BERLIN - Scientists have been wondering about why people kiss each other for more than 100 years. Psychoanalysis's founding father, Austrian neurologist Sigmund Freund (1856-1939) was convinced that kissing is innate – an instinct originated in every newborn's need to breastfeed that is never lost.

But in 1900, when Russian physiologist Ivan Pavlov (1849-1936) got his dog to drool at the sound of a bell, many psychologists took that as proof that behavior -- including human behavior -- was not a matter of instinct: it was learned. Human beings were the masters of their instincts, so kissing represented a conscious expression of love.

Modern science considers the will-or-nature debate outdated. Kissing is instinctive, but humans have different ways of behaving, is all.

Which would explain why some 650 million, or about 10% of humanity, don't kiss each other. On his travels, the father of evolutionary theory Charles Darwin (1809-1882) noticed that. So did French anthropologist Paul d'Enjoy who in 1897 noted that for the Chinese mouth-to-mouth kissing was considered an abomination, a form of cannibalism. Around this time Danish scientist Kristoffer Nyrop observed that among Finnish peoples couples enjoyed bathing together but never kissed each other.

In Mongolia, 90% of people kiss, 10% don't. Kissing is in our nature, whether we engage in kissing behavior or not. So where does it come from?

Where does kissing come from?

Freud's hypothesis is still relevant, along with a second one posited in 1960 by British zoologist Desmond Morris: kissing comes from early humans' habit of feeding mouth-to-mouth. Chimpanzees still do that. And when food was scarce, so the theory goes, ape parents pressed their lips to their offspring's mouths to reassure and calm them.

We know from the ancient Greeks that human beings used to feed each other mouth-to-mouth; and Austrian behavioral scientist Irenäus Eibl-Eibesfeldt, who founded the field of human ethology, has documented the behavior among certain African tribes such as the Himba in northern Namibia.

Just why the tendency to kiss, as opposed to not kissing, took the upper hand in evolutionary terms is another unresolved issue. Using up calories doesn't explain anything, although a normal kiss does use up 6.4 calories per minute or more depending on circumstances.

Research by neurologists and sexologists offers the possibility that kissing is a biological aid to partner selection. According to American scientist Sarah Woodley of the University of Duquesne in Pittsburgh, Pennsylvania, it is a way of running a check on a potential partner's immune system.

The illnesses to which a human is immune is coded in the MHC (major histocompatibility complex) genes -- and the smells and tastes of kisses pass this information on to us. The more different a kissee's MHC genes are to those of the kisser, the more likely the couple is to team up – because the wider the spread of immunity, the more diseases their offspring will be immune to and the higher the life expectancy of those children will be.

One of the tests scientists used to demonstrate the MHC theory was having men and women smell T-shirts that had been worn by others. Most subjects did in fact prefer the shirts worn by those with MHC genes different to their own. However, not all subjects did – which is why Woodley reached the conclusion that MHC genes are not the determining factor for why two people who like each other would mate or marry.

American sexologist Helen Fisher agrees that the MHC check helps in determining partner selection but that other bio-mechanisms are at play as well.

Gender differences in kissing

Another puzzle is the role played by the hormone oxytocin, which when released has a calming and relieving effect on humans and is considered to be the hormonal "glue" of partnership. Couples in love show higher levels of it. So it made sense to assume that levels of it would rise when people kissed.

Not so, say American psychologists Wendy Hill and Carey Wilson of Lafayette College in Easton, Pennsylvania. Levels only rise in men – not women. In fact, in women levels actually sink. From this, Hill and Wilson came to the conclusion that unlike men women need more than a kiss to feel bound to their partner.

Hill and Wilson also showed that, when men and women kissed, levels of cortisol went down sharply in both sexes. Cortisol causes feelings of stress, while kissing is a de-stresser – which may also explain why kisser and kissee are in no hurry to disengage.

The kiss will remain a subject of scientific research not least because lovers aren't the only ones kissing each other. The Pope kisses the ground of countries he visits. Churchgoers kiss the bishop's ring. East Germany's leader Erich Honecker and Russian leader Leonid Brezhnev famously kissed each other on the lips by way of greeting. And the French have traditionally kissed each other on the cheeks – with a kissing noise to accompany the ritual – to say hello and goodbye.

So it's entirely possible that the 10% of the world's non-kissers may soon join the 90% of those who do kiss. After all, we travel and emigrate more than ever before. In London-based American academic and writer Dr. Adrianne Blue's 1997 book On Kissing: Travels in an Intimate Landscape, she writes that a little over 20 years ago it would have been unheard of for British acquaintances meeting in public to kiss each other on the cheeks by way of greeting.

But by the late 1990s, Blue writes, the only issue in London is whether two or three smackers are the proper form. She believes in the globalization of the kiss – with the kissers winning out.

Read the original article in German.

Photo - happyhipposnacks