Imagine you’re at a bar with your friends. You notice someone you find strikingly attractive, stirring her drink. She notices you; she gets up and slowly saunters over. As she makes her way into earshot, she spits into her hand and whispers in your ear, “Would you be interested in tasting this?”
That’s how Rafael Wlodarski, an evolutionary behavioralist specializing in human sexual behavior at the University of Oxford describes his research on kissing (a field called “philematology,” by the way). Yes, it’s complicated, and a little gross.
The physical act of kissing is pretty well understood. On a biological level, when two people kiss, they’re actually engaging in a good, old-fashioned bodily fluid exchange. According to an editorial in the American Journal of Medicine, we swap “an average of 9 mL of water, 0.7 mg of protein, 0.18 mg of organic compounds, 0.71 mg of different fats, and 0.45 mg of sodium chloride” when we lock lips.
We probably also trade about 80 million bacteria (of about 300 species) in the act. That might sound bad, and certainly, some of these bacteria can be pathogens, ranging from strains of the coldsore-causing herpes virus to strep throat and even tuberculosis. But the majority of these tiny life forms are benign. And it really isn’t so appalling when you consider that giving someone a handshake (paywall) likely exchanges around 124 million bacteria.
Kissing can also burn calories, although probably not substantially: Estimates suggest that couples can burn anywhere between two and 26 calories per minute while kissing, and can use up to 30 different muscles. For comparison, walking at 3 mph burns about five calories per minute.
Kissing, in its most basic, objective form, is basically a saliva square-dance. But why humans engage in that dance is the subject of much debate.
Technically, what Westerners think of as kissing (locking lips) originated about 2,000 years ago, and it’s not ubiquitous. A 2015 study surveying 168 cultures reported that fewer than half engage in “lip-to-lip contact that may or may not be prolonged.” That said, versions of face-to-face touching and intimacy are seen in almost every culture, says Wlodarski. Inuit kissing, for example, ”involves placing the nose on the partner’s cheek and forehead and inhaling deeply.”
Studies on primates hypothesize that our animal relatives use kissing to facilitate conflict resolution and bonding. After an altercation, they kiss and make up. Chimps and other animals groom each other by licking. In theory, these behaviors could have stuck with us as evolution progressed. In studies on people, there is some evidence to suggest that kissing provides us a means of protecting ourselves from certain viruses (paywall). Or, it may just feel pleasant: our lips are one of the most sensitive parts of our bodies, right up there with our finger tips. It’s almost impossible to prove that any behaviors are carried down overtime for a genetic reason; some may be just culturally learned.
Wlodarski’s research seems to confirm that kissing—at least in a heterosexual context, and especially for women—may play a role in selecting mates. In a paper published (paywall) in 2013, Wlodarski and his team surveyed over 300 men and just under 600 women and found that women tended to prioritize a partner’s ability to kiss more than men did—especially when first assessing their partners for a relationship. Women also tended to view kissing as an important way to show affection in long-term relationships. The same study found that there is “very little evidence to support the hypothesis that the primary function of kissing is to elevate levels of arousal.” A later 2014 study found that being a good kisser can make some people more attractive for short-term relationships.
In a different 2013 survey (paywall) of 80 women, Wlodarski found that when first assessing a partner, women tended to place more importance on kissing right before they ovulate (when they are most fertile).
Kissing also “seems to be an activity that is very well placed to help individuals get close enough together to sample each other’s pheromones,” which may play a role in human attraction and mating behavior, Wlodarski says. Though pheromones—chemicals that give off different odors—have been shown to play a role in certain animal behaviors, their role in human interactions are a little more ambiguous.
Needless to say, the study of kissing is an inherently imperfect science, as Wlodarski notes in Smithsonian. Something we do know: A kiss really isn’t “just a kiss.”