The Science of Kissing: What Our Lips Are Telling Us

by Sheril Kirshenbaum

  If you think that’s impressive, consider another kissing record. In 2003, Joni Rimm paid $50,000 for the most expensive kiss ever sold at auction. She earned the privilege of kissing actress Sharon Stone at an AIDS charity event.

  On Valentine’s Day 2009 in Mexico City, couples, friends, and family members shared all kinds of kisses for 10 seconds. In total, 39,897 people kissed simultaneously, setting the latest world record.

  PART TWO

  Kissing in the Body

  How did it happen that their lips came together? How does it happen that birds sing, that snow melts, that the rose unfolds, that the dawn whitens behind the stark shapes of trees on the quivering summit of the hill? A kiss, and all was said.

  —Victor Hugo

  CHAPTER 5

  The Anatomy of a Kiss

  In recent decades, science hasn’t just cast light on the likely evolutionary origins of kissing. It has also taught us vastly more about the biology of a kiss as it occurs in our own bodies. By moving on from evolutionary biology and anthropology to physiology, we can start answering the kinds of questions that bear directly on our own romantic experiences: What happens to our bodies during a kiss? And what can we learn from this information about how to do it better?

  To begin, let’s trace a kiss from the moment of its inception between two partners all the way through the human body, paying close attention to the responses and stimuli that will determine whether we want to continue. I won’t distinguish between the male and female kissing experience yet, though obviously it differs greatly—that’s the topic of the next chapter. But for now, there’s so much to the physiology of a kiss that we’ll be busy enough even without introducing a gender divide.

  In determining how well a kiss is likely to go over, the environment in which it occurs is the first important factor. For example, the thrill of an erotic kiss, even from the man or woman of your dreams, would probably be greatly tempered during a solemn religious ceremony in a church, synagogue, or mosque (unless that’s your kind of thing).

  So picture it: The mood is set in a dark, candlelit, romantic setting and the person you adore looks into your eyes, holds you close, and you feel a rush of passion. It seems almost magical, and kissing is the first and possibly the only thing on both of your minds.

  Even before lips meet, a lot is going on here in the body. In particular, your eyes may be doing something incredible. Psychologist Arthur Aron of the State University of New York at Stony Brook has found that staring into a partner’s eyes has a tremendous impact on the feelings associated with falling in love. In his study, Aron paired male and female strangers together for an hour and a half, instructing them to first discuss intimate details of their lives and then, at the end of the time period, to stop talking and stare into one another’s eyes for four minutes. Afterwards, many of the research participants reported feeling a deep attraction for the other. In fact, two couples from the study were married within six months.

  Let’s assume that the eyes and the environment have done their seductive work and both partners move in for a kiss. Here an important motion happens, although we rarely ever think about it: We tilt our heads, either to the left or right. (Hopefully it’s the same way that our partner does coming from the opposite direction, lest there be an awkward collision.)

  According to psychologist Onur Güntürkün of the Ruhr-University of Bochum, Germany, about two-thirds of us tilt to the right when we approach for a kiss. In 2003, he reported as much in the journal Nature after conducting a rather voyeuristic experiment: Güntürkün watched lovestruck partners, aged roughly thirteen to seventy, kiss in public places like train stations, airports, parks, and beaches in Germany, Turkey, and the United States. To qualify, the kisses under observation required lip contact, a clear head tilt, and a lack of encumbrances, such as bags, luggage, or other objects that might influence the tilt direction.

  Interestingly, this tendency toward rightward head tilting does not seem to correlate with the percentage of us who are right-handed, because “righties” are almost eight times more common than “lefties.” It has been suggested by Güntürkün that instead, we may adopt our head tilt direction while in utero, as a fetus moves and tilts its head in the womb. However, others expect that the preference is set later in life, as we nurse. Studies show that up to 80 percent of mothers cradle their babies to the left, regardless of whether these moms are left- or right-handed. To nurse, the infants then have to turn their heads to the right, so it’s possible many of us learn to associate a rightward head tilt with feelings of affection early in life.

  Additionally, there is likely a subtle kind of interactive effect involved in determining a particular kissing alignment. Human gestures help us to interpret speech and understand language, so it’s possible that the initiator of a kiss subtly informs the other person about what to do through nonverbal cues. A slight head tilt to the right or left instantly provides visual, tactile, and other sensory signals about the situation. But if head tilt direction gets selected by one partner and shadowed by the other, these respective roles would be nearly impossible for an observer to distinguish. (It is worth noting, however, that following Güntürkün’s study, another experiment tested head tilt preference using dolls to eliminate the influence of social cues. The results revealed a similar bias to the right, independent of a partner’s influence. So although social clues are undoubtedly involved, other factors are also at work in determining our choice.)

  CREDIT: STUDIO WIM DELVOYE

  Kiss 4

  2000

  125 × 100

  Cibachrome on aluminium

  Even as we’re getting our heads aligned for a kiss, we also have to get our mouths ready—which means priming our facial muscles for action. The orbicularis oris muscle runs around the outside of our mouths, making it relatively easy to change the shape of our lips, especially when we’re inclined to pucker up. Meanwhile, the zygomaticus major, zygomaticus minor, and levator labii superioris work together to pull up the corners of the mouth and top lip; and the depressor anguli oris and depressor labii inferioris pull down the corners of the mouth and lower lip. And that’s just the beginning—an open mouth and tongue movement involves a far more complicated network of facial and postural muscles. Hopefully the recipient is worth all the trouble, as there’s significant coordination involved, not to mention the risk of wrinkle lines etched into your face over time from this repeated activity.

  But no matter how we get there, eventually—assuming we don’t bump foreheads or noses—there’s lip contact. That’s when things really start to heat up. Five of our twelve cranial nerves switch into high gear. These are the nerves that emerge directly from our brain stem, spreading intricately out to different parts of the face. They are responsible for all sorts of complex activities, helping us to hear, see, smell, taste, touch, and create facial expressions.

  During a passionate kiss, our blood vessels dilate and we receive more oxygen than normal to the brain. Our breathing can become irregular and deepen; our cheeks flush, our pulse quickens, and our pupils dilate (which may be one reason that so many of us close our eyes). It’s not exactly a workout, but kissing burns a few calories, with the quantity, of course, depending on the kissing session’s intensity and length.

  A long, openmouthed exchange also allows us to sample another person’s taste. The tongue is ideally designed to gather such information: It is covered with little bumps called papillae that feature our nine to ten thousand taste buds. (Let’s hope what they taste is our partner’s saliva, and not whatever he or she has been eating.)

  And still, that’s just a small sample of what’s going on. Whether we’re relaxed or nervous, our bodies are extremely busy, processing an incredible amount of detail about the situation so we know what to do next.

  PERHAPS MOST IMPORTANT OF ALL, when we kiss, all five of our senses are busy transmitting messages to our brain. Billions of little nerve connections are at work, firing away and distributing signals
around our bodies. Eventually, these signals reach what is called the somatosensory cortex: the region of the brain that processes feelings of touch, temperature, pain, and more. Here they are interpreted, resulting in “thoughts” such as: “Did he just have onions?” or “Where is that hand wandering?”

  The part of our body sending the most information to our brain during a kiss is, without a doubt, the lips. Packed with nerve endings, they are extremely sensitive to pressure, warmth, cold, and indeed to every kind of stimulus. In fact, one of the most remarkable things about the brain’s role in kissing is the disproportionate neural space associated with our lips compared with the rest of our body. Just a light brush on them stimulates a very large part of the brain—an area even more expansive than would be activated by sexual stimulation below the belt. This means our lips are our most exposed erogenous zone!

  To help get your mind around what this means, take a look at the sculpture below, which has been crafted to reflect the relationship between each part of our body and the proportion of brain tissue dedicated to processing sensory information that comes from it:

  © THE NATURAL HISTORY MUSEUM, LONDON

  Sensory homunculus

  In this image, the body surface is “mapped” to create a “brain’s-eye view.” As you can see, the lips and tongue look obscenely large compared with nearly every other feature, because they contain so many sensitive nerve endings. The brain area dedicated to other parts of the body, including the penis, is far smaller in proportion to their sizes. (Although there is not a corresponding sculpture for women, the proportions would be largely the same for most body parts, with the obvious exception of densely innervated organs like the clitoris and breasts. The lips would look enormous in both genders.)

  To date, science has barely begun to scratch the surface of the brain’s very intricate role in the act of kissing. Far and away the most complex (and mysterious) organ in our body, it is made up of about 100 billion nerve cells, connected at points called synapses and capable of transmitting signals to cells in other parts of the body. These neurons carry a dramatic variety of messages at dazzling speed, a feat they pull off thanks to little molecules called neurotransmitters—the chemical messengers of the brain and nervous system. Neurotransmitters make the leap across synapses between one nerve cell and another, bringing along a particular sort of information with them.

  Monkey See, Monkey Do

  A contemporary theory in neuroscience involves the possibility of so-called mirror neurons, excitable cells that fire messages in response to someone else’s experience as if it were happening to us personally. For example, watching another person get a pinprick on his or her hand would stimulate the same area in our brains as if we ourselves were pricked. It has been speculated that these cells are involved in how we interpret other people’s intentions and, therefore, it’s possible that mirror neurons may inform us how to respond to being kissed.

  In 2003, neuroscientists in Italy attempted to study this phenomenon in macaques, a medium-sized species of monkey. While the researchers did not examine kissing itself, they were interested in the motor neurons responsible for behaviors they termed “lip smacking,” “lip protrusion,” and “tongue protrusion.” They found that about one-third of these cells fired in macaques when they simply observed a human experimenter making one of these kissing-like actions.

  If mirror neurons indeed exist, watching another person move forward to kiss us might set off a “kissing response” in our brain as well, encouraging us to shadow the behavior and improving the odds that the kiss will be reciprocated. In the same manner, a partner’s excitement during a kiss may serve to increase our own, setting off a feedback loop of mutual anticipation.

  Since it is a strongly sensual experience, a kiss sends sensations directly to the limbic system, those parts of our brain associated with love, passion, and lust. As neural impulses bounce between the brain and the tongue, the facial muscles, the lips, and the skin during kissing, they stimulate our bodies to produce a number of neurotransmitters, and hormones including dopamine, oxytocin, serotonin, and adrenaline. Additionally, the right kiss can lead to the feeling of being on a natural “high” thanks to a rush of endorphins—substances produced by the pituitary gland and the hypothalamus that make us feel exhilarated.

  There is much to say about all these chemicals and how they work, but first a general comment. As I mentioned, their job is to transmit different kinds of signals between nerve cells, but while we have some idea as to how they influence our emotions and behaviors, keep in mind that there are legions of them coursing about our brains and bodies at any given time. More than sixty distinct neurotransmitters flow through the body’s neural network giving marching orders, in a concoction that has been aptly dubbed the “fluid brain” by endocrinologist Jean-Didier Vincent. So the most important thing to remember is that none of them are acting independently or solely controlling a behavior or experience. Rather, as physicist and science writer Stefan Klein aptly puts it, each neurotransmitter acts as “one voice in a choir.” Furthermore, our large cerebral cortex, which is involved in processing thoughts, allows us to make rational decisions that can be at odds with the changes in our bodies—so it is not as though we are entirely “ruled” by chemical signals.

  When it comes to kissing, one of the most important neurotransmitters is dopamine, a kind of natural drug associated with the expectation of a reward that brings us feelings of pleasure. Spiking during a passionate kiss, dopamine is responsible for a rush of elation and craving, and can also result in the obsessive thoughts that many of us experience in association with a new romance—almost like an addiction. And no wonder: This neurotransmitter is involved in stimulating the same part of our brain as a line of cocaine. It primes us to want more, making us feel energized. Some people pumping lots of dopamine even lose their appetites, or find that they cannot fall asleep—not surprisingly, the same “symptoms” commonly described when “falling in love.”

  Fortunately, dopamine does much more than lead to erratic behavior. It also allows us to recognize interesting situations, remember pleasurable experiences, and seek new ones. During the early period of a relationship, novelty triggers a rush of this neurotransmitter, and kissing more than does the trick. With a special person, that first lip contact can literally drug us on feelings of euphoria. Dopamine is probably the reason people say they feel like they are “on cloud nine” or “walking on air.” Without a doubt, it’s also often to blame for the addictive nature of extramarital affairs. As with many drugs, a person can become dependent on that high, even when he or she feels bad about cheating on a spouse.

  But in all relationships, illicit or otherwise, the novelty wears off relatively quickly, and our biology places a limit on how long the “high” conferred by dopamine can last. Studies have shown that levels of this intoxicating neurotransmitter decrease as we become more accustomed to a romantic partner, which might be why sexual desire tends to wane with the same person over time.

  Not everyone responds the same way. Humans vary in the number of receptors for dopamine dotting the tips of our nerve cells, and research suggests that a high number may predispose a person to sexual promiscuity or addictive behavior. For instance, geneticist Dean Hamer at the National Institutes of Health has reported a possible correlation between a gene that codes for dopamine receptors and erotic urges in men. Hamer reported that 30 percent of men possess this “promiscuity gene,” and they have, on average, 20 percent more sexual partners than average men.

  Women can probably experience a similar addiction to sexual novelty due to the increased uptake of dopamine, but the relationship has not been studied in detail (just one example of the way that, until relatively recently, female sexuality has garnered significantly less attention under the scientific microscope). But if a single stretch of genetic material may code for a “wandering eye” in men, the odds are good that it could encourage the same behavior in women.

  Granted, dopamin
e doesn’t act independently. It’s just part of Klein’s “chemical choir,” and must share its role with many other neurotransmitters, notably oxytocin, which fosters feelings of attachment and affection and is also associated with kissing. (I will return to oxytocin in chapter 8.) At the same time, a good kiss will also increase the body’s level of serotonin, another important chemical involved in regulating our emotions and the transmission of information in the brain. Like dopamine, serotonin can cause obsessive feelings and thoughts about another person. In fact, serotonin levels in someone who reports having just “fallen in love” rival those of patients suffering from obsessive-compulsive disorder (OCD). Meanwhile, a stress hormone called norepinephrine may be responsible for the sensation of feeling weak in the knees.

  Finally, the brain sends signals to the adrenal gland to secrete epinephrine (commonly known as adrenaline), boosting our heart rate, making us sweat, reducing stress, and priming our bodies for more physical contact. It also has the potential to distort our perception of a kiss itself. The rush we feel can enhance the experience (or even fool us into pursuing a poorly suited match). But when the mood, the emotions, and the chemical signals are right, a kiss might just be the start of a very intimate evening.

  AND WE’RE STILL NOT FINISHED with the brain and kissing, because this incredible organ isn’t merely involved in processing sensory data and responding. It’s also at work forming memories, and kisses are ideal for that purpose. Psychologist John Bohannon from Butler University has found that most of us can recall up to 90 percent of the details of a first romantic kiss. In his study of five hundred people, most remembered this experience more vividly than their first sexual encounter.