The last time you had sex, there was arguably not a thought in your head. O.K., if it was very familiar sex with a very familiar partner, the kind that--truth be told--you probably have most of the time, your mind may have wandered off to such decidedly nonerotic matters as balancing your checkbook or planning your week. If it was the kind of sex you shouldn't have been having in the first place--the kind you were regretting even as it was taking place--you might have already been flashing ahead to the likely consequences. But if it was that kind of sex that's the whole reason you took up having sex in the first place--the out-of-breath, out-of-body, can-you-believe-this-is-actually-happening kind of sex--the rational you had probably taken a powder.

Losing our faculties over a matter like sex ought not to make much sense for a species like ours that relies on its wits. A savanna full of predators, after all, was not a place to get distracted. But the lure of losing our faculties is one of the things that makes sex thrilling--and one of the very things that keeps the species going. As far as your genes are concerned, your principal job while you're alive is to conceive offspring, bring them to adulthood and then obligingly die so you don't consume resources better spent on the young. Anything that encourages you to breed now and breed plenty gets that job done.

But mating and the rituals surrounding it make us come unhinged in other ways too, ones that are harder to explain by the mere babymaking imperative. There's the transcendent sense of tenderness you feel toward a person who sparks your interest. There's the sublime feeling of relief and reward when that interest is returned. There are the flowers you buy and the poetry you write and the impulsive trip you make to the other side of the world just so you can spend 48 hours in the presence of a lover who's far away. That's an awful lot of busywork just to get a sperm to meet an egg--if merely getting a sperm to meet an egg is really all that it's about.

Human beings make a terrible fuss about a lot of things but none more than romance. Eating and drinking are just as important for keeping the species going--more so actually, since a celibate person can at least continue living but a starving person can't. Yet while we may build whole institutions around the simple ritual of eating, it never turns us flat-out nuts. Romance does.

"People compose poetry, novels, sitcoms for love," says Helen Fisher, an anthropologist at Rutgers University and something of the Queen Mum of romance research. "They live for love, die for love, kill for love. It can be stronger than the drive to stay alive."

On its good days (and love has a lot of them), all this seems to make perfect sense. Nearly 30 years ago, psychologist Elaine Hatfield of the University of Hawaii and sociologist Susan Sprecher now of Illinois State University developed a 15-item questionnaire that ranks people along what the researchers call the passionate-love scale (see box, page 60). Hatfield has administered the test in places as varied as the U.S., Pacific islands, Russia, Mexico, Pakistan and, most recently, India and has found that no matter where she looks, it's impossible to squash love. "It seemed only people in the West were goofy enough to marry for passionate love," she says. "But in all of the cultures I've studied, people love wildly."

What scientists, not to mention the rest of us, want to know is, Why? What makes us go so loony over love? Why would we bother with this elaborate exercise in fan dances and flirtations, winking and signaling, joy and sorrow? "We have only a very limited understanding of what romance is in a scientific sense," admits John Bancroft, emeritus director of the Kinsey Institute in Bloomington, Ind., a place where they know a thing or two about the way human beings pair up. But that limited understanding is expanding. The more scientists look, the more they're able to tease romance apart into its individual strands--the visual, auditory, olfactory, tactile, neurochemical processes that make it possible. None of those things may be necessary for simple procreation, but all of them appear essential for something larger. What that something is--and how we achieve it-- is only now coming clear.

The Love Hunt

If human reproductive behavior is a complicated thing, part of the reason is that it's designed to serve two clashing purposes. On the one hand, we're driven to mate a lot. On the other hand, we want to mate well so that our offspring survive. If you're a female, you get only a few rolls of the reproductive dice in a lifetime. If you're a male, your freedom to conceive is limited only by the availability of willing partners, but the demands of providing for too big a brood are a powerful incentive to limit your pairings to the female who will give you just a few strong young. For that reason, no sooner do we reach sexual maturity than we learn to look for signals of good genes and reproductive fitness in potential partners and, importantly, to display them ourselves.

"Every living human is a descendant of a long line of successful maters," says David Buss, an evolutionary psychologist at the University of Texas at Austin. "We've adapted to pick certain types of mates and to fulfill the desires of the opposite sex."

One of the most primal of those desires is that a possible partner smells right. Good smells and bad smells are fundamentally no different from each other; both are merely volatile molecules wafting off an object and providing some clue as to the thing that emitted them. Humans, like all animals, quickly learn to assign values to those scents, recognizing that, say, putrefying flesh can carry disease and thus recoiling from its smell and that warm cookies carry the promise of vanilla, sugar and butter and thus being drawn to them. Other humans carry telltale smells of their own, and those can affect us in equally powerful ways.

The best-known illustration of the invisible influence of scent is the way the menstrual cycles of women who live communally tend to synchronize. In a state of nature, this is a very good idea. It's not in a tribe's or community's interests for one ovulating female to monopolize the reproductive attention of too many males. Better to have all the females become fertile at once and allow the fittest potential mates to compete with one another for them.