Why Do Humans Kiss?

by | May 7, 2018

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Hipsters kissing

Kissing is weird if you think about it: putting your lips together with another person and exchanging saliva. It’s not exactly sanitary: one study estimated that between 10 million and 1 Billion bacteria from nearly 300 different species are exchanged during “active” kisses. According to an article in the American Journal of Medicine in 2013, some of the “pathologic organisms [which] can be transmitted by kissing, including upper respiratory infective viruses, herpes simplex, and Epstein-Barr viruses, as well as pathogenic streptococci, syphilitic spirochetes, and Mycobacterium tuberculosis bacteria.”

Humans are nearly unique in our romantic-sexual kissing. Chimpanzes and Bonobos are the only other animal species that have been observed kissing (and Bonobos do use tongue sometimes when kissing).

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Bonobos kissing

Even though humans kiss, it is not universal among cultures. In a study published in 2015, researchers found that only 46% of 168 cultures observed engaged in any romantic-sexual kissing. The study  found that more complex societies were likely to engage in kissing  vs. less complex cultures.

So, why do humans kiss? A clue about why we kiss is found by asking “why don’t other animals kiss?” Animals (including humans) produce pheromones, which are substances secreted by an individual that is received by another individual of the same species which affects the behavior or physiology of the receiving individual. Pheromones are important in sexual signaling. Many animals have a vomeronasal organ, which is an auxiliary nasal organ and processes pheromone signals.  Having a vomeronasal organ allows animals to detect pheromones from a distance. Humans have no vomeronasal organ. We detect and process pheromones using our noses, which aren’t nearly as sensitive to pheromones as the vomeronsal organ, so we have to be up close to detect pheromones.

The leading hypothesis is the main purpose of kissing is for humans to be close enough to each other to detect each other’s pheromones.  It is postulated that over time in western (and some other) cultures sniffing behavior turned into physical lip contact.

Just like other animals, we use pheromones to help in mate selection. An interesting study illustrating how pheromones can assist in mate selection was published in 1995 from the Proceedings of the Royal Society B. In the study, researchers had females smell t-shirts worn for a few days by different males. Both the female smellers and male t-shirt wearers were tested for various genetic markers. It turns out that individual females chose as most pleasant the smell of males who were genetically different from them. This makes sense from a mating perspective as genetic diversity makes for healthier offspring. Amazingly, the study found “this difference in odor assessment was reversed when the women rating the odors were taking oral contraceptives.”

In real life, it turns out that a kiss can be an important litmus test for a relationship – a survey from about 10 years ago found that 59% of men and 66% of women stated that there were times when they were attracted to someone only to find that this attraction dissipated after a first bad kiss and ended the romantic relationship right then and there.  There wasn’t anything necessarily wrong with the kisses – they just didn’t feel right. This survey is supportive of the pheromone-genetic selection hypothesis for kissing.

In addition to facilitating pheromone smelling, kissing can be enjoyable.  Our lips have the slimmest layer of skin on the body and are among the most densely populated with sensory neurons of anybody region.  Kissing unleashes a cocktail of chemicals that govern stress, motivation, social bonding and sexual stimulation.  In a recent study researchers compared the levels of two hormones in 15 college male-female couples before and after they kissed.  One hormone, oxytocin, is involved in social bonding, and the other, cortisol, plays a role in stress. The researchers predicted that kissing would boost the levels of oxytocin (bonding) and reduce cortisol (stress).  Surprisingly, the oxytocin levels only rose in the males and actually decreased in the females.  They concluded that females must require more than a kiss to feel emotionally connected or sexually excited and/or females likely require a more romantic setting than a psychology laboratory.  However, cortisol did drop in both males and females suggesting that kissing does reduce stress regardless of the intimacy level.

Finally, kissing can be good for you. Researchers at Oxford have found that people who kiss more frequently tend to be more satisfied in their relationships. In another study, men who habitually kissed their wives each morning lived 5 years longer on average.

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