Why do we kiss? ‘I am not sure we have anything close to an explanation’

We do it sitting in a tree, under the mistletoe, at midnight to ring in the new year. In fairytales, the act transforms frogs into princes and awakens heroines from enchanted slumber. We make up with it, seal with it, and – in Romeo Montague’s case at least – die with it.

Such is the supremacy of the kiss in our culture that we’ve extended the term to describe actions that don’t even involve lip contact – butterfly kisses, say, or the “Eskimo kiss”, a nose rub better known in Inuit culture as kunik. A similar Māori greeting, known as hongi, involves pressing noses together.

The earliest record of kissing dates back 4,500 years to ancient Mesopotamia (present-day Syria and Iraq), where cuneiform texts suggest smooching formed an ordinary part of romantic intimacy. Millennia later, the exact origins of kissing are still a matter of debate; a new theory published last month suggests snogging arose from primate grooming behaviour deep in our evolutionary past. How did the behaviour, which we do without a second thought (or, alternatively, obsess over and rank), come about?

“Freud, who was famously hung up on childhood complexes, took the view that kissing has something to do with the memory of the pleasures of sucking on your mother’s breast,” the renowned evolutionary psychologist Prof Robin Dunbar writes in his 2012 book, The Science of Love. “But the argument really doesn’t hold water. After all, if it really is a reversion to breast-sucking, why not just do that?”

Others hypothesise that osculation – the act of kissing – may have developed from premastication or “kiss feeding”, the act of pre-chewing food and pushing it into an infant’s mouth with the tongue. “It is certainly a possible precursor for kissing,” Dunbar tells Guardian Australia.

For the evolutionary psychologist Adriano Lameira, an associate professor at the University of Warwick, premastication doesn’t go far enough to explain the particular form kissing takes: a protrusion of the lips with a slight suction movement. “With premastication, people do indeed protrude their lips, but if anything they are taking things out of their mouths, not into,” he says.

Lameira has recently speculated that kissing may be a remnant final step of a fur grooming ritual. “You see grooming happening most frequently between family and friends in great apes. When the groomer finds [a] parasite or debris, the groomer will hold the fur … approach its lips protruded and do a slight suction movement to latch on to the parasite and debris,” he says.

Humans are not the only kissing apes. Bonobo, particularly amorous animals, are known to passionately tongue each other. Chimpanzees have been observed kissing and embracing after a fight. But most kissing in great ape species is more a case of one mouthing the other, with peck-like kisses, Dunbar says.

“Primates do do quite a lot of mouthing when grooming,” he says. “That is certainly one route to kissing, though I am not sure we have anything close to an explanation for how we got [here]”

The science of kissing

What’s in a kiss? About 80m bacteria, for starters, which are exchanged in the course of a 10-second pash. A locking of lips, one of most sensorily rich parts of the body, floods the brain with tactile information and lowers levels of the stress hormone cortisol. Saliva contains testosterone, leading some anthropologists to suggest that kissing is an unconscious means of stimulating sex drive in a potential mate.

“There is no question that mouth-to-mouth kissing allows the exchange of proteins and other cells in saliva and that this gives us very accurate information of both the immune system make-up of the other person and also their health,” Dunbar says. His research suggests that romantic kissing might be used to evaluate a potential mate’s suitability, as well as to mediate feelings of closeness between couples.

In unrelated work, a study of 26 couples found that upping the frequency of necking was associated with improved cholesterol levels, as well as perceived improvements in stress and relationship satisfaction.

Is there a right way to kiss? Yes, literally: repeated studies have found that most people tend to tilt their heads to the right when kissing romantic partners, which researchers have suggested may reflect an innate preference in humans for turning to the right – something seen as early as in utero. However, right-bias does not seem to be the case in non-romantic kisses – parent-child kisses tend mainly toward the left, which has led other scientists to hypothesise that the direction of head tilting is a learned behaviour.

‘It’s just not universal’

Work in the 1970s by the Austrian ethnologist Irenäus Eibl-Eibesfeldt found that kissing was present in about 90% of known cultures. It suggested that non-sexual kissing, such as adult to child, may be universal, says Prof William Jankowiak, an anthropologist at the University of Nevada, Las Vegas. “We’ve evolved as a human universal to care for our kids – I think this is just one of the many ways in which parents show involvement,” he says.

Some researchers, Jankowiak included, aren’t convinced that the romantic kiss is a product of evolution. If it were, they argue, the behaviour should also be, like non-sexual kisses, virtually universal.

But in 2015, Jankowiak and his colleagues looked at 168 cultures globally and found romantic kissing in less than half – 46% – of them. “It’s just not universal, which suggests it was discovered and rediscovered as a cultural event,” Jankowiak says.

By way of example, he cites the Mehinaku, an Indigenous people of Brazil, whose first reactions to observing Europeans kissing were to be disgusted at the behaviour.

If romantic kissing was something humans had evolved to do, Jankowiak questions, “what were the cultural forces to make people give up something that was genetically inevitable, responsible and pleasing?”

Jankowiak’s research also seemed to find a link between less clothing and less kissing. While romantic kissing was nearly ubiquitous in nine of 11 cultural groups living around the Arctic region, it was absent from the studied groups who lived in the tropics and subtropics.

“In a clothed environment, the only tactile thing you have is the face,” Jankowiak says. “Humans might be orientated to tactile touching, but the lips don’t necessarily have to be the primary zone.”