4 Fascinating Facts about Kissing

A scientist writes a book about the evolution of the kiss.

girl and guy kissing
(Image credit: Kateryna Govorushchenko)

The Science of Kissing, a new book by scientist Sheril Kirshenbaum, looks at a topic of interest to all of us — the touching of two person's lips. Kirshenbaum examines the evolutionary history of the kiss, as well as the (literal) chemistry of smooching. I asked her to give us a few fun facts about kissing.

1. Our pupils dilate during a passionate kiss, which may be part of the reason so many of us close our eyes.

When we smooch, the autonomic nervous system responds to the excitement, sexual arousal, or novelty by making our pupils bigger, which means they allow more light in — which causes our eyes to get more sensitive. This is my favorite kissing fun fact, and I've taken to squinting at Sweet Pants whenever we kiss, and saying, "Do my pupils look dilated?"

2. During a French kiss, a man passes testosterone to a woman through his saliva.

Over weeks and months, the repeated exchange may serve to enhance her libido. Why is that? Among other things, it seems testosterone stimulates the clitoris.

3. Scientific evidence suggests that a kiss can subconsciously reveal information about the genetics of the person being kissed, guiding us to instinctively sense whether or not he's a good match.

When we kiss another person, all of our senses are at work evaluating compatibility — even our sense of smell. Have you ever kissed someone only to realize that your romantic feelings disappeared? That's a fairly common reaction. According to evolutionary psychologist Gordon Gallup of the University of Albany, 59 percent of men and 66 percent of women have ended a budding relationship because of a bad kiss — and it turns out that our sense of smell may have influenced the decision. Why? Another person's natural scent — a smell which we may not necessarily register consciously, or may not necessarily notice as unpleasant — can provide clues about his reproductive status, or his DNA, that we respond to on a deep level. For instance, women are most attracted to men who possess genes that would create very different immune systems from their own — the idea being that a man with a different immune system would put a woman at a genetic advantage, enabling her to have a child with a versatile immune system. And on a subconscious level, we can pick up clues about a man's immune system by smelling him. So, a kiss is really a nice excuse to help us get close enough to sample the other person's scent!

4. A woman's breath can provide clues to a man about her reproductive status.

During menstruation, a woman typically has excess bacteria in her mouth, which often causes bad breath — and that has historically been helpful to men when trying to choose the most fertile woman to impregnate, or so the theory goes.