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

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. 

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.  

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.