Pucker Up! What Science Is Discovering About Why We Kiss

There's nothing like a good kiss. You lean into your beloved, close your eyes, hope your breath is OK, and...fade to black. But a kiss, as we're now learning from scientific research, is hardly just a kiss. Rather, it's a complex act of courtship that may help us pick our mate, perhaps even through unconscious chemical signals, and assess and maintain a relationship. "Kissing is not just lips meeting other lips," says Sarah Woodley, a biologist at Duquesne University in Pittsburgh.

Many philematologists—those who study kissing—agree that it began as a way for mothers to pass on pre-chewed food to their infants. Obviously it's come a long way since then, but scientists are beginning to tease out exactly what the prototypical long smooch between two adults means. What purpose does it serve?

As with so many other aspects of sexual behavior, the answer may be different for men and women. Gordon Gallup Jr., a psychologist at the State University of New York at Albany who in 2007 examined how kissing was perceived by more than 1,000 undergraduate students, found that the average female sees kissing as essential. Females "wouldn't dream" of having sex with someone without kissing first and were also much more likely to emphasize kissing during and after sex, says Gallup. It suggests that "females are much more prone to use kissing as a mate assessment device," he says. And even within an ongoing relationship, they use kissing as a way to update and monitor its status, he says.

Men, by contrast, use kissing as a means to an end—that is, sex. They were far more willing to have sex without kissing, the study found, and while a bad first kiss can be a deal-breaker for both genders, men are more willing to go ahead and have sex with someone whose kiss is unpleasant.

And how is all this assessment and evaluation being conducted? "At the moment of a kiss, there is a very complicated exchange of all kinds of different information," says Gallup. Some of it is in chemical form. Contained in male saliva, for example, are small amounts of testosterone, which is known to boost libido in both genders. It's possible, Gallup says, that repeated and prolonged exposure—that is, many make-out sessions over days and weeks and months—may increase the female's sex drive. (That might explain why men prefer sloppier kisses than women, too—they're trying to get her in the mood.)

There are probably also chemical messages being sent that we've barely begun to identify. While the term "pheromone" has been used to describe chemical messages detected by special sensory organs in many animals, its use to describe chemicals in humans has been more confusing, since we don't possess the same organs. But nomenclature aside, the idea that humans emit some kind of chemical signals, whether pheromones or something else, that are sensed by and may influence other humans, is pretty well accepted, says Woodley. The mechanism of transmission isn't nailed down; some may be transmitted by taste, while others might be present in saliva, then volatilized and sensed by the nasal cavity. We may able to smell some of these odors, or learn to, but others may be imperceptible, she says.

Some of these chemicals might play a role similar to the theorized role of testosterone in saliva, making females less tense and happier (and more prone to have sex). Or, the odors might help in assessing whether a potential mate is of high genetic quality or is genetically compatible, she says. Certain genes govern the "major histocompatibility complex," proteins involved in the immune system that help identify whether foreign bodies are part of us or not. (The similarity of an organ donor's and recipient's MHC plays a major role in determining whether the organ is accepted or rejected.) "These genes also make people smell different," says Woodley, and how you judge a potential mate's smell may indicate whether his immune system or general genetic makeup is similar to yours.

The evolutionary goal: to mate with someone whose genes are different enough to provide healthy diversity (i.e. not your brother) while still being in the same species. This notion hasn't always been borne out by research—which usually involves women sniffing men's sweaty T-shirts and rating the attractiveness of the odor—but it's still a viable theory, says Woodley. Something similar may be going on with kissing. These chemical signals might join other cues we get from the closeness and intimacy of kissing and combine to give an instant, hard-wired impression of whether this person is the right one with whom to mate. "When you get a highly intelligent, pair-bonding species that requires years to raise a baby, you evolve more and more brain mechanisms to weed out the losers and find out what works for you," says Helen Fisher, an anthropologist at Rutgers University. (Fisher has also looked into the broader question of what fuels romantic attraction.)