'It's in his kiss, that's where it is' – and now scientists know why

Study reveals the chemical effect unleashed when lips collide

The mutual caressing of lips and the exchange of saliva changes the levels of the bonding hormone oxytocin (Getty Images)

The ancient Greeks believed that it unleashed the soul upon a lover, whereas the Romans did it to test the sobriety of their wives. Now scientists have found that kissing alters the love chemicals of the body.

Whatever the reasons why generations of lovers have engaged in the act of open-mouthed snogging, it seems that the mutual caressing of lips and the exchange of saliva changes the levels of the bonding hormone oxytocin released into the bloodstream from the brain.

Intriguingly, oxytocin – which is also released during childbirth and is believed to be involved in the bonding of mother to newborn baby – actually decreases in women during a kissing bout, while it increases in their male partners. Scientists are unsure why there is this difference between the sexes, but they speculate that it may be connected with the fact that women have naturally higher levels of oxytocin in their bloodstream than men and that kissing just brings them nearer to the male level.

In addition to the changes in oxytocin levels, the scientists also found that a 15-minute bout of kissing with a loved one significantly lowers the levels of the stress hormone cortisol – which can only be good for a lover's sense of well-being.

"The science of kissing, known as philematology, is an under-researched area of study," said Wendy Hill, a professor of neuroscience at Lafayette College, Pennsylvania, who carried out the kissing experiments on 15 heterosexual couples who volunteered their time and their lips.

"Kissing is defined as a behaviour in which an adult male and female touch lips and engage in open mouth-to-mouth contact as a sign of greeting and affection. [It has been] proposed that kissing originated as an oral food exchange between mothers and infants, a behaviour known as pre-mastication," Professor Hill said.

"Pre-mastication is still common among some non-western societies. This behaviour closely resembles the kiss that is shared between adult pairs since both involve positive oral contact, neural stimulation and saliva exchange," she told the American Association for the Advancement of Science in Chicago.

Helen Fisher, an anthropologist at Rutgers University, New Jersey, said that kissing may stimulate any one of the three primary brain systems involved in mating and reproduction: the sex drive, romantic love and long-term attachment.

"The sex drive motivates you to seek a range of partners, romantic love motivates you to focus your mating energy on one individual at a time, feelings of attachment motivate you to sustain a pair bond at least long enough to rear a single child through infancy together," Dr Fisher said.

Kissing may have evolved as a fast-acting biological strategy to assess a potential mate quickly and to initiate a sexual partnership. "Men like sloppier kisses with more open mouths and more tongue movement.

The hypothesis is they're trying to get small traces of oestrogen to see where the woman is in her menstrual cycle to indicate the state of her fertility," Dr Fisher said. "There are others who think that women use smell as they are kissing to deduce some things about the man's immune system. That's not proven yet.

"There's some who suggest that by kissing a man a woman is unconsciously able to detect aspects of a particular complex of genes in the immune system, and that what they're doing is being turned on by someone with different variations in the system," she said.

By kissing, "you can the smell the health of their teeth and what they have been eating and drinking and smoking, and these are all devices we use to size up an individual before we do something like have sex with them," Dr Fisher said.

"This is the tip of the iceberg. We are going to find many other mechanisms we unconsciously use to size up a person's biological traits."