Woe is modern love, saddled with liberty and enchanted by romance. Long, long gone are the days of simple plans, of lifelong marriages to high-school sweethearts, and babies by the age of 22. Today’s courtships are beset with an endless series of options, negotiating absurd convergences of family and career, religion and tradition, style and fortune, sex and the city.

Everyone from Carrie & Co. to Oprah to Maureen Dowd has something to say on the subject, clever observations that attempt to explain the cultural shifts unfolding in the wake of women’s lib. This is the bumpy road to gender equality.

Yet the strides we’ve made since the pivotal days of Mary Wollstonecraft capture a mere 200-plus years of relatively slow change to the mating rituals of humans. In the annals of evolution, our cultural progress is but a drop in the bucket of biological influence. In today’s world, I can happily prevent pregnancy in any number of ways, but my genetic blueprint doesn’t know that.

While we mere mortals are busy focusing on, if not obsessing about, everything from voicemail to e-mail, vocation to vehicle, desperation to devotion, our powerful biological motivators are cruising under the radar. Mate selection is, however, both a social and a biological enterprise – so it might do us good to think about the unwitting sexual behaviors passed down to us through the ages. Because even in the 21st century, choosing a mate may have less to do with fate and more to do with natural selection for some trait or another, and the clusterfuck we call the dating scene may be nothing more than evolution on parade.

Given that the basic goal of human mating is to produce the most healthy and virile children possible, both men and women have an agenda when they’re deciding with whom they ought or ought not copulate. Women need to make sure to find men with resources to invest in their offspring (genetic resources as well as the more sociological kind, of course). Men need to either do it with a whole lot of women—hopeful that their progeny will survive—or they, too, need to find women who are going to bring home the genetic and child-rearing bacon.

A bit of context: In case you missed the movie and your college psych classes are a stonewashed memory, in the 1940s Dr. Alfred Kinsey began studying human sexuality at Indiana University. He published the findings of said research in such seminal works as Sexual Behavior in the Human Male (1948) and Sexual Behavior in the Human Female (1953) and, in effect, initiated a whole new field of biological and psychological study, as well as decades of sexual revolution.

Many researchers have followed in Kinsey’s footsteps, and the volume of sex studies continues to, er, swell. It’s no surprise that when offered, human sexuality courses are consistently among the most popular college classes in the United States and abroad. Human sexuality research may reveal quite a lot about our batty bar behavior, about the powerful underlying motivators affecting whom we sleep with, date and marry.

EXHIBIT A: From the Look of Things

Picture this: You’re in a bar. It’s Saturday night and the place is packed. Everywhere you look people are scoping and being scoped, flirting, flirting, flirting in booths, at the bar, from a distance. The entire scene is unfolding according to an archetypical series of cues. Nonverbal communication runs rampant.

Women actually initiate this coquetry, carefully glancing, primping, smiling and laughing according to a particular rhythm. Check out the girl at the bar casting a room-encompassing glance, smiling and smoothing her hair. She licks her lips, does a little dance in her chair and tosses her head, luring in the men around her, enticing them to synchronize their own behaviors, mirror her posture and echo her movements. Little does this vixen know, she’s playing out her seduction in a choreographed series of tried and true signals—all of which have been carefully studied and detailed by 40 years of nonverbal communication research.

But how does she select her prey? Like her male counterparts, she undergoes a heavy-duty test of the senses designed to make a whole lot of unconscious assessments long before cognitive analysis ever begins.

An early visual cue is waist-to-hip ratio (WHR), the formal scientific way to talk about “measurements.” Essentially, one’s waist-to-hip ratio is a key to fat distribution, which is a key to hormone circulation, which is a key to endocrine health, which is a key to long-term health—clearly an important factor in choosing a mate. Both men and women have ideal WHRs, which signal optimum health and are most attractive to potential mates.

Meanwhile a dude sits in the window of a café. A woman walks by and he asks himself, hot or not? This question is, is she symmetrical? Study after study has shown, invariably, the foxiest folks come with heavy doses of symmetry. Why? Because bilateral body symmetry contributes to attractiveness big time—and for good reason. It is well accepted among sexologists that symmetrical faces and bodies signal developmental stability, or a person’s biological ability to stand strong in the face of environmental adversity—i.e., disease, malnutrition, pollution, etc. We humans have an uncanny ability to gauge symmetry in a flash, without even knowing it.

EXHIBIT B: The Nose Knows

While women may have the upper hand in the flirting environment, everything changes as soon as a man and woman are close enough to smell each other. Enter pheromones.

The word “pheromone” comes from the ancient Greek words “pherein,” to carry, and “hormon,” to excite. These odorless secretions are made in the apocrine glands of the skin, e.g., genital areas, navel area, chest, breasts and all the other obvious places associated with hairs. Not only do these glands not become functional until puberty (a key clue that they’re involved in luring a mate), they cause all sorts of reactions in the course of human interaction.

Pheromones particularly affect ovulating women. Because, unlike some of our evolutionary cousins, human ovulation is visually and intellectually invisible, men are at a reproductive disadvantage compared with their monkey counterparts. Invisible ovulation puts good old-fashioned cuckoldry on the map and empowers women by motivating their mates to be caring all month long. Pheromones are just Mother Nature’s way of playing fair. They make ovulating women friskier and, in effect, let the cat out of the bag.

A range of research conducted over the last few decades has verified the power of pheromones. A study involving pheromone-filled pendants drew flocks of men to women who’d worn them. Another study, one in which subjects doped up on the pheromone androstenol gave extra-high ratings to photos of members of the opposite sex, proved that pheromones influence our visual judgments. It turns out our visual cues and our olfactory cues are linked in a more uncanny way: Subject judges in a third experiment most preferred the smells of T-shirts worn by highly symmetrical people. They aren’t called pheromones for nothing. Apparently the ancient Greeks were onto something.

But the real jaw-dropper came from a group of researchers from Vienna and Las Vegas who recently put it all together: Perhaps our visual ability to recognize facial and bodily symmetry (as well as genetic diversity, though that’s a whole other can of worms) is actually something that’s conditioned in a basic Pavlovian way. Perhaps the whole sensory assessment of potential mates has everything to do with an innate ability to detect pheromones, and their visual correlations are simply something we have learned to recognize. Perhaps tall, dark and handsome isn’t something we see, but something we smell.

EXHIBIT C: Size Does Matter

Enough about sights and smells. Our judgments of potential mates continue well beyond the precursory stages of attraction and flirtation. On to the boudoir.

The length and shape of a penis really does matter. Evolutionarily speaking, the last thing a man wants is for his sperm to be beaten out by the sperm of another man. So suppose a woman is sleeping with more than one man. Condoms aside, of course, each time a man ejaculates while having sex with her, his penis does everything it can to deliver his sperm to her fertile womb. According to research done at the State University of New York (Albany, Oswego), the longer his penis is, the closer said ejaculation will be to her cervix, gateway to the uterus, land of the legendary egg.

But that’s nothing. Imagine after sleeping with one guy, this woman winds up doing the nasty with another dude very shortly thereafter. That second dude is going to rely on the length of his penis to not only deliver the goods but to defeat the sperm already present in the woman’s vagina. How’s he gonna do that? Well according to the SUNY research, with a bit of good breeding, he’s going to insert his penis farther into her vagina than the first guy and use the ridge of its head, curved at just the right angle, to actually draw the first guy’s semen out of the ol’ love canal, thereby increasing his chances at being becoming a daddy. Ay, there’s the rub.

A man’s testicles are also involved in this competition. Imagine a man’s lady friend goes away to Whistler for the weekend. Even though he’s not necessarily fretting that she might cheat on him while she’s gone, his testicles are taking no chances. By sheer virtue of time apart—and no matter how much he masturbates—the man generates semen with an extra-high concentration of sperm. That way, when the lovely lady returns, he’ll be sure to thwart any cuckoldry with a high-density ejaculation designed to increase his shot at fertilization.

EXHIBIT D: Pandora’s Box

Once again, however, Mother Nature provides fair balance. It does, in fact, take two to tango. If men have the power to control whose sperm get fertilized, then so must women. Thus we must consider the mysterious female orgasm. For a long time scientists thought the female orgasm was useful in that it kept women lying down after sex and helped form a stronger pair bond. In the late 1960s, however, the British Medical Journal noted the muscular contractions and uterine suction involved in the female orgasm. Many years later, two British biologists looked into this phenomenon further and discovered that when a woman climaxes anytime between one minute before and 45 minutes after her lover, she retains considerably more sperm than she does during sex sans orgasm. The female orgasm has a function.

The question is, what causes a woman to orgasm? Some researchers at the University of New Mexico pursued an answer, testing the relationship between female orgasm and—that’s right—male symmetry. The result? The more symmetrical the man, the more orgasmic the woman. Amazingly enough, the same study showed romantic attachment had no effect on the frequency of female climax.

Alas, the University of New Mexico study also revealed that it is, in many ways, a player’s world. Orgasm-inducing, symmetrical men are also the least likely to invest in their relationship—with time or money. They have the shortest courtships before having sex and cheat more than their less symmetrical counterparts. The female orgasm has nothing to do with good guys and everything to do with good genes.

So don’t be fooled. The next time you find yourself inexplicably drawn to the hottie across the room, check yourself, your sly smile and your millennia of biological baggage. Ask yourself whether the object of your affection has something more to offer than genetics, or if all you’re really looking at is a symmetrical face with a rockin’ waist-to-hip ratio. The human power to reason is highly evolved, after all, and it may very well be just the thing to keep our sexual behavior in line. So keep on agonizing over voicemail and wondering about who will pay the check. It will all work out in the end. Mother Nature likes to play fair.