A kiss is more than just a kiss - it acts as screening tool to help us find the right partner, new research suggests.
When two people kiss they get very close, allowing each individual to assess the other through taste or smell.
The signals kissing provides may provide biological cues for compatibility, genetic fitness, or general health, scientists believe.
Researchers investigated the smooching truth by conducting an online survey in which more than 900 adults answered questions about the role of kissing.
"Kissing in human sexual relationships is incredibly prevalent in various forms across just about every society and culture," said psychologist Rafael Wlodarski, from Oxford University, who led the study.
"Kissing is seen in our closest primate relatives, chimps and bonobos, but it is much less intense and less commonly used," he said.
"So here's a human courtship behaviour 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."
The survey responses, reported in the journals Archives of Sexual Behaviour and Human Nature, showed that women rated kissing as generally more important in relationships than men.
Men and women who rated themselves as being attractive, or who tended to have more short-term relationships and casual encounters, also saw kissing as more important.
Previous studies have shown women tend to be more selective than men when initially choosing a partner.
Both men and women who are more attractive, or have many casual partners, are also known to be extra picky when looking for a long-term potential mate.
Since, according to the survey, both groups also place a high value on kissing, it suggests that kissing may help in mate assessment.
Co-author Professor Robin Dunbar, from the Department of Experimental Psychology at Oxford University, said: "Mate choice and courtship in humans is complex. 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.
"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?
"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."
The study also found that the significance of kissing changed according to the type of relationship people were in or seeking.
Women rated it as especially important in long-term relationships, suggesting that kissing also plays a key role in maintaining attachment between established couples.
While high levels of arousal might result from kissing, the researchers say sex does not appear to be the driving factor that explains why we kiss in romantic relationships.
