Why We Kiss: The Science of Sex

Pecking, smooching, Frenching, and playing tonsil-hockey—there are as many names for kissing as there are ways to do it. Whether we use it as an informal greeting or an intensely romantic gesture, kissing is one of those ingrained human behaviors that seems to defy explanation. Its many purposes—a blow and peck for good luck on dice, lips to ground after a rocky boat ride, kisses in the air to an acquaintance, and the long slow smooches of Hollywood—have different meanings yet are similar in nature. So why is it that we love to pucker up?

A Kiss Isn’t Just a Kiss

Philematologists, the scientists who study kissing, aren’t exactly sure why humans started locking lips in the first place. The most likely theory is that it stems from primate mothers passing along chewed food to their toothless babies. The lip-to-lip contact may have been passed on through evolution, not only as a necessary means of survival, but also as a general way to promote social bonding and as an expression of love.

But something’s obviously happened to kissing since the time of the chewed-food pass. Now, it’s believed that kissing helps transfer critical information, rather than just meat bits. The kissing we associate with romantic courtship may help us to choose a good mate, send chemical signals, and foster long-term relationships. All of this is important in evolution’s ultimate goal—successful procreation. 

Kissing allows us to get close enough to a mate to assess essential characteristics about them, none of which we’re consciously processing. Part of this information exchange is most likely facilitated by pheromones, chemical signals that are passed between animals to help send messages. We know that animals use pheromones to alert their peers of things like mating, food sources, and danger, and researchers hypothesize that pheromones can play a role in human behavior as well. Although the vomeronasal organs, which are responsible for pheromone detection and brain function in animals, are thought to be vestigial and inactive in humans, research indicates we do communicate with chemicals.

The first study to indicate that chemical signals play a role in attraction was conducted by Claud Wedekind over a decade ago. Women sniffed the worn t-shirts of men and indicated which shirts smelled best to them. By comparing the DNA of the women and the men, researchers found that women didn’t just chose their favorite scent randomly. They preferred the scent of man whose major histocompatibility complex (MHC)—a series of genes involved in our immune system—was different from their own. Having a different MHC means less immune overlap and a better chance of healthy, robust offspring. Kissing may be a subtle way for women to assess the immune compatibility of a mate, before she invests too much time and energy in him. Perhaps a bad first kiss means more than first date jitters—it could also mean a real lack of chemistry.

Men Sloppy, Women Choosy

Behavioral research supports this biological reasoning. In 2007, researchers at University of Albany studied 1,041 college student and found significant differences in how males and females perceived kissing. Although common in courtship, females put more importance on kissing, and most would never have sex without kissing first. Men, on the other hand, would have sex without kissing beforehand; they would also have sex with someone who wasn’t a good kisser. Since females across species are often the choosier ones when it comes to mate selection, these differences in kissing behavior make sense.

Men are also more likely to initiate French kissing and researchers hypothesize that this is because saliva contains testosterone, which can increase libido. Researchers also think that men might be able to pick up on a woman’s level of estrogen, which is a predictor of fertility.

Crazy for Canoodling

But kissing isn’t all mating practicality; it also feels good. That’s because kissing unleashes a host of feel-good chemicals, helping to reduce stress and increase social bonding. Researcher Wendy Hill and colleagues at Lafayette College looked at how oxytocin, which is involved in pair bonding and attachment, and cortisol, a stress hormone, changed after people kissed. Using a small sample of college couples that were in long-term relationships, they found cortisol levels decreased after kissing. The longer the couples had been in a relationship, the farther their levels dropped. Cortisol levels also decreased for the control group—couples that just held hands—indicating that social attachment in general can decrease stress levels, not just kissing.