Kissing is usually a show of affection between two people. A kiss can be passionate, aggressive, flirty or used as simple greeting. But why do we do it? What’s the science behind our desire to kiss one another?

Why Do We Kiss?

We can thank both social and evolutionary factors for our desire to exchange saliva. Our sociological desire to kiss stems from social bonding tendencies between adult humans, and can also be seen in other social mammals. Our desire to bond as adults is built off of the same basic hormonal and neurological framework which bonds mother and child. Primates, for example, will often kiss their babies to exchange pre-chewed food, an action which likely has been expanded upon, and generalized to adult affiliative behavior. Affiliative behavior can release oxytocin (often called the “love hormone”) which helps promote pair bonding and increases intimacy, making it easier for couples to remain together.

You Are Being Judged

Another reason why people may kiss is because it allows them to judge the immune system of a prospective mate. For offspring to have the best chances of survival, a strong immune system is desired. One way of ensuring this, is through maximum genetic diversity. Human females have a subconscious ability to detect MHC (major histocompatibility complex) compatibility, and in studies, have shown to prefer mates with a good match — this desire for diversity leads to a stronger and more robust immune system in their offspring.

Protection

It has also been shown that kissing does, in fact, give you cooties. A 2009 study published in the in the journal Medical Hypotheses concluded that kissing helps women build up a tolerance to cytomegalovirus, which lives in human saliva. The virus, normally innocuous, can be dangerous when its introduced during a pregnancy. It can kill unborn fetuses or cause birth defects like cerebral palsy and deafness. However, if cytomegalovirus is introduced to a woman in small amounts before she gives birth, she will build up an immunity to it, rendering the baby safe.

It should be noted that not all cultures kiss. This may mean that the behavior is not hardwired and can be modified by culture. If kissing in a particular culture is seen as taboo, people may not do it even if there is a biological urge.

While there is no one single reason we can point to as the cause for our urge to swap spit, one thing seems to be agreed upon – it certainly feels good.

Bonus Facts:

The first on-screen kiss was shot by the Edison Company in 1896. The film was 30 seconds long and consisted entirely of a man and a woman kissing close up.

Your lips have 100x more nerve endings than in your fingertips.

One British couple kissed for just over 31 straight hours in 2005, making it the longest kiss ever according to Guinness World Records.

The first mention of a kiss in writing was in the four Vedic Sanskrit texts which date back to 1500 B.C.





References:

Scientific American – January 2008

Western Journal of Communication 73 (2): 113–133. doi:10.1080/10570310902856071

Irenäus (1983). “A comparative approach to human ethology”. In Rajecki, D. W.. Comparing behavior: studying man studying animals. Routledge

C.A. Hendrie, G. Brewer: “Kissing as an evolutionary adaptation to protect against Human Cytomegalovirus-like teratogenesis” Medical Hypotheses 74 (2010) 222–224

