A Kiss Isn’t Just a Kiss

Philematologists, the scientists who study kissing, aren’t exactly sure why humans started locking lips in the first place. The most likely theory is that it stems from primate mothers passing along chewed food to their toothless babies. The lip-to-lip contact may have been passed on through evolution, not only as a necessary means of survival, but also as a general way to promote social bonding and as an expression of love.
But something’s obviously happened to kissing since the time of the chewed-food pass. Now, it’s believed that kissing helps transfer critical information, rather than just meat bits. The kissing we associate with romantic courtship may help us to choose a good mate, send chemical signals, and foster long-term relationships. All of this is important in evolution’s ultimate goal—successful procreation. 
Kissing allows us to get close enough to a mate to assess essential characteristics about them, none of which we’re consciously processing. Part of this information exchange is most likely facilitated by pheromones, chemical signals that are passed between animals to help send messages. We know that animals use pheromones to alert their peers of things like mating, food sources, and danger, and researchers hypothesize that pheromones can play a role in human behavior as well. Although the vomeronasal organs, which are responsible for pheromone detection and brain function in animals, are thought to be vestigial and inactive in humans, research indicates we do communicate with chemicals.
The first study to indicate that chemical signals play a role in attraction was conducted by Claud Wedekind over a decade ago. Women sniffed the worn t-shirts of men and indicated which shirts smelled best to them. By comparing the DNA of the women and the men, researchers found that women didn’t just chose their favorite scent randomly. They preferred the scent of man whose major histocompatibility complex (MHC)—a series of genes involved in our immune system—was different from their own. Having a different MHC means less immune overlap and a better chance of healthy, robust offspring. Kissing may be a subtle way for women to assess the immune compatibility of a mate, before she invests too much time and energy in him. Perhaps a bad first kiss means more than first date jitters—it could also mean a real lack of chemistry.

Men are also more likely to initiate French kissing and researchers hypothesize that this is because saliva contains testosterone, which can increase libido. Researchers also think that men might be able to pick up on a woman’s level of estrogen, which is a predictor of fertility.

 
Sniff, Snuggle, and Turn Right
 
Yet, not all cultures or mammals kiss. Some mammals have close contact with each others’ faces via licking, grooming, and sniffing, which may transmit the necessary information. And although chimps may pass food from mother to child, the notoriously promiscuous bonobos are apparently the only primates that truly kiss. And while it’s thought that 90 percent of the human population kisses, there’s still the 10 percent that doesn’t. So it seems that as much as we use kissing to gather genetic and compatibility information, our penchant for kissing also has to do with our cultural beliefs surrounding it.
Whether we live in a place where kissing is reserved for close acquaintances, or somewhere where a casual greeting means a one, two, or three cheeker, one thing does remain highly consistent: the side to which people turn while kissing. It’s almost always to the right. A 2003 study published in Nature found that twice as many adults turn their heads to the right rather than the left when kissing. This behavioral asymmetry is thought to stem from the same preference for head turning during the final weeks of gestation and during infancy.
One of the best things about kissing, however, is that we don’t have to think about any of this. Just close eyes, pucker up, and let nature takes its course.