The Differences in Gender -- Sealed With a Kiss

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 behavior. It's so common," said Susan M. Hughes, an assistant professor of psychology at Albright College in Pennsylvania, 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," said 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 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.

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 said.

A disproportionate amount of the brain, she noted, 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 said. "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."