. And symmetrical men view themselves the same way! Men, for their part, rate symmetrical women as more fertile, more attractive, healthier and better sex and marriage material, too-just as such women see themselves as having a competitive edge in the mating sweepstakes.
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