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Why Humans (And Only Humans) Kiss

Save for the bonobos that suck on each other’s tongues for up to ten minutes at a time, there aren’t any animals that kiss. And are we really going to count a tongue-suck as a kiss anyway?

Somehow, humans are actually the only species to kiss on the mouth, and the meanings of a kiss are plentiful. A parent might kiss a child goodbye upon dropping him off at school. A kiss might occur at the end of a date as either a polite “thank you, but I won’t be seeing you again” or as an “I like you. Let’s keep this up.” A kiss on the cheek is often a polite greeting. Kisses in longer relationships are a form of maintenance, and a particularly upset person might even demand a kiss upon his derrière.

The history of kissing is a fascinating one too. The first recorded kiss can be found circa 1500 B.C.E. in early Vedic scriptures, one of the primary Hindu texts. These texts mention people “sniffing” other people “with their mouths,” and kissing was described as a way to inhale someone’s spirit. This brings to mind the “Eskimo kiss,” where you nuzzle your nose to your partner’s cheek or forehead. In 200 C.E., the Kama Sutra, or the “Vatsyayana Kamasutra,” detailed lavish and novel ways to kiss, such as “licking” or “drinking [the] moisture of the lips.” Odysseus kissed his slaves upon returning home. In the Hebrew Bible, the Song of Solomon gets pretty hot and heavy, saying, “Let him kiss me with the kisses of his mouth: For thy love is better than wine.”

In 1872, Darwin hypothesized that, considering that kissing is a behavior humans all over the world partake in, there must be some innate desire driving us to kiss. Freud of course chimed in, claiming that kissing is a subconscious return to suckling at a mother’s breast. Some have even claimed that lips look similar to the labia, which explains why women put on deep red lipstick and plumper so they can “simulate the appearance of sexual arousal, like animals in heat,” as Joshua Foer cleverly notes.

During the Black Death in 14th-century Europe, kissing quickly fell out of vogue. Licking, sniffing, or nibbling eyebrows became a safer alternative, and while that sounds peculiar, it really accomplished the same thing.

That’s because, for all intents and purposes, sniffing someone’s face is the same as giving them a kiss.

Philematologists, or kissing researchers, have three main hypotheses for why we kiss: to arouse, to cement a relationship, or as a trial run for a potential mate. The third hypothesis has held the strongest, and the exchange of smells and saliva that a kiss facilitates allows for partners to quickly sort through a lot of genetic information very quickly.

Sheril Kirshenbaum, author of The Science of Kissing, found that women are attracted to men who have a different genetic code from their own immune system — the more different, the better. Having a different DNA set from a sexual partner increases the chances of having a healthy baby, and, after all, having healthy offspring is our main biological imperative. That’s why we’re also most attracted to symmetrical faces, healthy skin, and, in women, curves (child-bearing hips, essentially).

Kissing, therefore, is simply a means of measuring a partner’s health and biological compatibility. And although we could just ask our partner for a cheek swab to assess his/her DNA, it wouldn’t be quite as fun as a nice kiss.

Most recently, a study at the University of Oxford showed that the people who value kissing the most are women and then men who assessed themselves as “desirable” and “good-looking.” That’s because these cohorts are the pickiest about their partners (because they can be), thus they tend to use kissing as a sort of screening process. If the smell and taste is right, then the relationship can continue on. If it’s a little off, then they’ll skip along to the next date to try and sample some new saliva.

Interestingly, couples in the happiest relationships reported that kissing was more important than sex, and, according to the Oxford study, lasting relationships can be better predicted by kissing frequency than by frequency of intercourse. In fact, a study conducted in the 1980s showed that men who smooched their wives goodbye before heading out the door to work tended to have higher incomes, fewer car accidents, and lived longer than married men who passed up a morning kiss.

As for why animals don’t kiss, less research has been done. Essentially though, animals choose to skip the hour-long make-out session and just take a deep whiff to collect information. It’s perhaps a more practical way to get the biological and genetic details they need to choose a mate. But next time the wine glasses are empty, the movie is over, and the date is coming to a close, try giving your special someone a deep, spirited sniff. I have a feeling it won’t have quite the same effect as a kiss (even if it accomplishes the same thing).


  1. The hypothesis that checking the partners body odour before mating comes from animal studies. In fact, rodents and dogs can “smell” the so-called MHC genotypes, which are important modifiers of an individual immune system. It has been shown experimentally that mice prefer to mate with a partner of a divergent MHC genotype, which could make the offspring resistant against a wider spectrum of pathogenic infections. Dogs also recognize individual smells (of sweat, urine, saliva etc) by the admixed traces of MHC peptides.
    When behavioural psychologists tried to prove a similar ability in humans, however, they failed. Although people when presented odor samples of subjects of the opposite sex show individual preferences, there was no clear connection between the MHC types (or HLA in human) of the test person and the subject that provided the odor sample. It is not surprising, that humans with a > 100 fold less sensitive olfactory system can not do the same performance as some animals. So although the individual body odor might contain some markers of a persons genetic set-up, the human nose is by far not sensitive enough to detect this. So when some clever dating clubs came up with the idea to a-priory match femal and male partners by giving them samples of the potential partners, it perhaps worked much more on the basis of getting an idea of what the others are eating or drinking (apart from garlic and ouzo) or which parfume or body lotion they prefer. The smell of the sweat in people is also influenced to a large degree by the skin microbiom, i.e. the individual composition of bacteria which populate the skin.
    Coming back to the animal models: One theory says that kissing can be traced back to mouth-to-mouth feeding or beak-to-beak exchange of food in courting animals . Also during courtship, mated pairs of many bird species touch or clasp each other’s bills.

  2. Kissing is definitely nicer than asking for a cheek swab.
    Glad I discovered your blog, it is a pleasure reading your articles

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