(Your first kiss is a kind…)
Your first kiss is not just the first display of passion, but it is also a kind of screening process to discover whether a potential partner is compatible as a mate, according to scientists.
The researchers claim that the touching of lips is a "biological" quality control strategy for "mate assessment" which has evolved over millions of years.
Kissing also triggers certain hormones that reduce stress, increase attachment between a couple, and boosts the sex drive.
According to Helen Fisher, professor of anthropology at Rutgers University, New Jersey, men subconsciously use a kiss to gauge levels of oestrogen and in turn fertility.
On the other hand, women are sizing the man up to assess how strong his immune system and health is, and how well he looks after himself.
And the information is used by both sexes to make a choice before agreeing upon having sex because then the outcome could be too "expensive".
Fisher said that she believed both men and women used kissing for "mate choice, for sizing someone up, not only socially but chemically".
She even cited research by Gordon Gallup, of Albany University, and other researchers and said that perspective lovers unconsciously checked the fertility of the mate they were kissing.
In her opinion, men like "sloppier kisses" as they test the saliva to see how fertile their 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," the Telegraph quoted her as saying.
On the other hand, findings suggested that women used smell as they kissed 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. They're more attracted to a different immune system," she said.
"Over 90 per cent of societies around the world kiss and a great many animals.
"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 added.
The study was presented at the American Association for the Advancement of Science 2009 meeting.