Why do we kiss?

Why Do We… is a biweekly column written by Nicholas G Norwitz ’18.

“Did you kiss her?”

“Is he a good kisser?”

“You may now kiss the bride.”

Am I the only one who finds it odd that the act of pressing ones lips against the lips of another is so significant in our culture? While there are many social reasons that humans might invented the kiss, I personally find the evolutionary perspective the most interesting. So how is kissing evolutionarily adaptive?

Kissing promotes procreation by acting on women as an aphrodisiac. During a kiss, a man passes testosterone to his partner via his saliva, which supposedly increases his partner’s sex drive. Kissing can lead to sex, but social intuition tells me that saliva is a double-edged sword. So be careful guys, I don’t think most women appreciate copious amounts of spit in their mouth.

In addition to promoting procreation, kissing may help to maximize the fitness of a couple’s offspring. Generally, fitness improves in parallel genetic diversity. So if a woman had a way to sample her mate and determine if he was genetically complimentary to her, she would be able to bear healthier children. And, you guessed it, kissing serves exactly that purpose. While they may not be aware of it, women can taste a class of immune proteins major histocompatibility complexes (MHCs) that are also present in saliva. Thus, by kissing, women can determine the genetic compatibility of her partner.

The field of kissing biology (or philematology) is surprisingly complex and far from complete. And as an aside, the longest kiss lasted 58 hours and 36 minutes. I wonder what were they trying to do, sequence each other’s genomes?