What's in a kiss? A study by Oxford University researchers suggests kissing helps us size up potential partners and, once in a relationship, may be a way of getting a partner to stick around.

"Kissing in human sexual relationships is incredibly prevalent in various forms across just about every society and culture," says Rafael Wlodarski, the DPhil student who carried out the research in the Department of Experimental Psychology at Oxford University. "Kissing is seen in our closest primate relatives, chimps and bonobos, but it is much less intense and less commonly used.

"So here's a human courtship behavior which is incredibly widespread and common and, in extent, is quite unique. And we are still not exactly sure why it is so widespread or what purpose it serves."

To understand more, Rafael Wlodarski and Professor Robin Dunbar set up an online questionnaire in which over 900 adults answered questions about the importance of kissing in both short-term and long-term relationships.

Rafael Wlodarski explains: "There are three main theories about the role that kissing plays in sexual relationships: that it somehow helps assess the genetic quality of potential mates; that it is used to increase arousal (to initiate sex for example); and that it is useful in keeping relationships together. We wanted to see which of these theories held up under closer scrutiny."

The researchers report their findings in two papers, one in the journal Archives of Sexual Behavior and the second in the journal Human Nature, both published by Springer. They were funded by the European Research Council.

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The survey responses showed that women rated kissing as generally more important in relationships than men. Furthermore, men and women who rated themselves as being attractive, or who tended to have more short-term relationships and casual encounters, also rated kissing as being more important.

In humans, as in all mammals, females must invest more time than men in having offspring -- pregnancy takes nine months and breast-feeding may take up to several years. Previous studies have shown women tend to be more selective when initially choosing a partner. Men and women who are more attractive, or have more casual sex partners, have also been found to be more selective in choosing potential mates. As it is these groups which tended to value kissing more in their survey responses, it suggests that kissing helps in assessing potential mates.

It has been suggested previously that kissing may allow people to subconsciously assess a potential partner through taste or smell, picking up on biological cues for compatibility, genetic fitness or general health.

"Mate choice and courtship in humans is complex," says Professor Robin Dunbar. "It involves a series of periods of assessments where people ask themselves 'shall I carry on deeper into this 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."

He adds: "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 women waiting just for him? At what point do you have to compromise for the curate?

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"What Jane Austen realised is that people are extremely good at assessing where they are in the 'mating market' and pitch 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.

"We see some of that coming out in the results of our survey, suggesting that kissing plays a role in assessing a potential partner," Professor Dunbar explains.

Past research has also found that women place greater value on activities that strengthen long-term relationships (since raising offspring is made easier with two parents present). In the current study, the team found that kissing's importance changed for people according to whether it was being done in long-term or short-term relationships. Particularly, it was rated by women as more important in long-term relationships, suggesting that kissing also plays an important role in mediating affection and attachment among established couples.

While high levels of arousal might be a consequence of kissing (particularly as a prelude to sex), the researchers say it does not appear to be a driving factor that explains why we kiss in romantic relationships.

Other findings included:

* In short relationships, survey participants said kissing was most important before sex, less so during sex, was less important again after sex and was least important at other times. In committed relationships, where forming and maintain a lasting bond is an important goal, kissing was equally important before sex and at times not-related to sex.

* More frequent kissing in a relationship was linked to the quality of a relationship, while this wasn't the case for having more sex. However, people's satisfaction with the amount of both kissing and sex did tally with the quality of that relationship.

* In a companion paper in the journal Human Nature, the researchers report that women's attitudes to romantic kissing also depend on where in their menstrual cycle and their relationship they are. Women valued kissing most at initial stages of a relationship when they were in the part of their cycle when they are most likely to conceive. Previous studies have shown that hormonal changes associated with the menstrual cycle can change a woman's preferences for a potential mate. When chances of conceiving are highest, women seem to prefer men who display supposed signals of underlying genetic fitness, such as masculinized faces, facial symmetry, social dominance, and genetic compatibility. It appears that kissing a romantic potential partner at this time helps women assess the genetic quality of a potential mate, the researchers say.