The Science of Kissing: What Our Lips Are Telling Us

The Science of Kissing

Grand Central Publishing

PART ONE

The Hunt for Kissing’s Origins

I wonder what fool it was that first invented kissing.

—Jonathan Swift

CHAPTER 1

First Contact

When it comes to humanity’s first kiss, or its predecessor in another species, we have no way of knowing exactly how and why, once upon a time, it happened. After all, there are kisses of joy, of passion and lust, of love and endearment, of commitment and comfort, of social grace and necessity, of sorrow and supplication. It would be silly to assume all these different types of kisses developed from a single behavior or cause; in all likelihood, we kiss as we do today for multiple reasons, not just one. In fact, scientists suspect that kissing arose and disappeared around the globe at different times and different places throughout history.

So while there are certainly some convincing theories out there about how kissing may have emerged, nobody claims that they represent absolute truth. At best, they possess a degree of plausibility that makes them persuasive. In this chapter, we’ll survey four such theories, each of which has a basis in the scientific literature.

Scientists have proposed two separate relationships between kissing and our feeding experiences in infancy and early childhood. They have also suggested that kissing may have emerged from the practice of smelling another individual of the species as a means of recognition. I will examine each of these theories, but will begin with perhaps the most intriguing one of all: the idea that the behavior arose due to a complex connection between color vision, sexual desire, and the evolution of human lips.

A WOMAN’S LIPS make an indelible impression. They draw attention to her face, advertising her assets in deeply hued and rosy colors. The effect is further enhanced because human lips are “everted,” meaning that they purse outward. This trait sets us apart from other members of the animal kingdom. Unlike other primates, the soft, fleshy surface of our lips remains exposed, making their shape and composition intensely alluring.

But what makes them so attractive that we want to kiss the lips of another person?

A popular theory takes us back millions of years, when our ancestors had to locate food among leaves and brush. Calories were hard to come by, and wandering far into the jungle could be dangerous. In this context, some of our ancestors evolved a superior ability to detect reddish colors, giving them the advantage of locating the ripest fruits, which in turn helped them survive long enough to pass on their color-detecting genes to their offspring. Over many generations, the signal “red equals reward” became hardwired into our ancestors’ brains. Indeed, the color continues to grab our attention today—something marketing professionals know and exploit regularly.

Contemporary psychologists report that looking at red quickens the heart rate and pulse, making us feel excited or even “out of breath.” In fact, red seems so important to humans that time and again, across early cultures, it is one of the first colors to be named. In their 1969 book Basic Color Terms: Their Universality and Evolution, anthropologist Brent Berlin and linguist Paul Kay studied twenty languages and determined that after cultures develop words for black and white (probably because these help to determine day from night), red is frequently the third.

But how does this relate to kissing? Neuroscientist Vilayanur S. Ramachandran of the University of California, San Diego, suggests that once our ancestors were primed to seek red for a food reward, they were probably going to check out the source of this color wherever it occurred—including on parts of the female anatomy. Eventually, red likely served as a flashy signal to help facilitate another essential and enjoyable behavior besides eating: sex.

Comparative evolutionary research has demonstrated that in primates, skin and hair coloring evolved after color vision. In other words, once our ancestors developed the ability to detect this color, it became emphasized on their bodies and particularly in the labial region, serving to indicate a female’s peak period of fertility, called estrus. Those with the most conspicuous sexual swellings were probably also most successful at attracting males and passed their flamboyantly endowed posteriors on to their daughters. Today, there’s no mistaking the females of many species when they are ready to mate. As Duke University primate scientist Vanessa Woods puts it, “Female bonobos look like they are carrying their own bright red bean bag attached to their bottoms to sit down on when they get tired.”

But how did an attraction to the color red move from our nether regions to our facial lips? The most likely scenario is that when our ancestors stood upright, their bodies underwent many associated changes in response, including a shift in the location of prominent sexual signals. Over time, the delectable rosy color, already so attractive to males, shifted from our bottoms to our faces through a process called evolutionary co-option. And the male gaze followed.

That’s why human females do not have to advertise our reproductive cycle on our rear ends. We exhibit what’s called “hidden estrus” instead. But following this theory, our lips are quite literally a “genital echo,” as the British zoologist Desmond Morris put it, resembling the female labia in their texture, thickness, and color. Indeed, when men and women become sexually excited, both our lips and our genitals swell and redden as they are engorged with blood, becoming increasingly sensitive to touch.

To test the “genital echo” hypothesis, Morris showed male volunteers photographs of women wearing various lipstick colors and asked them to rate the attractiveness of each. The men consistently chose those featuring the brightest (most aroused-looking) red lips as most appealing. To quote Morris, “These lipstick manufacturers did not create an enhanced mouth; they created a pair of super labia.”

And if a plump, rosy smile gets noticed, it probably means men themselves are rewarded for paying attention—in an evolutionary sense. A woman’s naturally large, reddish lips may provide clues about her fertility. They swell when she reaches puberty, and thin with age. Multiple studies have linked full lips to higher levels of the hormone estrogen in adult women, meaning that they serve as a reliable indicator of her reproductive capacity.

No wonder that across cultures, men report that fuller lips on women are an asset, and in turn women have recognized for millennia that there’s power in highlighting them. The first record of lipstick dates back five thousand years to the Sumerian region, and ancient Egyptians, Greeks, and Romans used dyes and strong wines to tint their lips.

Today’s men continue to respond to the stimulus of a sexy mouth, and many women are eager—even desperate—to achieve Angelina Jolie–like proportions. Not only do 75 to 85 percent of American women wear lipstick, but we are taking the obsession to new extremes. We purchase plumpers to achieve the “beestung effect,” and purposely irritate our outer lip membranes with everything from cinnamon to alpha hydroxy acids and retinol. We coat our mouths with formulas from sheep glands and regularly inject fillers and fat. Some women even insert Gore-Tex strips through painful lip implant procedures, which are increasing in popularity (even though a partner can sometimes feel them during a kiss). In the end, women are paying billions of dollars for a result that may be driven by the same impulses that first attracted our primate ancestors to ripe fruit.

Granted, the science suggests that all those fancy creams and glosses actually work… up to a point, anyway. According to psychologist Michael Cunningham of the University of Louisville, men really do prefer larger lips. However, they also report that fake-looking lips are a turnoff, suggesting that the size of a woman’s mouth in relation to her other facial features is most important. Therefore, when those natural proportions are upset through cosmetic surgery, the result may not be as attractive as the original package.

So it’s true: Our lips probably did evolve to look the way they do because they elicit a magnetic sexual attraction. But in the quest to understand the origins of kissing, there’s a lot more ground (and face) to cover.

FOR THE NEXT THEORY, we need to consider the development of a human baby. During the first trimester in the womb, it will develop recognizable lips, and even before birth, fetuses have been observed to suck their thumbs. Upon delivery, newborns immediately form their mouths as if to nurse, which in a mechanical sense is the movement associated with kissing.

Noting as much, Desmond Morris had another idea about our lips. Aside from a “genital echo,” he was interested in the way their shape makes them very well suited to suckle milk from the distinct human breast.

When our ancestors began to stand upright, red lips weren’t the only sexual signal to migrate across our bodies. Female breasts also became more pronounced, mirroring the look of buttocks. While all mammals provide milk to their young through nursing, the rounded human breast has a unique contour. Unlike in species covered with hair, a woman’s naked breasts stand out, drawing attention to her nipples.

During pregnancy, the breasts become swollen and tender. When the baby is born, a mother holds it close during feedings and the newborn responds by engulfing the nipple in his or her mouth. This encourages sucking behavior as the infant ingests vital nutrients needed to grow. Nursing is enormously pleasurable for the child, and new mothers quickly recognize that it’s a great way to soothe and calm a fussy baby.

Given the critical importance of keeping infants well nourished, it’s not surprising that evolution would shape the human nipple and lips so that they fit comfortably together. Furthermore, breast-feeding promotes a deep bond between a caregiving mother and her completely dependent baby through a flood of chemical messengers in their brains called neurotransmitters (which I’ll discuss further in chapter 5). Here is the first encounter with safety and love, and Morris hypothesized that throughout our lives we associate lip pressure with these feelings. Later in life, we will seek similar experiences in other relationships, and kissing will come to promote a special connection between family members as well as between lovers. It allows us to convey warmth and affection through an expression we began to experience in infancy.

There has been a great deal of literature about the importance of the relationship between mother and child, and how this may dictate other encounters throughout our lives. Some readers will note that with this theory of kissing’s origins, we are approaching terrain popularized by Sigmund Freud. He, too, proposed that the drive to kiss begins during infancy. The child, once deprived of its mother’s breast, seeks similar pleasurable sensations throughout its lifetime through thumb-sucking and other behaviors. As Freud put it, “The inferiority of this second region [the thumb] is among the reasons why at a later date he seeks the corresponding part—the lips—of another person (‘It’s a pity that I can’t kiss myself,’ he seems to be saying).” According to Freud, we spend our lives trying to return to our mother’s breast.

The big difference between the views of Freud and Morris is that while Freud viewed kissing as a symptom of breast deprivation, Morris described it as a way of rekindling positive experiences from infancy. Although we do not have clear memories of our earliest years, puckering up to nurse in a mother’s comforting embrace probably does have a lasting impact on us, as lip contact itself becomes entwined with feelings of love and trust.

OF COURSE, THERE’S MORE to feeding a growing child than breast milk or formula, and kissing theories also abound that originate in the next stage of development following infancy.

For thousands of years, “premastication”—the prechewing of a meal for another individual—served as a central means of delivering food to young, largely helpless toddlers. A premasticating mother places her mouth over her child’s and parts both lips. Then, using her tongue, she presses soft food between them.

While this practice may sound unappetizing to some, it’s important to remember that for most of our existence, mothers had far fewer options than they do now. Daniel and Dorothy Gerber didn’t start hand-straining solids in their kitchen until 1927, and grocery stores stocking prepared jars of mashed peas haven’t speckled our landscapes until rather recently. Premastication was the most practical way to wean children off breast milk before they had a full set of teeth.

Written records of prechewing food date back to ancient Egypt. But humans have probably been feeding each other this way since prehistoric times and, indeed, the behavior likely comes to us from our nonhuman ancestors, like great apes. It occurs in other parts of the animal kingdom as well, as I’ll discuss in the next chapter.

In fact, premastication persists in human cultures even today. A recent survey reported that people in 39 out of 119 modern cultures studied premasticate a wide array of substances, citing food exchange, healing rituals, disease prevention, and more. However, it is important to note that kissing is not necessarily present in all cultures where the prechewing of food occurs. For example, premastication was long practiced among the Ituri Pygmies of the Congo, yet mouth-to-mouth kissing was apparently unknown among these peoples until Europeans arrived.

Nevertheless, just as with nursing, premastication may lay the foundation for kissing behaviors later in life. We’ve already seen how oral stimulation in infancy helps foster loving feelings and strong attachments. The premastication theory is, in essence, just an extension of this logic. Past the nursing stage, the child continues to develop and receive care from a loving mother, and now the oral stimulation occurs in a mouth-to-mouth fashion. The intense bond comes to center on contact at the lips—and quite possibly a pattern of behavior and emotional response is established that will help promote kissing much later in the child’s life.

In this way, it is possible that through repeated puckering between mothers and children, the passionate romantic kiss between lovers could have emerged.

THE NURSING AND PREMASTICATION hypotheses suggest that our ancient guide for intimacy may lie in something far less romantic—suckling a mother’s breast, sharing prechewed food along with our saliva, or some combination of both. However, there is also evidence that kissing may originate with a very different facial feature: our noses. The friendly and familial variant of the kiss could have started with a sniff.

Humans have powerful scent glands under our skin, giving each of us a distinct smell. Scientists have observed that even in infancy, human beings use their noses to keep track of important relationships. Breast-fed newborns, for example, seem able to recognize their mother’s natural odor, while bottle-fed babies do not develop the ability.

In a similar way, many anthropologists believe that the first “kisses” may have been delivered via our noses rather than our lips, as we closely inhaled the scent of our loved ones’ cheeks. Many early cultures became accustomed to what’s called the “oceanic kiss,” so named to describe a traditional greeting in Polynesia. Such a “kiss” involves going back and forth across the nose to smell another person for the purpose of identification, and probably served as a reliable means to recognize and reconnect with relatives and friends, and perhaps even provide clues about a person’s health.

Over time, a brush of the lips may have come to accompany this practice—eventually leading to the evolution of kissing as a greeting. This could have started the tradition of the social kiss, in which we welcome friends and community members, sending the message that we’re glad to see them or have missed their company.

It’s important to note that whether or not your intentions are romantic, to kiss another person on the cheek or elsewhere—or to sniff him or her—it’s necessary to move into that individual’s “personal space.” To get this close, there must be some level of trust or expectation. Thus delivering a friendly kiss or sniff, or receiving one, amounts to an unspoken gesture of acceptance.

What’s particularly powerful about the sniffing theory is that we have many accounts of this greeting among indigenous peoples. In 1883, for instance, the British South Seas explorer Alfred St. Johnston published Camping Among Cannibals, in which he described how a tribesman in Fiji smelled his hand in a “courteous and respectful” salutation and farewell. Another example comes from Charles Darwin’s description of the so-called Malay kiss:

The women squatted with their faces upturned; my attendants stood leaning over them, laid the bridge of their noses at right angles over theirs and commenced rubbing. It lasted somewhat longer than a hearty hand-shake with us. During this process they uttered a grunt of satisfaction.

Even today, many cultures continue to show affection by smelling a loved one on the cheek. The traditional Canadian Inuit kunik, or “Eskimo kiss,” does not actually involve rubbing noses together as commonly believed. Instead, it’s a kind of nuzzle-sniff. To properly bestow a kunik, you press your nostrils against the skin of a loved one and breathe in, thereby suctioning the skin of the recipient against your nose and upper lip. The Maori of New Zealand practice a similar custom.

So might older habits of sniffing really be one reason we kiss today, particularly when it comes to delivering greetings? It’s not all that outlandish a possibility when you consider that sampling another person’s scent is a primal urge, even if no longer in accordance with polite manners. As we’ll see later, laboratory research has found that human subjects report preferring the scent of a partner or their children to that of strangers, suggesting that smell provides important clues into our relationships. As humans developed better language skills, smell probably became less necessary for recognizing one’s relatives, but remained an important means to strengthen bonds between people.

Today, of course, blatant sniffing wouldn’t generally go over well. It might come across as offensive, embarrassing, or worse. Yet over much of our collective past, sniffing may have been considered a perfectly normal behavior among friends and acquaintances. In fact, it’s something many people still do in greeting someone new or entering an unfamiliar home—even though we don’t admit it.

AS WE HAVE SEEN, there are a great many possible evolutionary pathways that might explain the origins of kissing. The hypotheses I’ve reviewed may have worked individually to promote kissing, or may have complemented one another and overlapped. But no matter when or where it began, there is little doubt that it was dramatically reinforced once it started.

The work of Rutgers University anthropologist Helen Fisher suggests that kissing’s prevalence is ultimately fostered by our brains. She proposes that the behavior likely evolved to facilitate three essential needs: sex drive (lust), romantic love (attraction), and a sense of calmness and security (attachment). Our sex drive encourages us to find partners, romantic love leads us to commit to one person, and attachment keeps us together long enough to have a child. These are not phases, but brain systems that can act together or independently. Each is involved in promoting reproduction, and kissing bolsters all three by encouraging close relationships.

Fisher’s reasoning suggests that whatever the means by which kissing arrived among us, its persistence can be traced to its advancement of key human social and reproductive needs. With each distinct human culture, it’s likely that kissing emerged in part out of instincts rooted in our evolutionary past, but was also influenced by unique social norms among peoples, giving it a very diverse cast in different places.

And humans aren’t the only ones exchanging saliva and affectionate gestures, or engaging in kissing and kissing-like behaviors. Many other species were licking, nuzzling, caressing, and more, long before we arrived—and in many ways their behaviors parallel ours, and often seem to serve a similar purpose. The next chapter, then, looks to the animal kingdom for “kissing” among the furry, slimy, prickly, and aquatic creatures with whom we share the planet. From them, we find additional proof that however kissing originated, similar behaviors are shared not only among human cultures but across species—strong evidence that, despite all the variability, affectionate nibbling and muzzling may be rooted in our common evolutionary lineage with the rest of life on earth.

Not All Cues Are Hidden

In 2007, a team of psychologists from the University of New Mexico published a paper suggesting that even though estrus—what we call “being in heat at peak fertility”—is concealed in humans, men may still be able to detect it on a subconscious level. The researchers came up with an ingenious way of studying this by examining the tips earned by eighteen exotic dancers at gentlemen’s clubs.

These women recorded the onset of their periods, shift hours, and tip earnings for two months (or some 5,300 lap dances), and the results were intriguing. Dancers earned, on average, $70 per hour when ovulating, $35 per hour while menstruating, and $50 per hour during the weeks in between. Notably, women on birth control pills did not show the earnings peak.

Although the New Mexico scientists aren’t entirely sure what to make of this result, and the sample size was limited, it suggests that although modern women do not visually display rosy bottoms, estrus in our own species may not go completely unnoticed.

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Excerpted from The Science of Kissing by Kirshenbaum, Sheril Copyright © 2011 by Kirshenbaum, Sheril. Excerpted by permission.
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