Researchers in the United States have determined what many of us already knew: women think of long-term relationships while kissing, and men think of sex.
"A lot of the findings might seem self-evident but what's self-evident to females is not always to males," Gordon Gallup, an evolutionary psychologist who led the study at the State University of New York (SUNY), told AFP.
Lesser-known phenomena that the researchers hit on in the study, in which 1,041 US university students took part, included the fact that men like their kisses wetter and with more tongue contact than women, and that women place more importance than men on the state of their kissing partner's teeth.
The authors of the study explain the findings, and even the act of kissing, in scientific terms that might put off die-hard romantics.
"We see kissing as an evolved courtship ritual. At the moment of a kiss there's a complicated exchange of information -- everything from postural to tactile to chemical cues," Gallup said.
"That information may tap into hard-wired subconscious evolved mechanisms that allow people to unwittingly make reproductive choices that are in their long-term interest."
The researchers found that a bad first kiss can kill a relationship, Gallup said.
"A clear majority of male and female college students said they have been attracted to someone only to find they were no longer interested after the first kiss," Gallup said.
"What that tells us is that a kiss won't necessarily make a relationship, but it can clearly kill a relationship," he said.
The study puts it this way: "Perhaps kissing in these instances may activate evolved mechanisms that function to discourage reproduction among individuals who could be genetically incompatible."
The fact that men show a greater preference for tongue contact when kissing could have its basis not in eroticism -- although the study showed that men found kissing more erotic than women -- but science.
"It is possible that kissing styles that maximize salivary exchange provide subtle information about a females reproductive status since saliva and breath odor change across the menstrual cycle," it said.
"In addition, male preference for salivary exchange could function to introduce substances such as hormones or proteins into womens mouths that may influence their mating psychology, and even make them more sexually receptive."
In other words, men's pronounced taste for French kissing comes down to them wanting to bed their partner.
Unfortunately for them, the study also found that women are less likely to kiss someone they think is only after sex, and that they "place a greater emphasis on kissing to induce bonding."
Women were also "more likely to initiate kissing after sexual intercourse."
"This supports other findings in showing that males tend to engage in a hasty post-copulatory departure and demonstrate an emotional shift after sexual intercourse to reduce the likelihood of bonding and investment in short-term mating partners," the authors of the study wrote.
Kissing between sexual or romantic partners occurs in over 90 percent of human cultures, according to the study, which was published in the scientific journal "Evolutionary Psychology."