Image courtesy U.S. National Archives and Records Administration This famous photograph captures a celebratory kiss in New York City's Times Square at the end of World War II.

When you really think about it, kissing is pretty gross. It involves saliva and mucous membranes, and it may have historical roots in chewed-up food. Experts estimate that hundreds or even millions of bacterial colonies move from one mouth to another during a kiss. Doctors have also linked kissing to the spread of diseases like meningitis, herpes and mononucleosis.

Yet anthropologists report that 90 percent of the people in the world kiss. Most people look forward to their first romantic kiss and remember it for the rest of their lives. Parents kiss children, worshippers kiss religious artifacts and couples kiss each other. Some people even kiss the ground when they get off an airplane.

So how does one gesture come to signify affection, celebration, grief, comfort and respect, all over the world? No one knows for sure, but anthropologists think kissing might have originated with human mothers feeding their babies much the way birds do. Mothers would chew the food and then pass it from their mouths to their babies' mouths. After the babies learned to eat solid food, their mothers may have kissed them to comfort them or to show affection.

In this scenario, kissing is a learned behavior, passed from generation to generation. We do it because we learned how to from our parents and from the society around us. There's a problem with this theory, though: women in a few modern indigenous cultures feed their babies by passing chewed food mouth-to-mouth. But in some of these cultures, no one kissed until Westerners introduced the practice.

Image courtesy Jan Roger Johannesen/Stock.xchng Research suggests that mothers kiss their babies because of the way prehistoric mothers fed their children.

Other researchers believe instead that kissing is instinctive. They use bonobo apes, which are closely related to humans, to support this idea. Bonobos kiss one another frequently. Regardless of sex or status within their social groups, bonobos kiss to reduce tension after disputes, to reassure one another, to develop social bonds and sometimes for no clear reason at all. Some researchers believe that kissing primates prove that the desire to kiss is instinctive.

Image courtesy Tom Jude/Stock.xchng Many animal species exhibit kissing-like behaviors.

Several other animal species have behaviors that resemble kissing. Many mammals lick one another's faces, birds touch one another's bills and snails caress one another's antennae. In some cases, the animals are grooming one another rather than kissing. In others, they're smelling scent glands that are located on faces or in mouths. Regardless, when animals touch each other in this way, they're often showing signs of trust and affection or developing social bonds.

Scientists don't entirely agree on whether kissing is learned or instinctive. There's support for both arguments, just as there's support for the different theories of why people started doing it in the first place.

The Effects of Kissing

What an Incredible Smell You've Discovered! People in some cultures rub one another's noses or cheeks rather than, or in addition to, kissing. Anthropologists theorize that this "Eskimo kiss" grew from people smelling one another's faces much the way animals do.

While researchers aren't exactly sure how or why people started kissing, they do know that romantic kissing affects most people profoundly. The Kinsey Institute describes a person's response to kissing as a combination of three factors:

• Your psychological response depends on your mental and emotional state as well as how you feel about the person who is kissing you. Psychologically, kissing someone you want to kiss will generally encourage feelings of attachment and affection. If you're kissing someone you don't like, or you're kissed against your will, your psychological response will be completely different.

• Your body physically reacts to being kissed. Most people like to be touched, and that's part of your body's response to kissing. But kissing also affects everything from your blood to your brain. We'll look at your body's biological reactions to kissing in detail in a later section.

• The culture in which you grew up plays a big part in how you feel about kissing. In most Western societies, people are conditioned to, look forward to and enjoy kissing. The behavior of the people around you, depictions in the media and other social factors can dramatically affect how you respond to being kissed.

Image courtesy Esther Seijmonsbergen/Stock.xchng Whether, why and how people kiss depends largely on psychological, biological and social factors.

These factors play a part in all kisses, not just those that are romantic in nature. In other words, when a mother kisses her child's bruise to make it feel better, psychological, physical and social factors play a part in both people's reactions. The same is true when friends kiss as a greeting, worshippers kiss religious symbols or siblings kiss and make up after an argument. Even though some kisses are platonic and others are romantic, they generally have one thing in common - they are inspired by and tend to inspire feelings we think of as positive.

Regardless of exactly how people got the idea to kiss or what they mean when they do it, anthropologists are pretty sure that people started kissing thousands of years ago. We'll look at the history of kissing in more detail next.

Do We Have to Hear the Kissing Part? Modern research suggests that just about every culture on the planet kisses. However, anthropologists and ethnologists have described a few cultures in Asia, Africa and South America that do not kiss at all. Some of these cultures view kissing as disgusting or distasteful. However, other researchers point out that these societies may view kissing as too private to discuss with strangers. In other words, they might kiss but not talk about it.