Ask Smithsonian: Why Do We Kiss?

Love snuggling up to a sweetie and smooching? That's romantic, but—spoiler alert—kissing can be a disgusting and dangerous activity.

While kissing, couples exchange 9 milliliters of water, 0.7 milligrams of protein, 0.18 mg of organic compounds, 0.71 mg of fats, and 0.45 mg of sodium chloride, along with 10 million to 1 billion bacteria, according to one accounting. Many pathological organisms can be transmitted through mouth-to-mouth contact, including those that cause colds and other respiratory viruses, herpes simplex, tuberculosis, syphilis and strep.

That last part doesn’t sound too romantic, but romance has very little to do with why we, as a species, are drawn to this very intimate contact. Humans are biologically driven to push their faces together and rub noses or touch lips or tongues.

At its most basic, kissing is a mating behavior, encoded in our genes. We share the vast majority of those genes with the mammalian species, but only humans (and occasionally our close primate relatives like chimps and bonobos) kiss.

But the reason for kissing is still mostly a mystery, even to scientists who have spent decades studying the behavior. It’s not possible to say which is the overriding factor: that people kiss because of a psychological attraction, or because of a subconscious urge to mate with the chosen kiss-ee. Most likely, it’s a combination of the two. “You can’t have psychology without a biological brain,” says Rafael Wlodarski, who has devoted much of his career to philematology—the science of kissing.

Wlodarski, a postdoctoral researcher with Oxford University’s social and evolutionary neuroscience research group, has found that kissing helps heterosexuals select a mate. Women in particular value kissing early on. Saliva is full of hormones and other compounds that may provide a way of chemically assessing mate suitability—that’s the biological brain stepping in.

Women are also more likely to say that a first kiss could be the decider for selecting a mate. Can the biological drive overcome the perception that your chosen one is a bad kisser? Wlodarski says it’s hard to separate the two, but that “I would hazard a guess that if someone thinks someone is a bad kisser it’s because their smell wasn’t right,” he says. Women have to be more selective because they face greater consequences when they make a poor mating decision—like having to carry a baby for nine months, says Wlodarski.

Kissing in heterosexual relationships—for both men and women, but particularly women—also cements the intimacy bond over the length of a relationship, says Wlodarski. Interestingly, Wlodarski and his Oxford colleagues have found that people who kiss more frequently seemed to be happier and more satisfied in their relationships, whereas intercourse frequency did not make a difference.

Wlodarski says he’s hoping to determine why kissing makes people feel more bonded. That is one of many unanswered questions about kissing—and that’s only for heterosexuals. Researchers are just scratching the surface in understanding kissing behavior in homosexuals, he says. “It’s an extra level of complexity.”

And what about non-sexual kissing? Even though it may not be a mating device, it still probably arose out of that biological imperative, says Wlodarski. A kiss on the cheek is an evolutionary modification that’s shown up in larger, more complex societies where it’s a sign of respect or admiration. 

Not every culture is down with the full-on mouth kissing enlivened by a wandering tongue. That seems to be a modern, and Western, convention, perhaps from the last 2,000 years, says Wlodarski. A study published in 2015 found that less than half of the cultures surveyed engage in romantic, sexual kissing.

There’s evidence—at least from written history—that in the past, kissing was primarily mutual face or nose rubbing, or even sniffing in close proximity. In Hindu Vedic Sanskrit texts, kissing was described as inhaling each other’s soul.

Now that does sound romantic.

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Alicia Ault | | READ MORE

Alicia Ault is an independent journalist whose work has appeared in the New York Times, the Washington Post, Medscape Medical News, WebMD and Wired. She splits her time between St. Petersburg, Florida, Washington, D.C., and New Orleans, exploring the rich variety of food, music, art and culture on offer in those cities.