Kissing is actually pretty weird when you think about it. Of course, we’re often too distracted by excitement (or nerves) when we kiss someone to think about it that hard. But if you pause to consider it, you’ll realize that humans are among the only animals that naturally kiss.
What purpose does kissing serve, and why have we been doing it for so long? For that matter, have we always been kissing, or is it a relatively new invention? While researchers may never be able to answer those questions with certainty, they’ve come up with some interesting theories—here are the most convincing ones so far.
It Provides Us with a Rich Sensory Experience
One niche theory suggests that we desire kissing because our brains desire stimulation of our most sensitive body parts—like the lips.
The most sensitive parts of the body activate greater parts of the brain when touched. When you use your super-sensitive lips and tongue, the brain responds in a big way, even though these body parts are small. The somatosensory, or tactile, part of the brain devotes more neurons to signals from the lips than to signals from duller body parts, like your legs or back.
Therefore, kissing stimulates lots of neurons for two people at the same time. It could be that we seek it out simply because it’s such a deep sensory experience—and because we get to share that experience with another person.
It Gives Us Good Hormones
While this theory doesn’t explain why humans started kissing in the first place, it would explain why we keep doing it.
When we kiss, our brain releases hormones like oxytocin that make us feel good. Oxytocin is partially responsible for the feelings of closeness that we usually know as “love.” Dopamine is another feel-good hormone that gets released when we kiss someone we care about. Kissing also appears to reduce our levels of cortisol, the stress hormone.
We naturally crave these hormones and seek them out where we can get them. Kissing offers a fast track to releasing them, so we are motivated to keep doing it.
It Helps Us Choose Sexual Partners
In many modern cultures, kissing on the lips can be a precursor to sex. And, in that role, kissing actually seems to serve a very important purpose.
In some studies, for example, women have reported that kissing helps them choose new partners. There might be a biological reason for that: the chemicals in saliva, some researchers think, have the power to tell us who will make a good mate. Saliva is rich with information that our brains could be subconsciously responding to when we kiss someone.
Other researchers think that kissing takes the place of the strong-smelling pheromones that other animals use to find mates. Most animals can detect each other’s pheromones from far away, so there’s no need for kissing. But humans have weak senses of smell, so kissing might be the only way to get close enough to get the information we need.
It Came from Early Humans
Another theory is that modern kissing “evolved” from the early-human practice of feeding infants by mouth, or transmitting food that had already been chewed up. Over time, this mouth-to-mouth contact may have developed from a practical way to feed a toothless infant, to a meaningful way to show someone you care.
Do All Humans Kiss?
While all of these theories are interesting, they’re all complicated by one fact: not all humans actually kiss.
In fact, some research suggests that less than half of all cultures have romantic kissing. So, either we are natural kissers and some cultures repressed the desire, or we aren’t natural kissers and some cultures developed kissing anyway.
We may never know exactly how or why kissing developed, but science has certainly uncovered some powerful motivations to keep doing it. So, don’t worry about the millions of (mostly harmless) bacteria that kissing can transmit: at least you’re getting fun hormones and valuable biological information in the deal.