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.

Looking at oxytocin levels, the researchers found that they increased only in the males, whereas the researchers thought it would increase in both sexes. They hypothesized that it could be that women need more than a kiss to stimulate attachment and bonding, or that the sterile environment of the research lab wasn’t conducive to creating a feeling of attachment.

Kissing, therefore, plays a role not only in mate selection, but also in bonding. At an Association for the Advancement of Science meeting on the science of kissing, Helen Fischer, an evolutionary biologist, posits multiple reasons for lip locking. She believes that kissing is involved in the three main types of attraction humans have: sex drive, which is ruled by testosterone; romantic love, which is ruled by dopamine and other feel-good hormones; and attachment, which involves bonding chemicals like oxytocin. Kissing, she postulates, evolved to help on all three fronts. Saliva, swapped during romantic kisses, has testosterone in it; feel-good chemicals are distributed when we kiss that help fuel romance; and kissing also helps unleash chemicals that promote bonding, which provides for long term attachment, necessary for raising offspring.

Sniff, Snuggle, and Turn Right
Yet, not all cultures or mammals kiss. Some mammals have close contact with each others’ faces via licking, grooming, and sniffing, which may transmit the necessary information. And although chimps may pass food from mother to child, the notoriously promiscuous bonobos are apparently the only primates that truly kiss. And while it’s thought that 90 percent of the human population kisses, there’s still the 10 percent that doesn’t. So it seems that as much as we use kissing to gather genetic and compatibility information, our penchant for kissing also has to do with our cultural beliefs surrounding it.

Whether we live in a place where kissing is reserved for close acquaintances, or somewhere where a casual greeting means a one, two, or three cheeker, one thing does remain highly consistent: the side to which people turn while kissing. It’s almost always to the right. A 2003 study published in Nature found that twice as many adults turn their heads to the right rather than the left when kissing. This behavioral asymmetry is thought to stem from the same preference for head turning during the final weeks of gestation and during infancy.

One of the best things about kissing, however, is that we don’t have to think about any of this. Just close eyes, pucker up, and let nature takes its course.