Lips are the most exposed erogenous zone, which makes kissing feel very good.
Scientists in the Netherlands have reported that we share about 80m bacteria during a passionate ten-second kiss; a finding that makes puckering up seem cringe-worthy - and downright unsanitary at the start of cold and flu season.
But take heart: we're more likely to get sick by shaking hands throughout the day than through kissing. And the science behind this behaviour reveals that along with all of those germs, we share plenty of benefits with a partner as well.
Kissing is not all about bacterial exchange or romance. Our first experiences with love and security usually involve lip pressure and stimulation through behaviours that mimic kissing, like nursing or bottle feeding. These early events lay down important neural pathways in a baby's brain that associate kissing with positive emotions that continue to be important in throughout his life.
Our lips are the body's most exposed erogenous zone. Unlike in other animals, human lips are uniquely everted, meaning they purse outwardly. They are packed with sensitive nerve endings so even the slightest brush sends a cascade of information to our brains, which can feel very good.
Kissing activates a very large part of the brain associated with sensory information because we're at work making sense of the experience in order to decide what to do next. Kisses work their magic by setting off a whirlwind of neurotransmitters and hormones through our bodies that influence how we think and feel.
If there's real "chemistry" between two people, a kiss can set the stage for a new romance. A passionate kiss puts two people in very close proximity - nose to nose. We learn about each other by engaging our sense of smell, our taste buds and sense of touch. And through that information all sorts of signals are being sent to our brain informing us about the other person. In fact, the scent of man can provide subconscious clues about his DNA to his partner.
Evolutionary psychologists at the State University of New York at Albany found that 59% of men and 66% of women say they have ended a budding relationship because a kiss didn't go well. It's nature's ultimate litmus test, nudging us to be most attracted to the people that may be the best genetic partners.
Research by Swiss biologist Claus Wedekind found that women are most attracted to the scents of men who carry a different genetic code for their immune system in a region of DNA known as the major histocompatibility complex or MHC.
Scientists suspect that when a couple who carry distinctly different genetics for fighting disease, their children are likely to benefit by having a strong immune system. We may not exactly be thinking about parenthood when we connect with someone at the lips, but kissing provides clues to help us decide whether to take a relationship further. (However, it's important to add that women who take the birth control pill show the opposite preference toward men with MHC genetics most like their own. This suggests that when we are on contraceptives, we may be fooling our bodies in more ways than we realise.)
Getting hot under the collar
Aside from helping us find a great match, kissing has other perks as well. It sets off a cascade of neural impulses that bounce between the brain and the tongue, lips, facial muscles, and skin. Billions of little nerve connections distribute information around the body, producing chemical signals that change the way we feel.
A passionate kiss can spike the neurotransmitter dopamine, which is linked to feelings of craving and desire. Oxytocin, known as the "love hormone," fosters a sense of closeness and attachment. Adrenaline boosts our heart rate and can make us start sweating as our bodies begin to anticipate what might occur later. Cortisol, known as the stress hormone, also dips to reduce uneasiness. Blood vessels dilate, breathing can deepen, cheeks flush and our pulse quickens.
Kissing fosters the sensations we often describe when we are falling in love. In this way, a kiss can herald in a new romantic relationship. It can also solidify the strong bonds we share with family members and friends. Kisses come in many varieties and are inherently tied to the most meaningful and significant moments of our lives by providing a means to communicate beyond what words can convey.
Science has barely begun to study kissing, despite its obvious evolutionary and personal significance, but what we already know demonstrates that there's a lot more to going on than meets the eyes - and lips.