What Science is finding out about why we kiss.

You may be surprised what messages you're sending and receiving when you lock lips.
By Katherine Hobson, U.S. News & World Report

There's nothing like a good kiss. You lean toward your beloved, close your eyes, hope your breath is okay, 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, Ph.D., 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?

The role of testosterone
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.

Kissing and stress reduction

Kissing may also reinforce pair bonding, helping to maintain relationships. Research by Wendy Hill, Ph.D., a neuroscientist at Lafayette College, in Easton, Pa., 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 co-authors 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."

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