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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.
But how does one female signal the rest? The answer is almost certainly smell. Pheromones—or scent-signaling chemicals—are known to exist among animals, and while scientists have had a hard time unraveling the pheromonal system in humans, they have isolated a few of the compounds. One type, known as driver pheromones, appears to affect the endocrine systems of others. Since the endocrine system plays a critical role in the timing of menstruation, there is at least a strong circumstantial case that the two are linked. "It's thought that there is a driver female who gives off something that changes the onset of menstruation in the other women," says chemist Charles Wysocki of the Monell Chemical Senses Center in Philadelphia.
It's not just women who respond to such olfactory cues. One surprising study published last October in the journal Evolution and Human Behavior showed that strippers who are ovulating average $70 in tips per hour; those who are menstruating make $35; those who are not ovulating or menstruating make $50. Other studies suggest that men can react in more romantic ways to olfactory signals. In work conducted by Martie Haselton, an associate professor of psychology at UCLA, women report that when they're ovulating, their partners are more loving and attentive and, significantly, more jealous of other men. "The men are picking up on something in their partner's behavior that tells them to do more mate-guarding," Haselton says.
Scent not only tells males which females are primed to conceive, but it also lets both sexes narrow their choices of potential partners. Among the constellation of genes that control the immune system are those known as the major histocompatibility complex (MHC), which influence tissue rejection. Conceive a child with a person whose MHC is too similar to your own, and the risk increases that the womb will expel the fetus. Find a partner with sufficiently different MHC, and you're likelier to carry a baby to term.