Quotes of the Day

One thing you can say about lust, it sure shows up early. Talk all you want about the honey-sweet face of an innocent newborn, the fact is, from the moment we appear in the world, we're not much more than squalling balls of passion. Our needs aren't many: to sleep, to eat, to be held, to be changed. Satisfy these, and there won't be any trouble. Fail to, and you will hear about it.

Of all the urges that drive us, it's the passion to be held that makes itself known first. If a baby is startled fresh from the womb, German pediatrician Ernst Moro discovered in 1918, its arms will fly up and out, then come together in a desperate clutch. Holding is good, and floating free is bad—a lesson that's not so much learned after birth as preloaded at the factory. In fact, doctors have long known that babies who aren't held simply fail to thrive. Not surprisingly, it's a need we never outgrow. In one way or another, we spend the rest of our lives in a sort of sustained Moro clinch.

Physical contact—the feeling of skin on skin, the tickle of hair on face, the intimate scent drawn in by nose pressed to neck—is one of the most precious, priceless things Homo sapiens can offer one another. Mothers and their babies share it one way, friends and siblings share it another, teams and crowds in a celebratory scrum share it a third. And of course lovers share it in the most complex way of all.

Of all the splendidly ridiculous, transcendently fulfilling things humans do, it's sex—with its countless permutations of practices and partners—that most confounds understanding. What in the world are we doing? Why in the world are we so consumed by it? The impulse to procreate may lie at the heart of sex, but like the impulse to nourish ourselves, it is merely the starting point for an astonishingly varied banquet. Bursting from our sexual center is a whole spangle of other things—art, song, romance, obsession, rapture, sorrow, companionship, love, even violence and criminality—all playing an enormous role in everything from our physical health to our emotional health to our politics, our communities, our very life spans.

Why should this be so? Did nature simply overload us in the mating department, hot-wiring us for the sex that is so central to the survival of the species, and never mind the sometimes sloppy consequences? Or is there something smarter and subtler at work, some larger interplay among sexuality, life and what it means to be human? Can evolution program for poetry, or does it simply want children?

If there's indeed much more than babies involved in the reasons for sex, we're clearly not the first species to benefit from that fact. Even among the nonhuman orders, sex appears to be regularly practiced for a whole range of nonreproductive reasons with a wide range of community-building benefits. How else to explain the fact that homosexual behavior occurs in more than 450 species? How else to explain kissing among bonobos, nuzzling among zebras, literal necking among male giraffes? How else to explain the fact that some sexually active animals seem to avoid reproduction quite deliberately, mating at times that are unlikely to produce young or picking partners that are unable to do so? From 80% to 95% of a species of sea lion rarely or never reproduce, though they continue to couple. And so of course do many of us, chasing sex as passionately as the most prolific of breeders.

"How many times in your life do you think about being sexual," asks clinical psychologist Joanne Marrow of California State University, Sacramento, "and how many of those times are you thinking about reproduction?" So what gives? And don't say simply that sex is fun. So are gardening and traveling and going to the movies, but when was the last time you woke up in the middle of the night with your heart pounding and your breath catching because of a dream you were having about a trip to Barcelona? Just as there's more to sex than babies, there's also more to it than fun.

Part of what makes touch—and by extension, sex—such a central part of the species software is that hedonism simply makes good Darwinian sense. It's not for nothing that hot stoves hurt and caresses feel nice, and we learn early on to distinguish between the two. "All creatures do things that feel good and avoid things that feel bad," says J. Gayle Beck, professor of psychology at the University of Buffalo. "The individuals who learn that best live the longest."

But mastering even so basic an idea can be a slow process, often too slow when survival is on the line. And so nature provides us with a head start. Before we have a chance to practice our first little Moro grab—before we leave the womb, in fact—our pleasure engine is humming. "Little boys can have erections from the day they're born, sometimes even in utero," says Marrow. "Both sexes get pleasure from touching themselves without having to be taught." Once we're in the world, both nature and experience reinforce that need for physical contact, turning us into full-blown tactile bacchanalians.

Nursing alone is a powerful reinforcer. The mechanics of animal nursing can be a utilitarian business, with wobbly-legged newborns standing up to drink from Mom as if she were a spigot. Human nursing, by contrast, requires flesh-on-flesh cuddling. What's more, a mother's metabolism ensures that this contact occurs more or less all day long. Anthropologist Sarah Blaffer Hrdy, professor emeritus at the University of California at Davis, points out that human beings produce very dilute breast milk, which necessitates frequent nursing sessions and therefore provides loads of opportunities for mother and child to touch.

The whole-body rapture found in Mom's arms lasts only through infancy, but children become expert at seeking the same security as they grow older, and good parents have a sixth sense about what the priorities are. A wailing child with a cut knee gets a long hug first, even though it's the bleeding wound that needs attention. In uncounted thousands of such tactile transactions, kids learn to use touch as a means of connection at least as expressive as—and certainly more satisfying than—anything so detached as speech. With the pump thus primed, they are ready for the next, exponentially bigger step: the moment, at age 12 or so, when the glands engage, the hormones flow and a childhood of simple physicality becomes a lifetime of sexuality.

From the moment the bodies of boys and girls are able to conceive, nature is very clear that it wants these mere babies to go about making babies of their own, and so it makes the impulse almost irresistible. There's a reason for the fabled sexual stamina of teens: the more frequent the pairings, the more likely the offspring. What's more, the pleasure of sex can often lead to long-term bonding, something else nature wants if babies and children—with their long years of dependency—are going to survive into adulthood.

But even at this unsophisticated stage of sexual maturation, there's more going on in kids than simply developing an exquisite reproductive itch and learning the wonderful ways it can be scratched. "More and more in our field, we don't even talk about sex anymore," says anthropologist Gil Herdt, director of the Program in Human Sexuality Studies at San Francisco State University. "We talk about sexuality. It's something that involves the entire person, the whole life course, not just the sexual acts."

Marrow agrees and takes the notion even further with the belief that human sexuality is a form of communication as much as it is of procreation. Nearly all creative acts are at least in part communicative. Songs are written to be sung to somebody else; pictures are painted to be hung for somebody else. Is it any surprise that sex—an act infinitely more intimate than any type of art—is also a creative way of communicating complex ideas and deep feelings? "The biologists think the biology comes first," Marrow says. "I think consciousness is the first part of sex, and exploring that consciousness with another person is one of its purposes." If Marrow is right, it's no wonder that poetry and music are often included in the business of romance, if only to make that message richer.

Of course, artistry—even something as small as a well-chosen greeting card or a romantic setting for dinner—may open the sexual door, but something else must keep it from closing again. What sustains a physical relationship after the early romantic rounds end is something more nuanced than seduction and more enduring than passion. Often it's something as wonderfully ordinary as stability. Partners who maintain a robust sex life are simply more likely to remain partners than those who don't, something almost any couple knew long before the sex researchers thought to quantify it. If it is hard to be physical with a mate you've stopped loving, it can be equally hard to get to that cold point with a person with whom you still share the intimacy, exclusivity and, especially, vulnerability of sex. This is particularly true as the intoxication of a new relationship begins to fade and partners start to notice flaws they were too romantically tipsy to see before.

Not only does the relationship benefit from a steady sex life, but so can the physical and emotional health of the partners themselves. Research suggests that married people may live longer than singles, that happily marrieds do best of all, and that couples who remain at least somewhat sexual, even into their dotage, report a better level of satisfaction both with their relationships and with their lives as a whole. Certainly, it's hard to say if people who start off happy and satisfied simply have more sex or if it's the sex that makes them happy and satisfied. Whatever the answer, it's clear that human beings would not be fully Homo sapiens—at least not as we've come to understand ourselves—without the great, mysterious, preposterous pageant of our sexuality.

• Jeffrey Kluger