The Science of Kissing: What Our Lips Are Telling Us

by Sheril Kirshenbaum

  Cortisol, meanwhile, is known as the “stress hormone,” and has a central role in our body’s responses to anxiety or threat. When released, it raises blood pressure and blood sugar, while suppressing our immune response. Cortisol is the reason we can be amped to do well on a test or in a public presentation, and then crash afterward. Too much of the hormone can be a bad thing, but in more moderate amounts it helps to restore stability in our bodies after stressful periods. In healthy people, cortisol levels rise and fall on a daily cycle.

  Hill and Wilson are interested in learning more about how concentrations of oxytocin and cortisol in the body change before and after kissing a partner—and how this, in turn, may promote bonding between lovers. At the outset of their research, the scientists hypothesized that kissing would enhance bonding through hormonal changes, and expected to find that it leads to increased levels of oxytocin and decreased levels of cortisol.

  To that end, their team at Lafayette College recruited fifteen heterosexual couples aged eighteen to twenty-two who were in long-term relationships (averaging 560 days). The experiment took place at possibly the least romantic setting on campus, the university’s student health center. At the outset, each study subject had blood drawn and spit in a cup to provide what’s called “baseline information” (the body’s levels before the experiment) on both hormones. Next, the couples were organized into two groups. Half were instructed to kiss their partner with mouths open, while the rest were told to hold hands and talk. In my favorite anecdote from the experiment, one relieved female participant assigned to kiss her boyfriend remarked to Hill, “Thank God, because I didn’t have anything to talk to him about if we were put in the other group!”

  The experimental group followed their kissing orders, and the couples in the control group carried on a conversation with each other for sixteen minutes. Afterward, everyone provided another saliva and blood sample and filled out questionnaires about their personality, current levels of stress, and the degree of intimacy in their relationship. The women were also asked about their menstrual cycles and whether they used birth control, since these details might impact the results (for instance, women on birth control had higher baseline levels of oxytocin than those not taking the pill).

  Hill and Wilson found that the stress hormone, cortisol, decreased across both groups, regardless of whether couples were kissing or holding hands. Thus it appears that affectionate behavior has real health benefits; it cuts stress. What’s more, when we’re relaxed, we’re also more likely to want to take things to the next physical level.

  Curiously though, oxytocin did not behave at all as expected. It appeared to decrease in women, while rising in men. The length of a couple’s relationship did not affect the strength of response. The scientists had anticipated that they would see an increase in oxytocin across sexes, and were very puzzled by this result. But then, if experiments always went according to plan, it wouldn’t be science.

  So what might have been going on? In research, we must always look for factors outside of a study’s explicit design that could bias the results. In this particular kissing study, there may have been one large inherent flaw: location. The scientists theorized that women might require more than just kissing to feel sexually excited or connected to a partner; they may need other mood-inducing elements. The sterile, unromantic health facility environment could explain why they experienced the opposite response from what was expected.

  The Lafayette team decided to repeat their study, paying more attention to the ambiance. They set the scene, providing jazz music, flowers, and even electric candles for an added touch. They moved a couch into a secluded room in the back of an academic building on campus, rather than conducting trials at the health facility.

  The second experiment included nine heterosexual couples and three lesbian couples. This time, the researchers also considered a third hormone called alpha-amylase, which provides another measure of stress and is related to our sympathetic nervous system. The average relationship length for the participants during these trials was 564 days. As before, the couples were divided into two groups. The “control” couples spoke, but did not touch each other, during their encounter, while the experimental pairs kissed for a set duration of time.

  The outcome was even more perplexing this time. Once again, cortisol levels decreased for everyone, but both women and men had lower levels of oxytocin at the end of the experiment than at the beginning—precisely the opposite of what was expected. Levels of alpha-amylase did not change. Interestingly, the heterosexual women reported feeling a greater increase in intimacy with their partners than either the men or homosexual women did, but the extremely small sample size makes it impossible to draw any conclusions about the reasons for this.

  Overall, these results pose intriguing questions about the way kissing influences our bodies, especially since neither study reflected the trends anticipated. The team has not yet conducted any follow-up research, but they are interested in returning to the topic with a larger sample size in the future. For now, they are working to improve their means of analyzing oxytocin through saliva samples, and are considering running follow-up studies in college residence halls. That way, kissing might take place in a more natural and comfortable setting, and couples would only need to provide a spit sample afterward. (These factors might limit anxiety.)

  This experiment represents a classic example of how science isn’t always predictable, and sometimes surprises the researchers involved. It also demonstrates how difficult it is to study such an emotional subject as kissing. We know oxytocin increases from feelings of closeness, and that kissing promotes bonding between lovers, so it’s a true puzzle why these studies didn’t turn out differently. When the experimental procedure improves, I suspect scientists will observe that oxytocin rises in men and women from kissing—just as we already know it does during sex.

  Nearly every scientific study raises new questions, and Hill and Wilson’s is no exception. For example, how much does the kissing environment really matter in terms of our hormonal response? Would similar trends appear in couples who have been together for much longer or shorter periods of time? If future analyses determine that oxytocin rises from kissing a partner as expected, could kissing soothe a troubled relationship? Quite possibly it could.

  But there’s a darker side, too, to all that saliva exchange. Not every kiss benefits its practitioners, and kissing isn’t always healthy, or even sanitary. In the next chapter, we’ll explore the thing we all feared most in grade school: cooties.

  The Effects of Testosterone on Women

  When women are exposed to artificially high levels of testosterone through steroid use or gender reassignment, they develop more body hair and a deeper voice, and may feel more aggressive. The clitoris can extend a couple of centimeters as well.

  The average heterosexual woman needn’t be concerned, though. The amount of testosterone transmitted in a kiss is many orders of magnitude smaller.

  CHAPTER 9

  There Are Such Things as Cooties

  An intensive care unit nurse I know sees all sorts of strange and disturbing illnesses. When I told her that I was writing this book, she quipped, “Maybe the germs are the ones making us do the kissing.” Swapping spit is such an ideal way of spreading germs that it may be more in their interest than in ours.

  While I wouldn’t necessarily go that far, it’s certainly true that there are many possible illnesses associated with kissing. The human mouth is a filthy place and serves as the breeding ground for legions of bacteria—the microscopic one-celled organisms that keep the antibacterial soap industry booming. Evolutionary biologists Atomz and Avishag Zahavi even suggest that merely accepting a kiss might indicate a partner’s high level of commitment. After all, it means he or she is willing to risk acquiring someone else’s illness in order to embrace and connect.

  This chapter considers the utterly unromantic side of kissing, centered on hygiene and disease. In these days of pandemic fe
ars and H1N1 flu, it’s a very necessary part of the story. The goal isn’t to frighten you: Generally speaking, readers have little to fear, health-wise, from a kiss. Between most individuals, even the French variety is highly unlikely to prove dangerous. But it’s valuable to know just what we’re exposing ourselves to when we decide that someone’s worth the germs—as many of our loved ones certainly are.

  THE AVERAGE BACTERIA’S LIFE is pretty dull. It consumes nutrients from its environment and grows to twice its size; then it divides into two. No big party, simply an exceedingly successful strategy practiced by the oldest living organisms on earth.

  We host an overwhelming number of these foreign bacterial cells inside of us, without which we could not hope to survive. Our bodies are made up of about a trillion human cells, but at any moment we also have about ten trillion bacterial cells in or on us. When you add up all of the DNA living in a person, it turns out that we contain around thirty thousand human genes and three million bacterial genes. As Princeton University molecular biologist Bonnie Bassler points out, this means we’re arguably just 1 percent human and 99 percent bacteria! Most of these bacteria help us by absorbing nutrients, digesting food, producing vitamins, and supporting our immune system, but when the “bad” bacteria take up residence, we can become sick.

  Kissing is a very effective way to share these little guys. Our saliva has many important functions—it allows us to taste food (and our kissing partner) by lubricating our taste buds with a protein called mucin—but it is also an ideal conduit for bacteria, of both the good and bad variety. During the 1950s, Baltimore City College’s Dr. Owen Hendley determined that 278 colonies of bacteria could be passed between kissers, although over 95 percent were of the harmless sort. Nevertheless, the numbers game is pretty stunning when you consider that our saliva contains about one hundred million bacteria per milliliter. For reference, one milliliter of saliva is about the size of a die on a Las Vegas craps table.

  One of the first risks from these bugs is dental decay, the most common human malady around the world. A particularly nasty strain in this respect is Lactobacillus acidophilus, which feasts on chewed but unswallowed starch and sugar (the morsels stuck between our teeth after a meal). Through the process of fermentation, the bacteria transforms them into a substance called lactic acid, which in turn eats away at our enamel and encourages more bacterial reproduction, perpetuating the cycle of tooth decay.

  Susceptibility to cavities is highly varied among individuals, meaning that some of us are far more affected than others. So if your partner’s mouth harbors an above-average bacteria count, you may actually run the risk of getting additional cavities yourself as a result. That doesn’t mean you should shy away from an otherwise healthy relationship based on the number of fillings you spot. Regular brushing and flossing should keep your own bacterial concentration under control regardless.

  Tooth decay is just one of the risks posed by bacteria that travel through our mouths. While we’re still on the subject of oral hygiene, let’s consider something a bit more disturbing, though fortunately mostly benign: a so-called fuzzy tongue.

  You may occasionally notice a thin, colored film carpeting your partner’s tongue. The soft grassy layer may look disconcerting (especially if it’s yellow or brown). This condition can occur when heavy doses of antibiotics kill off the beneficial bacteria in our mouths, allowing a nastier variety to take up residence. Should you ever encounter this phenomenon, the bearer of fuzz probably deserves the benefit of the doubt—so wait a few days before making any judgment about his or her hygiene habits. Still, it’s advisable to avoid mouth-to-mouth contact until things clear up. You wouldn’t want to contract any kind of lingering illness that required the antibiotics to begin with.

  And there are still other, lesser-known types of bacteria that can exploit the human kiss. In 1982, Drs. Barry Marshall and Robin Warren discovered that a bacterium called Helicobacter pylori is commonly responsible for causing ulcers. This bug weakens the protective coating of the stomach and the upper part of the small intestine, allowing stomach acid to get through. Thus while ulcers have multiple causes—they can result from stress or spicy food, among other things—we now know one prominent trigger is a microscopic organism lurking in our mouths. Scientists are still not entirely sure how H. pylori moves between people, but it has been found in saliva, leading many doctors to speculate that kissing may be one cause. (Fortunately, even though nearly one out of five people under the age of forty carries H. pylori, most do not develop ulcers.)

  Moving beyond these relatively harmless conditions, there’s also a troubling correlation between a teenager’s number of kissing partners and the likelihood that he or she will develop dangerous bacterial meningitis. This frightening disease causes inflammation of the meninges (membranes covering the brain and spinal cord) and a condition known as septicemia (blood poisoning). Meningitis symptoms include a high fever, vomiting, severe headache, joint and muscle pains, stomach cramps, diarrhea, cold hands and feet, and sensitivity to light. The condition can be fatal.

  A 2006 study published in the British Medical Journal examined 144 teenagers aged fifteen to nineteen who had been diagnosed with meningitis. The researchers found that openmouthed kissing of multiple partners was associated with a heightened risk of illness. Still, bear in mind that there are many correlated factors not linked directly with kissing that also increase the chance of contracting meningitis. Furthermore, I suspect that the statistics cited above are heavily influenced by the lifestyles of the demographic being studied, which poses a higher risk than average. For example, a crowded dormitory with shared bathrooms probably provides more opportunities than average for exposure to disease.

  Perhaps the most worrisome thing about bacteria is not the conditions they currently cause, but those they might cause in the future. In our germophobic society, bacteria are known to be growing stronger. Through an onslaught of antibacterial hand soaps, cleansers, and unnecessary prescription antibiotics, humans have unwittingly bred resistant strains of bacteria that survive and reproduce when their weaker compatriots perish. We have created superbugs that are increasingly immune to medical treatments, and deadlier than ever before.

  Today many microbiologists fear another pandemic like the Black Death will occur once we can no longer cure the bacterial infections that have evolved a resistance to our best drugs. Staphylococcus and streptococcus (“staph” and “strep” infections) are two bacterial strains that are becoming increasingly resistant to antibiotics, and ever more worrisome to health care professionals. Streptococcal bacteria caused the untimely death of Muppets creator Jim Henson at the age of fifty-three. A growing number of people are walking around carrying colonies of dangerous antibiotic-resistant strains like methicillin-resistant staphylococcus aureus (MRSA), oxacillin-resistant staph aureus (ORSA), and vancomycin-resistant enterococcus (VRE). Should these enter the bloodstream through breaks in the skin (including in the mouth) they can become very dangerous.

  BUT BACTERIA are only one type of germ that can be transmitted through a kiss. Viruses also enter our bodies, where they grow, reproduce, and spread to make us sick. About a hundred times smaller than bacteria, they too can cause all sorts of mayhem. Many are capable of invading and ultimately killing off individual cells. Some viruses reproduce without causing a malignancy, but other strains have been proven to cause diseases like cervical cancer, smallpox, human immunodeficiency virus (HIV), and polio.

  The odds are good that at least one viral kissing-related danger will be intimately familiar to you. Many people reading this book right now already carry this virus in their bodies, and once you do, you become a host for life. I’m referring to herpes simplex 1 (HSV-1), which is easily transmitted and causes reddish or purple cold sores on the outer edge of the lip. Sometimes these blisters occur in bunches, and they can be filled with fluid before scabbing over and disappearing. They may be unsightly and embarrassing, but are not otherwise dangerous except in very rare c
ases. (Another strain called HSV-2 can cause oral sores as well, but is more commonly associated with genital herpes, and also lasts for life.)

  While HSV-1 is commonly associated with romantic kissing, it can also be obtained by the sharing of eating utensils, toothbrushes, or even by social kisses between friends and relatives. Once we’re infected, you might say the virus unpacks and gets comfortable. While many people who are carriers never show the symptoms of HSV-1, lesions can burn before breakout and emerge painfully. An eruption can be triggered by getting a cold, excessive exposure to the sun, stress, a lip injury, or even dental work.

  In truth, it’s nearly impossible to avoid this virus as we go about our lives: An estimated 50 percent of us have acquired HSV-1 by the time we reach our teens, and 80 to 90 percent of the population tests positive by age fifty. With figures like that, there shouldn’t be any stigma associated with HSV-1, but the statistics were of no consolation to a good friend of mine in high school. Her cold sores forced her to suffer through years of taunting by boys who very likely also carried the same virus. I advised her not to pay them any mind because, after all, the real “freaks” are those of us who do not wind up as hosts.

  Of course, it’s the Epstein-Barr virus (EBV), another type of herpes virus, that’s responsible for the “kissing disease,” better known as mononucleosis, or “mono.” It’s also very common. Up to 95 percent of American adults have been infected, and carry and spread the virus, on and off, throughout their lives.

  In childhood, EBV symptoms are generally indistinguishable from those of other illnesses, and may disappear quickly. But during adolescence, EBV causes infectious mononucleosis, accompanied by a fever 35 to 50 percent of the time. The condition brings about inflated lymph glands, a sore throat, and sometimes a swollen liver or spleen. It can make a person feel achy all over and tired. Occasionally there are more serious problems, but the virus is rarely fatal. While it’s carried in saliva and can be spread through kissing, you can get it in all sorts of other ways—by sharing straws, pillows, food, spoons, and forks.