The Kiss

We celebrate kisses in literature and art. On screen, it’s the moment we’re always waiting for, and the climax of every great love story. And in our own lives, it’s the ultimate way to express how we feel.

A passionate kiss causes our blood vessels to dilate as the brain receives more oxygen than normal. Our cheeks flush, our pulse quickens, and breathing becomes irregular and deepens. Our pupils dilate, which may be the reason so many of us close our eyes. We also activate five of our twelve cranial nerves that spread out intricately to different parts of the face. The nerve pathways guide the way we interpret the world by helping us see, smell, hear, taste, and touch.

On top of that, our lips are associated with a disproportionately large part of the brain. Sex researcher Alfred Kinsey even reported that some women could reach orgasm from prolonged deep kissing without genital contact. While this sounds unusual, it likely has to do with the way our lips are packed with sensitive nerve endings so that even the slightest brush sends a cascade of information to our brains that often feels very good. Although we often don’t think of them in this way, our lips are the body’s most exposed erogenous zone.

The kiss is a universal language that transcends time and boundaries. Decades ago, anthropologists estimated that over 90 percent of cultures practiced the custom, and with the rise of the Internet and ease of travel in the 21st century, it’s fair to assume that nearly all of us are doing it. Today we see kissing practically everywhere. It is a perfect example of how both “nature” and “nurture” can complement each other to create a single complex and variable behavior. Humans seem to have an instinctive drive to kiss, but the way kisses are expressed is influenced tremendously by individuals’ culture and personal experiences. Yet unlike other human behaviors, science has barely begun to put kissing under the microscope despite its clear evolutionary and personal significance.

How does a kiss work? It acts like a drug by stimulating the natural chemicals in our bodies. When there’s real “chemistry” between two people, the right kiss can spark the magic of true romance by triggering a cocktail of hormones and neurotransmitters to course through our brains and bodies.

Kissing keeps our bodies extremely busy interpreting an enormous amount of information as billions of little nerve connections distribute signals to help determine what happens next. As neural impulses bounce between the brain and the tongue, facial muscles, lips, and skin, these impulses produce a number of neurotransmitters that influence how we feel. The right kiss can lead to the feeling of being on a natural “high.”

One of the most important neurotransmitters kissing can promote is dopamine, which is involved in helping us feel rewarded and experience pleasure. Dopamine is likely the chemical basis for terms like “walking on air.” This is the neurotransmitter responsible for the type of intense desire that makes a new romance feel addictive. Dopamine spikes during novel experiences, and a first kiss with someone special more than fits the bill. It causes us to crave more and can even lead to a loss of appetite, insomnia, or euphoria.

Of course, dopamine is only one of many chemicals that guide our emotions during a kiss. Involved with strong feelings of attachment and intimacy, oxytocin is a hormone that can be triggered by kissing. Endocrinologists believe that it is the substance that helps keep love alive in long-term relationships after the initial novelty has subsided.

Meanwhile, a passionate kiss can also raise our level of serotonin, which leads to obsessive thoughts about the kissing partner. Scientists have observed that serotonin levels in subjects involved in new relationships look a lot like those in patients suffering from obsessive-compulsive disorder. Kissing also stimulates adrenaline, which helps our bodies to anticipate what might occur next. It boosts our heart rate, which reduces stress, and can make us break into a sweat. In other words, the body’s chemical response to a good kiss mirrors many of the same feelings frequently associated with falling in love.

Love isn’t just romantic. Our brains are primed to associate kissing with feelings of attachment and security from birth. A newborn’s earliest feeding experiences at his mother’s breasts involve movements and mouth pressure similar to kissing. These actions lay down the neural pathways in a baby’s brain that continue to be important in other powerful, bonding relationships throughout his life.

Under comfortable conditions, a kiss from someone we love lowers levels of the stress hormone cortisol, reducing uneasiness and making us feel secure. However, under pressure or in the wrong setting, it can have the opposite effect, which brings up kisses that do not go so well.

Research in evolutionary psychology reports that 59 percent of men and 66 percent of women have ended a budding relationship because of a bad first kiss. When the chemistry feels wrong, both partners instinctively sense they should back off.

How might this work? A kiss puts two people in very close proximity. Our sense of smell allows us to pick up subconscious clues about the other person’s DNA or reproductive status. Biologist Claus Wedekind found that women are most attracted to the scent of men who have a very different genetic code for their immune system in a region of DNA known as the major histocompatibility complex. Pairing off with a male who has a different set of genes for immunity can lead to children that will have a higher level of genetic diversity, making them healthier and more likely to survive. (However, it’s important to note that women who take the birth control pill exhibit the opposite preference.) So even though we may not be consciously aware of it, we use behaviors like kissing to judge whether to take a relationship further, based on genetic evidence. In this manner, it’s fair to say that the act of kissing serves as nature’s litmus test.

Not surprisingly, all of the intense stimulation that a positive exchange initiates can create a very vivid memory. When Butler University psychologist John Bohannon studied over 500 subjects, he discovered that most people remember more of the detail about their first kiss than their first sexual encounter, no matter if it took place five months or 50 years ago.

Whether the experience evokes fireworks and violins, or doesn’t quite live up to expectations, the kiss serves as the single most universal and humanizing practice we all share. It is a reliable way to gauge our relationships and express our emotions, far beyond that which words can convey.

Sheril Kirshenbaum is a science writer and research associate at the Center for International Energy and Environmental Policy at the University of Texas at Austin. She is also the author of The Science of Kissing: What Our Lips Are Telling Us. Kirshenbaum was a TEDGlobal 2011 speaker.

Illustration by Lynda Lucas

The Stuff of Life

Issue 15

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