As Valentine's Day approaches, the kiss goes under scrutiny. A recent study, one of the first to investigate kissing in depth, sheds some light on this common show of affection, writes Rob Stein
A kiss, it turns out, is definitely not always just a kiss.
As Valentine's Day approaches, research has begun shedding light on that most basic of all human expressions of love - the smooch - which has received surprisingly little scientific scrutiny.
"You'd think there would be a lot of research on kissing behaviour. It's so common," says Susan Hughes, an assistant professor of psychology at Albright College in Pennsylvania in the US, whose recent study is one of the first to probe snogging in depth. "But there isn't. It's really been ignored."
In fact, much about love and attraction remains mysterious. "This is a seminal paper," says Helen Fisher, a Rutgers University anthropologist who studies love. "It's remarkable that we don't know more about these things. But love has not really been well studied until recently."
In people, kissing to express affection is almost universal. About 90 per cent 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 says.
But, she adds, "I've never believed that," adding that similar behaviour is found in many species. Birds tap beaks. Elephants shove their trunks in each other's mouths. Primates called bonobos practise their own version of French kissing.
Fisher believes kissing is all about choosing the right mate.
"There's so much information exchanged when you kiss someone that I just thought it must play a vital role in mate choice, and this paper is elegantly showing that," Fisher says.
A disproportionate amount of the brain, she notes, is geared toward interpreting signals from the mouth.
"When you look at the brain regions associated with picking up data from the body, a huge amount of the brain is devoted to picking up information from the lips and tongue," she says. "Very little of the brain is built to pick up what happens to, say, your back. There have been case reports of people being stabbed in the back without even knowing it. But even the lightest brush of a feather on your lips and you feel it intensely."
Hughes and her colleagues set out to probe some of the mysteries of lip-smacking by conducting a series of three in-depth interviews with 1,041 students at the University at Albany, in New York.
"This was a fishing expedition," Hughes admits. "We didn't know what to expect."
But Hughes and her colleagues had three hypotheses. "People may use kissing as a sort of mate assessment," she says. "You can tell a lot of information about a person by being in close proximity - from their breath, the taste of their saliva, things like that."
Their second hypothesis was that kissing promotes bonding.
"If you are accepting a kiss, you are putting yourself at risk of contracting an illness. And we suspect it raises levels of a hormone called oxytocin, which is related to interpersonal bonding," Hughes says.
The third hypothesis was that kissing is simply a way of inducing sexual arousal, increasing the chances of having sex.
"Men might use this more to seduce their partners more than women do," she says.
The researchers found support for all three theories, Hughes and her colleagues reported in the journal Evolutionary Psychology, as well as provocative differences between men and women.
Women place more emphasis on the taste and smell of the person they kiss than men do, the researchers found.
"That clues us in that females may be using it more to make mate assessments than men," she says.
Women were also more likely to refuse to have sex with a partner unless they kissed first. More than half of the men said they would have sex without kissing first, but fewer than 15 per cent of the women said the same.
Moreover, kissing is clearly a much bigger potential deal-breaker for women than for men. Women were much more likely to say they would refuse to have sex with a bad kisser.
"Women are definitely using kissing to make an assessment about the male. If he's a bad kisser, than she's not going to want to have sex with him. She's getting a lot of information from that kiss," she says.
Men were also more likely to expect kissing to lead to sex. Men assumed that would be the case about half the time; women only about one-third of the time. And it made no difference to men if they were in a short- or long-term relationship.
"Men tend to think kissing should lead to sex no matter what," Hughes says.
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 says. "We didn't expect that."
Men tend to have less acute senses of taste and smell than women, which could explain that finding, she says.
"Perhaps males need more saliva to make subtle mate assessments," she adds, 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."
Women in Hughes's study were more likely to say kissing was important before, during and after sex, as well as throughout a relationship.
"That supports the idea that females are using kissing to create a bond," Hughes adds.
"No wonder we remember our first kiss," Fisher says. "No wonder we're nervous about our first kiss. We haven't known why. This helps explain it."