The Science of Kissing

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Table of Contents

Copyright Page

Preface

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.

XX

Sheril Kirshenbaum, January 2011

Introduction

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.