Exactly how do we signal our amorous interest and intent in each other? It's been trivialized, even demonized, but the coquettish behavior indulged in by men and women alike is actually a vital silent language exchanging critical-and startling-information about our general health and reproductive fitness.

"She was," he proclaimed, "so extraordinarily beautiful that I nearly laughed out loud. She... [was] famine, fire, destruction and plague... the only true begetter. Her breasts were apocalyptic, they would topple empires before they withered... her body was a miracle of construction... She was unquestionably gorgeous. She was lavish. She was a dark, unyielding largesse. She was, in short, too bloody much... Those huge violet blue eyes... had an odd glint... Aeons passed, civilizations came and went while these cosmic headlights examined my flawed . Every pockmark on my face became a crater of the moon."

So Richard Burton described his first sight of a 19-year-old Elizabeth Taylor. He didn't record what happened next, but a growing cadre of scientists would bet their lab coats and research budgets that sometime after that breath-catching, gut-gripping moment of instant mutual awareness, Liz tossed her hair, swayed her hips, arched her feet, giggled, gazed wide-eyed, flicked her tongue over her lips and extended that apocalyptic chest, and that Dick, for his part, arched his back, stretched his pecs, imperceptibly swayed his pelvis in a tame Elvis performance, swaggered, laughed loudly, tugged his tie and clasped the back of his neck, which had the thoroughly engaging effect of stiffening his stance and puffing his chest.

What eventually got these two strangers from across the fabled crowded room to each other's side was what does it for all of us-in a word, , the capacity to automatically turn our actions into sexual semaphores signaling interest in the opposite as predictably and instinctively as peacocks tan their tails, codfish thrust their pelvic fins or mice twitch their noses and tilt their backs to draw in the object of their .

Long trivialized and even demonized, flirtation is gaining new respectability thanks to a spate of provocative studies of animal and human behavior in many parts of the world. The capacity of men and women to flirt and to be receptive to flirting turns out to be a remarkable set of behaviors embedded deep in our psyches. Every come-hither look sent and every sidelong glance received are mutually understood signals of such transcendent history and beguiling sophistication that only now are they beginning to yield clues to the psychological and biological they encode.

This much is clear so far: flirting is nature's solution to the problem every creature faces in a world full of potential mates-how to choose the right one. We all need a partner who is not merely fertile but genetically different as well as healthy enough to promise viable offspring, provide some kind of help in the hard job of and offer some social compatibility.

Our animal and human ancestors needed a means of quickly and safely judging the value of potential mates without "going all the way" and risking with every possible candidate they encountered. Flirting achieved that end, offering a relatively risk-free set of signals with which to sample the field, try out sexual wares and exchange vital information about candidates' general health and reproductive fitness.

"Flirting is a negotiation process that takes place after there has been some initial ," observes Steven W. Gangestad, Ph.D., an evolutionary psychologist at the University of New Mexico in Albuquerque who is currently studying how people choose their mates. "Two people have to share with each other the information that they are attracted, and then test each other" on an array of attributes. Simply announcing, 'I'm attracted to you, are you attracted to me?' doesn't work so well. "It works much better to reveal this and have it revealed to you in smaller doses," explains Gangestad. "The flirting then becomes something that enhances the attraction."

It is an axiom of science that traits and behaviors crucial to survival-such as anything to do with attraction and sex-require, and get, a lot of an animal's resources. All mammals and most animals (including birds, fish, even fruit flies) engage in complicated and energy-intensive plots and plans for attracting others to the business of sex. That is, they flirt.

From nature's standpoint, the goal of life is the survival of our DNA. Sex is the way most animals gain the flexibility to healthfully sort and mix their . Getting sex, in turn, is wholly dependent on attracting attention and being attracted. And flirting is the way a person focuses the attention of a specific member of the opposite sex. If our ancestors hadn't done it well enough, we wouldn't be around to discuss it now.

A silent language of elaborate visual and other gestures, flirting is "spoken" by intellect-driven people as well as instinct-driven animals. The very universality of flirting, preserved through evolutionary history from insects to man, suggests that a flirting plan is wired into us, and that it has been embedded in our genes and in our brain's operating system the same way and for the same reasons that every other sexual trait has been-by trial and error, with conservation of what works best.

Like any other language, flirting may be deployed in ways subtle or coarse, adolescent or suave. Nevertheless, it has evolved just like pheasant spurs and lion manes: to advertise ourselves to the opposite sex.

Flirtation first emerged as a subject of serious scrutiny a scant 30 years ago. Irenaus Eibl-Eibesfeldt, now honorary director of the Ludwig-Bohzmann Institute for Urban Ethology in Vienna, was already familiar with the widespread dances and prances of mate-seeking animals. Then he discovered that people in dozens of cultures, from the South Sea islands to thc Far East, Western Europe, Africa and South America, similarly engage in a fairly fixed repertoire of gestures to test sexual availability and interest.

Having devised a special camera that allowed him to point the lens in one direction while actually photographing in another, he "caught" couples on film during their flirtations, and discovered, for one thing, that women, from primitives who have no written language to those who read Cosmo and Marie Claire, use nonverbal signals that are startlingly alike. On Eibl-Eibesfeldt's screen flickered identical flirtation messages: a female smiling at a male, then arching her brows to make her eyes wide, quickly lowering her lids and, tucking her chin slightly down and coyly to the side, averting her gaze, followed within seconds, almost on cue, by putting her hands on or near her mouth and giggling.

Regardless of language, socioeconomic status or religious upbringing, couples who continued flirting placed a palm up on the table or knees, reassuring the prospective partner of harmlessness. They shrugged their shoulders, signifying helplessness. Women exaggeratedly extended their neck, a sign of vulnerability and submissiveness.

For Eibl-Eibesfeldt, these gestures represented primal behaviors driven by the old parts of our brain's evolutionary . A woman presenting her extended neck to a man she wants is not much different, his work suggested, than a gray female wolf's submissiveness to a dominant male she's after.

Since then, researchers have turned up the intensity, looking, for example, at compressed bouts of flirting and courtship in their natural habitat-hotel bars and cocktail lounges. From observations at a Hyatt hotel cocktail lounge, researchers documented a set of signals that whisks a just-met man and woman from barroom to bedroom. Her giggles and soft laughs were followed by hair twirling and head-tossing; he countered with body arching, leaning back in the chair and placing his arms behind head, not unlike a pigeon puffing his chest.

If all went well, a couple would invariably progress from touching themselves to touching each other. The first tentative contacts could be termed "lint-picking." She would lift an imaginary mote from his lapel; he would brush a real or imaginary crumb from her lips. Their heads moved closer, their hands pressed out in front of them on the table, their fingers inches from each other's, playing with salt shakers or utensils. Whoops! An "accidental" finger touch, then perhaps some digital "dirty dancing," more touching and leaning in cheek to cheek. By alone, the investigators could predict which pairs would ride up the elevators together.

Social psychologist Timothy Perper, Ph.D., an independent scholar and writer based in Philadelphia, and anthropologist David Givens, Ph.D., spent months in dimly lit lounges documenting these flirtation rituals. Like the ear wiggles, nose flicks and back arches that signal "come hither" in rodents, the women smiled, gazed, swayed, giggled, licked their lips, and aided and abetted by the wearing of high heels, they swayed their backs, forcing their buttocks to tilt out and up and their chests to thrust forward.

The men arched, stretched, swiveled, and made grand gestures of whipping out lighters and lighting up . They'd point their chins in the air with a cigarette dangling in their mouth, then loop their arms in a wide arc to put the lighter away. Their swaggers, bursts of laughter and grandiose gestures were an urban pantomime of the prancing and preening indulged in by male baboons and gorillas in the wild. Man or monkey, the signals all said, "Look at me, trust me, I'm powerful, but I won't hurt you." And "I don't want anything much... yet."

All the silent swaying, leaning, smiling, bobbing and gazing eventually brought a pair into full frontal alignment. Face to face, they indulged in simultaneous touching of everything from eyeglasses to fingertips to crossed legs. Says Perper, "This kind of sequence-attention, recognition, dancing, synchronization-is fundamental to courtship. From the Song of Songs until today, the sequence is the same: look, talk, touch, kiss, do the deed."

The fact that flirting is a largely nonexplicit drama doesn't mean that important information isn't being delivered in those silent signals. By swaying her hips, or emphasizing them in a form-fitting dress, a flirtatious woman is riveting attention on her pelvis, suggesting its ample capacity for bearing a child. By arching her brows and exaggerating her gaze, her eyes appear large in her face, the way a child's eyes do, , along with giggles, her youth and "submissiveness." By drawing her tongue along her lips, she compels attention to what many biologists believe are facial echoes of vaginal lips, transmitting sexual maturity and her interest in sex. By coyly averting her gaze and playing "hard to get," she communicates her unwillingness to give sex to just anyone or to someone who will love her and leave her.

For his part, by extending a strong chin and jaw, expanding and showing off pectoral muscles and a hairy chest, flashing money, loudly or resonantly, smiling, and doing all these things without accosting a woman, a man signals his ability to protect offspring, his resources and the -driven vitality of his sperm as well as the tamer side of him that is willing to stick around, after the sex, for fatherhood. It's the behavioral equivalent of "I'll respect you in the morning."

"I can't tell you why I was attracted to her the instant she walked into my office," recalls a 32-year-old screenwriter. "It was chemistry. We both flirted and we both knew it would lead nowhere. I'm happily married." The statement is almost stupefyingly commonplace, but also instructive. Each of us "turns on" not to mankind or womankind but to a particular member of the opposite sex. Certain stances, personal styles, gestures, intimations of emotional compatibility, perhaps even odors, automatically arouse our interest because they not only instantly advertise genetic fitness but they match the template of Desired Mate we all carry in our mind's eye.

As with Dick and Liz, or any couple, the rational, thinking part of their brains got them to the place where girl met boy; they had the event on their calendars, planned what they would wear, arranged for transportation. But in that first meeting, their capacity to react with their instinct and hearts, not their heads, overrode their cognitive brains. Otherwise, they might: not have had the nerve to look at each other.

The rational brain is always on the lookout for dangers, for complexities, for reasons to act or not act. If every time man and woman met they immediately considered all the possible risks and vulnerabilities they might face if they mated or had children, they'd run screaming from the room.

It's no secret that the brain's emotionally loaded limbic system sometimes operates independently of the more rational neocortex, such as in the face of danger, when the fight-or-flight response is activated. Similarly, when the matter is sex-another situation on which survival depends-we also react without even a neural nod to the neocortex. Instead, the flirtational operating system appears to kick in without conscious consent. If, at the moment they had met, Dick and Liz had stopped to consider all the possible outcomes of a relationship, they both would have been old before they got close enough to speak.

The moment of attraction, in fact, mimics a kind of brain damage. At the University of Iowa, where he is professor and head of neurology, Antonio Damasio, M.D., has found that people with damage to the connection between their limbic structures and the higher brain are smart and rational-but unable to make decisions. They bring commitment to a whole new level. In attraction, we don't stop and think, we react, operating on a "gut" feeling, with butterflies, giddiness, sweaty palms and flushed faces brought on by the reactivity of the emotional brain. We suspend intellect at least long enough to propel us to the next step in the mating game-flirtation.

Somewhere beyond flirtation, as a relationship progresses, courtship gets under way, and with it, intellectual processes resume. Two adults can then evaluate potential mates more rationally, think things over and decide whether to love, honor and cherish. But at the moment of attraction and flirtation, bodies, minds and sense are temporarily hostage to the more ancient parts of the brain, the impulsive parts that humans share with animals.

If flirting is a form of self promotion, nature demands a certain amount of truth in advertising. "For a signaling system to convey something meaningful about a desirable attribute, there has to be some honesty," explains Gangestad, "so that if you don't have the attribute you can't fake it." Just as the extravagant colors of birds that figure so prominently in their flirting rituals proclaim the health of animals so plumed, humans have some signals that can't be faked.

Waist-hip ratio is likely one of them. It's no secret that men snap to attention and even go dry at the mouth at the sight of a shapely woman. Science has now calculated just how curvy a woman has to be to garner such appreciation: the waist must measure no more than 60 to 70% of her hip circumference. It is a visual signal that not only figures powerfully in attraction but is a moving force in flirtation. And unless steel-boned corsets stage a comeback, it is an attribute that just can't be put into play unless it is real.

In simplest terms, says Gangestad, waist-hip ratio is an honest indicator of health. Studies have shown that hourglass-shaped women are less likely than other women to get diabetes and cardiac disease. They are also most likely to bear children, as hips take their shape at from the feminizing hormone .

"The literature shows that women with a 0.7 waist-hip ratio have a sex-typical hormone profile in the relationship of estrogen to testosterone, and that women with a straighter torso, meaning a waist-hip ratio closer to 1:1, indeed have lower fertility," Gangestad reports. "It appears that males have evolved to pay attention to this cue that ancestrally was related to fertility."

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 signal that their bearer has biological power to spare; after all, he or she has survived despite the "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 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 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 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.

Flirtation, it turns out, is most successful among the most symmetrical. Men's bodily symmetry matches up with the number of lifetime sex partners they report having. Symmetrical men also engage in more in their -"extra-pair copulations" in the language of the lab. And they get to sex more quickly after meeting a romantic partner compared to asymmetrical men. They lose their virginity earlier in life, too.

When women flirt with symmetrical men, what their instincts are reading might once have been banned in Boston. Male symmetry is also shorthand for female sexual satisfaction. Gangestad and Thornhill surveyed 86 couples in 1995 and found that symmetrical men "fire off more female copulatory orgasms than asymmetrical men." Women with symmetrical partners were more than twice as likely to climax during intercourse. Thrills are only a short-term payoff, however; female is really a shill for fertilization, pulling sperm from the vagina into the cervix.

Successful as symmetrical men are at flirtation, it's only their presumably better genes that women really want. Women definitely do not prefer symmetrical men for long-term relationships. There's a definite downside to getting someone with really good DNA. Symmetry, Gangestad explains, affords those men who possess it to take a dastardly mating strategy. His studies show that symmetrical men invest less in any one romantic relationship-less time, less attention, less money and less fidelity. They're too busy spreading around their symmetry. "They also tend to sexualize other women more," Gangestad reports. "It may be that males who can have the most access without giving a lot of investment take advantage of that."

A guy who will stick around and help out with parenting is on most women's wish list of qualities in a mate, Gangestad concedes. "I wouldn't exclude the possibility that men have been doing some direct parental care for some time, and so a preference for that might also have an evolutionary basis." But also on a woman's wish list from an evolutionary standpoint would be someone who is going to provide good genes for healthy babies. Unfortunately, says the Albuquerque researcher, "what can and does happen in a mating market is that those things don't all come in the same package."

Although the signals and semaphores of flirting are largely devoid of explicit content, the style with which one flirts can be downright revelatory. "How a person flirts honestly reveals some important qualities about an individual," says Gangestad. Symmetry isn't everything; there are signals of more subtle skills.

In some species, the females watch the males fight each other and then choose the one who can hold the central territory But we humans are more differently evolved creatures with more complex lives in which our higher faculties presumably contribute something to success, whether it's surviving in primitive equatorial caves or sophisticated urban ones.

Enter , and . Deployed in flirting, they disclose more about an individual person than all the antlers do about leching animals. "They are likely saying something important about our very viability," says Gangestad. "When we can engage in humor and creativity, they act as an honest signal that we've got a reasonably well put together . They may indicate there's some developmental integrity underneath our brain." And a certain ability to withstand whatever challenges life throws a person's way.

What's more, our basic social ability to "read" another's facial gestures and emotional expressions acts as a fact-checking system in flirtation. It enables us to glimpse the tone of a prospective mate's inner life and to check for the presence or absence of psychological weakness. And in fact, women are pretty good at doping out information about such important attributes-even when they get very little time to make a judgment.

In a recent set of studies, Gangestad and a colleague extracted one-minute segments from more extensive videotaped interviews with men not in committed relationships. The brief segments were then shown to women who were asked to rate the men on a variety of characteristics, including how attractive they'd be in a pair relationship. The women were able to make judgments about each man's intelligence, ability to be caring and how nice he seemed. They also paid attention to another set of characteristics-how effective a man was likely to be with other males, how socially influential he was.

The men who were rated most attractive for long-term relationships scored high on both sets of characteristics. But what may be most notable about the study was that women's observations, from a mere snippet of videotape, were remarkably accurate. They correlated closely with the men's ratings of their own personality.

After two people share the information that they are attracted, then, through the way they flirt, they may unwittingly let on more about themselves. "It becomes a testing ground as well as an information-revealing process," says Gangestad.

Thus, while we appear to be pre-programmed with an urge to wile or wiggle our way onto another's mental radar screen, we also seem psychologically constituted to pay rapt attention to looks and actions intended to be sexually appealing. Otherwise, neither Liz and Dick nor any two contenders would have a reliable, safe or peaceful means of communicating attraction and getting to the more durable business of courtship, mating and commitment to the offspring that will carry our DNA into the next generation.