Why Do Humans Kiss Each Other? Here's The Science Of Kissing

Written by Amari D. Pollard
Amari D. Pollard is a writer and audience development strategist. Her writing focuses on politics, culture, relationships, and health, and she has been published at The Week, Bustle, PopSugar, Reader's Digest, and more.
Expert review by Kristina Hallett, Ph.D., ABPP
Board-certified Clinical Psychologist
Kristina Hallett, Ph.D., ABPP is a board-certified clinical psychologist, Director of Clinical Training at Bay Path University, and an associate professor in Graduate Psychology. She has a private practice in Suffield, Connecticut.
Couple Embracing in a Field

Image by Caleb Gaskins / Stocksy

We've been taught that kissing is something you do when saying hello and goodbye, when dating potential suitors, or continuing on in a long-term relationship. Yet, for all the kisses we receive throughout our lifetime, does anyone know why we kiss in the first place? While the act of kissing may feel normal and instinctive to many, it's not practiced in every culture—and when you think about it, it's actually kind of gross transferring all that saliva and bacteria. According to the experts, here's why we kiss.

Why do humans kiss when other animals don't?

Evolutionary psychologist and University at Albany professor Gordon Gallup Jr., Ph.D., says kissing may have evolved as a primitive feeding gesture between mother and child, where the mother chews up small portions of food and then transfers it to her baby. Biological anthropologist Helen Fisher, Ph.D., adds that another very important part of human evolution is the courtship and mating process. "If you never fall in love with somebody and never form a partnership, you're never going to have any babies," she explains.

There are a vast array of kisses, from perfunctory to serious, but there is something particularly special about romantic and sexual kisses. One of the benefits of kissing is that it allows for loads of data to be exchanged, which then makes it possible for people to unconsciously assess their potential and permanent partners. The lips are one of the thinnest layers of skin on the human body and densely populated with nerve endings, explains Fisher. This allows people to pick up on the temperature, taste, and smell of someone. Through smell, people are able to assess all kinds of information about others, such as the health of their immune system or their fertility.

Evolutionary anthropologist and University of Kent professor Sarah Johns, Ph.D., suggests that's why some cultures may not need to kiss in order to gain certain knowledge. When people are wearing less clothing or bathing less, they are able to tell that information without needing to get too close. She also notes that "exposure to showing people kissing in a romantic way may have an impact on people thinking it's more desirable," along with exposure to people of higher statuses practicing kissing because people are more likely to emulate them. 

When looking to humans' closest relatives, primates, kissing actually is a common practice, says Fisher. And while there isn't enough data collected on all animal habits, there is enough evidence to show that face touching, face rubbing, or face licking is involved across many species. Those actions put "participants into such close contact that there is still exchange of intimate, potentially relevant genetic and reproductive information. So it does appear to be a hardwired courtship strategy," confirms Gallup. 

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How kissing influences our relationships.

A kiss is so powerful that it can determine whether a relationship begins, ends, or continues. A University of Alabama study conducted by Gordon and his colleagues showed that there is such a thing as a deal-breaking kiss—or, as Fisher likes to call it, "a kiss of death." In a survey of over 1,000 college students, the majority of both male and female students found themselves attracted to someone in one or more instances, only to discover they were no longer interested after kissing for the first time.

However, just because there are deal-breaking kisses does not mean the people in question are bad kissers. Gordon asserts that someone who "may be a good kisser for one person may be a bad kisser for another." In other words, if you find someone to be a good kisser, that may be a reflection of the fact that that particular person is a good genetic match for you.  

According to Match's 2018 Singles in America study, which Fisher helps lead, 81% of men and 62% of women felt it was appropriate to kiss on the first date. This again demonstrates that kissing is used as a mate assessment tool. That said, men and women tend to report kissing for very different reasons. Men tend to kiss as a means of gaining sexual favors or as a way to achieve reconciliation, says Gordon. Women on the other hand tend to kiss as a means of establishing a romantic relationship and monitoring the status of a relationship. Fisher adds that women use kissing as a way to look further down the road, meanwhile asking themselves important questions such as: Do they like this person well enough? Would they make a good partner? Are they patient?

Why does kissing feel good?

When the millions of nerve endings on your lips are activated with a kiss, it makes you feel good because of the signals it transfers to your brain. Gordon says there is evidence that some people remember the details of their first kiss far better than they remember the details of their first sexual encounter. So it's very possible that a novel kiss drives up the dopamine system in the brain, affirms Fisher, and then pushes people over the threshold of falling in love because dopamine is associated with feelings of intense romantic love. 

According to Fisher, humans have evolved three distinctively different brain systems for mating and reproduction: sex drive, feelings of intense romantic love, and feelings of deep attachment. Saliva contains a lot of testosterone, which helps to trigger the sex drive. And since the internal cheek cells are well built to absorb testosterone, wet kisses cause arousal. As explained earlier, romantic love is triggered by dopamine. And then the third brain system, attachment, is impacted by oxytocin, sometimes referred to as the "cuddle hormone." Oxytocin brings about feelings of emotional closeness and "cosmic union," explains Fisher. She also says that in long-term couples, a kiss not only increases levels of oxytocin but reduces cortisol, the body's main stress hormone. That makes kissing a powerful tool for mating.

To put it plainly, there is nothing simple about a kiss. "It's a hardwired, evolved, largely unconscious interaction that occurs and subserves a basic reproductive biological function that most people have no insight into whatsoever," says Gordon. And like so many other things in life, humans have no control over how kissing affects our behavior. We're just along for the ride.

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