Kissing helps us assess potential partners if, like a Jane Austen heroine, we cannot wait forever for Mr Darcy to come along, a study suggests.
Scientists believe kissing helps people judge the quality of a potential mate through taste, smell and fitness.
Once in a relationship, the Oxford University study found kissing was a way of getting a partner to stick around.
Women were found to value kissing more highly in long-term relationships.
An online survey of 900 adults by the Oxford team, published in the journal Archives of Sexual Behaviour, showed that men and women who were more attractive or had more casual sex partners were more selective in choosing mates, and those groups valued kissing more highly.
This suggests that kissing helps in sizing up a potential partner, the study says.
Professor Robin Dunbar, from the department of experimental psychology at Oxford University, said courtship in humans was complex and involved a whole series of assessments before men and women decided to carry on their relationship.
"Initial attraction may include facial, body and social cues. Then assessments become more and more intimate as we go deeper into the courtship stages, and this is where kissing comes in.
"In choosing partners, we have to deal with the 'Jane Austen problem': How long do you wait for Mr Darcy to come along when you can't wait forever and there may be lots of you waiting just for him? At what point do you have to compromise for the curate?"
Prof Dunbar said that Jane Austen, whose works of romantic fiction included Pride and Prejudice and Sense and Sensibility, realised that people were extremely good at assessing where they were in the "mating market" and pitching their demands accordingly.
"It depends what kind of poker hand you've been dealt.
"If you have a strong bidding hand, you can afford to be much more demanding and choosy when it comes to prospective mates," he said,
Feelings of affection
If kissing plays a part in selecting a partner then it also plays an important role before sex in short relationships and at a range of different times in committed relationships, the study found.
The study found that kissing was particularly important to women in long-term relationships.
This may be because it plays a role in increasing feelings of affection and attachment among couples, the researchers suggest.
Previous research had found that women placed greater value on activities that strengthen long-term relationships because being pregnant and raising children is easier when two parents are present.
In another study in Human Nature, researchers from Oxford suggest that women's attitude to romantic kissing also depends on where in their menstrual cycle and their relationship they are.
Women valued kissing most at the start of a relationship and around the time they were most likely to conceive in their cycle.