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.)
Kissing may also reinforce pair bonding, helping to maintain relationships. Research by Wendy Hill, a neuroscientist as well as provost and dean of the faculty at Lafayette College, has shown that kissing reduces levels of the stress hormone cortisol. In a study to be presented on Valentine's Day at the annual meeting of the American Association for the Advancement of Science, Hill and her coauthors report that a second study, also small, found that the longer the relationship between the two parties, the more cortisol levels declined. (So did cortisol levels in the control group of couples who talked softly rather than kissing.) Hill is also studying the role of oxytocin, a social-bonding hormone that promotes calm and attachment; previous research has suggested it rises during kissing, at least in men.
Whatever we have yet to learn about the science of kissing, there's no denying that it seems to act as an indicator of how a twosome is doing. "In an established relationship, the frequency of kissing is a good barometer as to its status," says Gallup. "If it's no longer featured prominently or is entirely absent, there's a much higher probability that the relationship is in trouble."