The Science of Kissing

What Our Lips Are Telling Us


By Sheril Kirshenbaum

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From a noted science journalist comes a wonderfully witty and fascinating exploration of how and why we kiss.

When did humans begin to kiss? Why is kissing integral to some cultures and alien to others? Do good kissers make the best lovers? And is that expensive lip-plumping gloss worth it? Sheril Kirshenbaum, a biologist and science journalist, tackles these questions and more in The Science of a Kiss. It’s everything you always wanted to know about kissing but either haven’t asked, couldn’t find out, or didn’t realize you should understand.

The book is informed by the latest studies and theories, but Kirshenbaum’s engaging voice gives the information a light touch. Topics range from the kind of kissing men like to do (as distinct from women) to what animals can teach us about the kiss to whether or not the true art of kissing was lost sometime in the Dark Ages. Drawing upon classical history, evolutionary biology, psychology, popular culture, and more, Kirshenbaum’s winning book will appeal to romantics and armchair scientists alike.


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Table of Contents

Copyright Page


A kiss is one of the most significant exchanges two people can have, serving as an unspoken language to convey our deepest feelings when words simply will not do. From a symbol of love and desire to a perfunctory greeting between family and friends, this act can have innumerable meanings and resonances. For many of us, it is part of our earliest introduction to planet earth, and is often involved in our final exit as well. Some kisses are sealed forever in our minds and hearts, while others are forgotten as quickly as they occur. Across continents and time, kissing is one of the most important activities in our lives, yet its real nature has been too often overlooked by scientists and laypeople alike.

When I first told friends and colleagues I was working on this book, many of them wondered aloud what would inspire a project on osculation—the scientific term for kissing. But I turned the question around: Why not? After all, decades ago, anthropologists estimated that kissing was practiced by over 90 percent of cultures around the world. The figure has probably grown thanks to globalization, the Internet, and the ease with which we now course across hemispheres. Even in societies in which couples do not traditionally kiss, they frequently engage in similar behaviors, such as licking or nibbling at one another's faces and bodies. This makes kissing a practice with obvious evolutionary significance, whose study could provide insight into our collective past and current physiology. And given that kissing also leaves such an indelible mark on the human experience, why not further explore this behavior from as many angles as possible?

My journey toward writing this book began in 2008. The week before Valentine's Day, I composed a short piece, entitled "The Science of Kissing," at The Intersection, the Discover magazine blog I share with science journalist Chris Mooney. To our surprise, readership spiked as the page was linked widely around the Internet. We received thousands of visitors over the next several days, and emails poured in with questions. They never stopped.

By Valentine's Day 2009, I had co-organized a panel discussion on "The Science of Kissing" for the normally staid annual meeting of the American Association for the Advancement of Science. The press went wild scheduling briefings, and our kissing symposium was covered by major news outlets ranging from National Geographic to CNN, making headlines in countries all around the world. Everyone seemed curious to hear what a bunch of scientists had to say about something so obviously relevant to each of our lives.

As the kissing queries continued, I dug into some books to see what was out there. The answer was, not much. The standard how-to manuals contained few answers to my growing list of questions. I wanted some solid explanations about why we kiss, what happens to our bodies when we do, and what this information might teach us about kissing in relationships. So I began interviewing experts, reading the scientific literature, and collecting theories. Some focused on chemical interactions during a kiss that may help us determine whether we have made a good match. Others tried to uncover kissing's origins by looking to our ancient ancestors' sexual exploits and preferences. It turned out there was a lot of interesting research related to kissing, but it was all in fragments.

As my investigation progressed, though, the science from different fields began to converge. Neuroscientists trying to understand how our brains function were interested to hear what endocrinologists reported on hormonal changes related to kissing. In turn, those same endocrinologists asked what I was hearing from anthropologists about similar behavior in other primates, like chimpanzees and bonobos. The anthropologists were curious to know what physiologists were finding about the body's physical response to a kiss. And so on.

Accessing the scientific literature on this subject, however, posed challenges of its own. I frequently found myself in awkward conversations with petite, elderly librarians that went something like this:

"Can you please help me find the article called 'Fetishes and Their Associated Behaviors'?"

"I'm sorry, did you say 'Fetishes'?"

"Yes, please."

"Here's the reference, dear. I won't ask what this is for, but be careful."

My research also garnered endless curious and at times incriminating glances from strangers as I reviewed related art and historical accounts on my laptop. On top of that, I discovered that mountains of misinformation have been circulating for years on kissing, with no scientific basis. And then there were the pursuits that led me to wander into very unfamiliar territory, such as when I interviewed a sex-robot engineer and viewed my brain in the laboratory. Needless to say, I could not have possibly imagined what I was getting into when I set out to write this book.

Fortunately, as I began composing the actual manuscript, a serendipitous coincidence brought me to San Francisco just in time to catch Mary Roach, author of Bonk: The Curious Coupling of Science and Sex, on her book tour. I was relieved to hear about Roach's struggles through similarly embarrassing situations, and listened intently as she discussed the challenges of writing on related topics. I took her words to heart, feeling inspired to press on. I had already been influenced by the works of sex research pioneers like Alfred Kinsey, William Masters, Virginia Johnson, and many others. If these brave individuals could go all the way when it came to exploring sexuality, surely I could at least aim for first base.

If a book on kissing raises some eyebrows, I can live with that. So with an open mind, several scientists in tow as allies, and a few inspired ideas, I embarked on the journey to understand the kiss—and learned more than I could have possibly imagined.

This book tells the true story of humanity's most intimate exchange.


Sheril Kirshenbaum, January 2011

Any man who can drive safely while kissing a pretty girl is simply not giving the kiss the attention it deserves.

—Albert Einstein


Scientists are not exactly sure why we kiss. This may be in part because they have not even definitively decided what a kiss is. Unlike most other areas of scientific investigation, there's no accepted "taxonomy," or classification system, for different kinds of kisses and closely related behaviors. What's more, you don't find the experts crunching the numbers and figures on kissing across world cultures, as researchers would surely do if they wanted to get a handle on the available data. Why so little analysis of osculation? Perhaps kissing seems so commonplace that few of us have paused to reflect on its deeper significance. Or it's possible the subject has been intentionally avoided under the microscope given the challenges of interpreting what a kiss really means.

Yet the behavior we recognize as kissing simply cries out for better scientific explanation. Just think: From a completely clinical perspective, microbiologists will tell you that it is a means for two people to swap mucus, bacteria, and who knows what else. Picturing all those tiny organisms swishing through our saliva isn't just unromantic, it inspires a question: Why would this mode of transferring germs evolve? And why is it so enjoyable when the chemistry is right?

When it comes to kissing, there are also immediate and personal reasons for wanting to explore the science. It can help us understand how much kissing really matters in relationships, and whether we can enhance them by improving our technique. Are we born knowing how to kiss, or does practice make perfect? Do men and women experience kissing the same way? Why can a bad kiss stop a promising relationship cold, whereas the right one can begin something special with the person we least expect?

Because a kiss brings two individuals together in an exchange of sensory information by way of taste, smell, touch, and possibly even silent chemical messengers called pheromones (odorless airborne signals), it has the potential to provide all kinds of insight into another person. So even when our conscious minds may not recognize it, the act can reveal clues about a partner's level of commitment and possibly his or her genetic suitability for producing children.

The human body's response to kissing is just one of many intriguing aspects of the science involved. From an evolutionary perspective, scientists can't fully make up their minds whether humans kiss out of instinct, or if instead it's a learned behavior for expressing affection. The dispute traces back to none other than the father of evolutionary biology, Charles Darwin. In his 1872 book The Expression of the Emotions in Man and Animals, Darwin noted with interest that kissing "is replaced in various parts of the world, by the rubbing of noses." Here I introduce a distinction that will be important in later chapters. This is the difference between kissing with the lips and various "kissing-like behaviors" that may appear related and could serve similar purposes, or even represent a precursor to modern romantic kissing.

The definition of a kiss is relatively simple: It is either the mouth-to-mouth orientation of two individuals or the pressing of one's lips on some other part of another's body (or on an object). But "kissing-like behaviors" is a much broader category, and should include a large array of exchanges between people (or animals) that focus on the use of the lips and face, and perhaps some other body parts. For example, Darwin described the practice—very common in many cultures—of sniffing another human being in close contact, in search of recognition or to establish a rapport. Despite such cultural diversity, however, he suggested that the many different types of kisses and related behaviors found around the globe all reflected an innate desire to receive "pleasure from close contact with a beloved person."

In its broadest sense, then, Darwin surmised that the drive to "kiss" was innate and perhaps hereditary—or as we would now say, encoded in our genes.

Today some anthropologists disagree, maintaining that kissing is a purely cultural phenomenon—a learned behavior that we pick up merely by watching others do it. The majority of experts, though, seem to share Darwin's original view, especially when using his broad definition, which groups together kissing with practices that included "the rubbing or patting of the arms, breasts, or stomachs," as well as "one man striking his own face with the hands or feet of another." From this vantage point, kissing-like behaviors appear nearly universal among human beings. And as we'll see, they have so many analogs in other species that they are likely part of our common evolutionary inheritance.

To fully explore the scientific kiss, this book takes its inspiration from an approach originally popularized by the late Dutch ethologist Nikolaas Tinbergen. Tinbergen emphasized that in order to understand a particular behavior, we should ask a specific set of questions about it. The answers to these questions are not mutually exclusive but instead serve to inform one another.

Part 1 of this book will explore what Tinbergen called the "ultimate" explanations for kissing—those that center on the behavior's evolutionary history and purpose. Here I will describe the leading theories that might account for how and why early humans started locking lips. Were we the first species on the planet to do so, or have we inherited the behavior from a shared ancestor with other mammals? By comparing human kissing with similar behaviors in other animals, we'll gain insight into how and why kissing emerged.

Next I will move on to examine kissing throughout human history and across modern cultures. At the end of this survey, you will see that while kissing-like behaviors take a vast number of forms, and while kissing norms in the world today vary greatly across societies, the basic desire to embrace another individual using the face, mouth, and sometimes other related parts of the body does appear to be universal, just as Darwin concluded. I'll come to terms with the famous "nature/nurture" debate by showing that the way we kiss is conditioned both by our biology and culture—the result being a fascinating variety of unique kissing styles, customs, and techniques.

But in a sense, that's just the prelude to the heart of the book. Part 2 explores how kissing is actually experienced in our bodies, an analysis that will allow us to consider what Tinbergen called the "proximate" explanations for this behavior. That means looking at kissing in its immediate context among individuals, and seeking to understand the neurological, biological, or psychological reasons underlying the motivation to kiss. Here I will also explore how the act of kissing directly affects an individual and the role it plays in the relationships he or she chooses, and chooses not, to have. We'll also learn about some major differences in how men and women perceive kissing, and the hidden information that kisses can convey.

Part 3 builds upon the lessons learned by moving into an actual laboratory setting to try to make some new discoveries about the science of kissing. In this, I'll enlist the help of a group of brave neuroscientists from New York University, who set up a novel MEG (magnetoencephalography) experiment using a cutting-edge scientific machine whose interior nevertheless looks remarkably like a toilet. From there, we'll glimpse at what the future of kissing itself may look like in our increasingly interconnected, digitized, and even robotic world. Finally, I'll synthesize themes throughout the book to provide some practical advice based on the best kissing research to date.

The ideas and theories on kissing that you'll read in these pages may be numerous, but unlike on popular reality television shows, we don't necessarily need to eliminate all the alternatives in order to isolate a winning contender. Rather, we'll explore kissing through many lenses at once, and you'll soon see that it's possible to tie seemingly unrelated fields of science together in unexpected and intriguing ways. By the end of the journey, you'll know vastly more about what's behind a kiss—but I promise, this knowledge won't take any of the magic away.



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.



On Sale
Jan 5, 2011
Page Count
272 pages

Sheril Kirshenbaum

About the Author

Sheril Kirshenbaum is a research scientist at The Center for International Energy and Environmental Policy at the University of Texas at Austin . She blogs on Discover magazine’s website, The Intersection, and contributes to a variety of blogs and science publications. Visit her website at

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