Kissing was developed 'to spread germs'

They say the gesture allows a bug named Cytomegalovirus, which is dangerous in pregnancy, to be passed from man to woman to give her time to build up protection against it.

The bug is found in saliva and normally causes no problems. But it can be extremely dangerous if caught while pregnant and can kill unborn babies or cause birth defects.

Writing in the journal Medical Hypotheses, researcher Dr Colin Hendrie from the University of Leeds, said: "Female inoculation with a specific male's cytomegalovirus is most efficiently achieved through mouth-to-mouth contact and saliva exchange, particularly where the flow of saliva is from the male to the typically shorter female."

Kissing the same person for about six months provides the best protection, he added.

As the relationships progresses and the kisses become more passionate, the woman's immunity builds up, cutting her odds of becoming ill.

By the time she becomes pregnant, the odds of her unborn baby becoming infected are much lower.

Previously scientists have claimed that kissing acts as a form of evolutionary quality control, with saliva holding clues to fertility, health and genes.

But the psychologists from Leeds and the University of Central Lancashire said these things can be judged without getting quite so intimate.

Dr Hendrie said: "Information concerning body tone, smell, reproductive condition, disease state and, of course, personal physical and oral hygiene can all be gained solely from close physical proximity.

"The small amount of additional information from kissing is an unlikely pressure for its development."