In people, kissing to express affection is almost universal. About 90 percent of human cultures do it.
One traditional view is that kissing, known scientifically as osculation, evolved from women chewing food for their children and giving it to them mouth-to-mouth, Fisher said.
But, she said, "I've never believed that," adding that similar behavior is found in many species. Birds tap beaks. Elephants shove their trunks in each other's mouths. Primates called bonobos practice their own version of French kissing.
"Men tend to think kissing should lead to sex no matter what," Hughes said.
"When the woman is first kissing the man, she's not necessarily sending the signal, 'Let's go to the next stage' -- but the man is reading it that way," Palmer said. "So both can get themselves into difficulties if they don't verbalize their true intentions."
Men were also much more likely to want to exchange more saliva during a kiss.
"Males like the very moist, wet open-mouth kisses," Hughes said. "We didn't expect that."
Men tend to have less acute senses of taste and smell than women, which could explain that finding, she said.
"Perhaps males need more saliva to make subtle mate assessments," she said, noting that previous research has suggested that a woman's breath changes across the menstrual cycle. "He may be subconsciously detecting whether she's fertile or not."