First kiss is screening process

Kissing triggers certain hormones that reduce stress, increase attachment between a couple and increases the sex drive. Photo: GETTY

A first kiss is a screening process to discover whether a potential partner is compatible as a mate, claim scientists at the American Association for the Advancement of Science 2009 meeting.

Researchers believe that the touching of lips is a "biological" quality control strategy for "mate assessment" which has evolved over millions of years.

It also triggers certain hormones that reduce stress, increase attachment between a couple and increases the sex drive.

Dr Helen Fisher, professor of anthropology at Rutgers University, New Jersey, said men use a kiss to subconsciously assess levels of oestrogen and therefore fertility.

Women on the other hand are sizing the man up to see how strong his immune system and health is and how well he looks after himself.

Both sexes use the information to make a choice before committing to sex because then the outcome could be too "expensive".

Dr Fisher said she believes both men and women use kissing for "mate choice, for sizing someone up, not only socially but chemically".

Citing research by Gordon Gallup, of Albany University, and other researchers she said unconsciously perspective lovers checked the fertility of the mate they were kissing.

She said men like "sloppier kisses" because they are testing the saliva to see how fertile a woman is.

"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," she said.

She said women used smell as they are kissing to deduce some things about the man's immune system.

"There's some who suggest 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 my someone with different variations in the system," she said.

"They're more attracted to a different immune system"

Dr Fisher said that the kiss was too common across all cultures not to have a serious underlying objective.

"Over 90 per cent of societies around the world kiss and a great many animals," she said.

"These are all devices we use to size up an individual before we do something like have sex with them which is very metabolically expensive and very time consuming.

"It is almost parsimonious to think that kissing would be an adaptive mechanism for assessing the quality of an individual."

She said that once a couple had decided they wanted each other that the body then produced hormones that produced "romantic love".

She said: "Part of the brain becomes active like a rush of cocaine. Romantic love is not an emotion. It comes from the motor of the mind, the craving part of the mind, where you reach for that chocolate. It's a drive."

Asked if the first kiss could make or break a relationship, she said: "In one study, 66 per cent of women and 59 per cent of men reported they had become attracted to somebody and then with the first kiss they ended it."

By Richard Alleyne, Science Correspondent at the AAAS meeting in Chicago