The Science of Kissing: What Our Lips Are Telling Us

by Sheril Kirshenbaum

  More recently, research has demonstrated that when women are ovulating, male partners tend to be more attentive, affectionate, and romantic than on average. And as it happens, they are also more likely to be jealous of other men during this period. Scientists have proposed several triggers for this behavior, which are not mutually exclusive and probably serve to reinforce one another.

  For example, at peak fertility, a woman’s mood and behavior often reflect her increased interest in sex. She is more likely to be attracted to dominant masculine features like a square jaw, to pay greater attention to the way she dresses, and to go out to social functions. There is also research suggesting she may have an enhanced likelihood of cheating on her partner. So it’s possible the man in her life will be responding to her change in behavior. However, pheromones may also be at work, sending him subtle signals about her body’s reproductive status and fostering the urge to care for and protect his partner.

  Studies also suggest that a reason for enhanced libido may be that during ovulation, women become increasingly sensitive to androstenone, a chemical in human male sweat (the same one that’s in pigs). In the famous 1980 “dentist’s chair experiment,” for instance, androstenone was sprayed on a chair in a dentist’s waiting room, and scientists then observed where test subjects chose to sit. They noted that women tended to sit in the pheromone-covered chair, whereas men avoided it. In another study, the same pheromone was applied to a specific restroom stall, and once again, men avoided the treated door. When women were tested in that scenario, however, they did not seem to exhibit any obvious preference.

  Some have proposed a possible relationship between the amount of androstenone secreted by men and the levels of the sex hormone testosterone in their bodies. If it exists, then perhaps during a woman’s most fertile time of the month, she can detect silent signals that draw her to men who are pumping the most testosterone. Kissing them would then put her in close proximity to all that male sweat and androstenone—which would further increase her arousal and receptivity to sex.

  Another human pheromone suspect is androstadienone, also found in male sweat, and also involved with testosterone. This chemical has been reported to influence the mood of heterosexual women, making them feel more relaxed. Meanwhile, about one-third of women also release substances called copulins in their vaginal secretions. Some studies suggest these compounds may work to increase libido and raise testosterone levels in male partners, which would make them pheromones, but scientists still aren’t sure how strong an influence they may have.

  Still, there’s a huge problem with all this human pheromone talk. For while scientists have identified substances secreted by both men and women that may act as pheromones, it’s unclear whether we even have a special organ to detect them. Researchers have suggested that humans, too, might have a kind of VNO. Small holes just inside our nasal opening, on either side of the septum, have been observed in many individuals, varying in size, shape, and location for each person. However, the cells in this region of the body do not appear to connect with the nerves that would presumably be necessary to transmit pheromonal information.

  And there are other reasons for hedging. The current near-consensus of scientists studying pheromones is that a single chemical compound cannot ensure a specific response under every circumstance. Many hormonal and physiological factors influence our behavior, so regardless of whether we can detect pheromones, we just aren’t as predictable as pigs.

  Despite all of this, however, we can’t discard the possibility that we may be constantly transmitting chemical cues to other people, and especially when we’re up close and personal, attached at the lips. But that doesn’t mean we should fall for all those perfume and cologne companies bombarding us with advertisements and claiming to include human pheromones in their scents. It sure sounds enticing, but the bottom line is that science has yet to unravel the mystery of human pheromone exchange. Without more definitive research, women are probably better off spending their money on lipstick, while men might benefit most by keeping breath mints on hand.

  THE TENOR OF THIS discussion raises an interesting question: Have the customs of modern society caused us to shroud our most attractive asset—our scent—under a blanket of soap, perfumes, and other commercially created products? And would we behave differently without so many of these products and the artificial aromas they create? Maybe that’s why long-term human relationships are so important—a more natural scent inevitably pokes through as we spend extended periods of time with someone, and may cue us in to the value of the match. Further, what’s unpleasant to one person probably tantalizes another.

  In any case, I expect we will hear a great deal more about pheromones, and about genetic compatibility in relationships, in the coming years. In fact, a Boston-based Internet dating website called ScientificMatch.com already offers to find you a date based not only on attractiveness or your personal values, but also on your genetic composition.

  ScientificMatch.com uses DNA from a cheek swab and runs a “genetic matching algorithm” to locate a partner. According to the website, a lifetime membership costs $1,995.95, but ScientificMatch promises that with a good DNA-based pair-up, customers can achieve a more satisfying sex life, a higher rate of orgasm in women, less cheating, higher fertility, and healthier children. Or as their online banner reads, “Thinking of bringing the relationship to the ‘next level’? Not sure if they’re really ‘the one’? Consider another piece of the puzzle—check your DNA.”

  This may be the way things are going, but it sounds like our noses have evolved to do a pretty good job without going through all the trouble.

  What His Nose Knows

  In 2000, psychologists in Texas conducted a study similar to Wedekind’s experiment. Seventeen women—who, importantly, were not using hormonal contraceptives at the time—wore the same T-shirt to bed for three consecutive nights, first during the fertile ovulatory phase of their menstrual cycle and then again during their non-ovulatory period. Afterward, the shirts were collected and frozen.

  When the shirts were thawed, fifty-two male participants volunteered to rate their scents based on intensity and pleasantness. And sure enough, the shirts worn during the women’s ovulatory phase were consistently rated as more attractive and sexy.

  Furthermore, when the same shirts were retested a week later after sitting at room temperature, the results were similar. This suggests that the relevant odors do not dissipate immediately, so it’s possible men may be employing their noses more than they realize when choosing a partner. In turn, their preferences may be influenced by female fertility, and kissing provides the time and proximity for an adequate sample of a woman’s scent.

  CHAPTER 8

  Close Encounters

  Remember that scene in Pretty Woman when prostitute Vivian Ward (played by Julia Roberts) explains she’ll have sex with—but not kiss—her clients? Apparently the writers for the film did their research: Refraining from mouth-to-mouth kissing has been common among women of the “oldest profession” for a very long time. Social scientists Joanna Brewis and Stephen Linstead report that prostitutes often won’t kiss because it requires a “genuine desire and love for the other person.” By avoiding the lips of a client, they are best able to keep emotions out of their work.

  What about the preferences of those on the flip side of the experience—the so-called johns? Sex therapist Martha Stein watched as sixty-four prostitutes had sex with their clients for a total of 1,230 encounters. Using hidden one-way mirrors and tape recorders, she ensured the men could not detect her presence. Stein reported that just 36 percent of johns wanted to kiss the prostitute somewhere on her body, and only 13 percent were interested in French kissing.

  These examples demonstrate that kissing seems fully removable, in certain contexts, from sex for sheer pleasure. Both prostitutes and their clients instinctively seem to sense there is more to a kiss than to other sexual acts—that it’s in a different category. And indeed, in social
surveys, people rate kissing as more intimate than almost every other kind of activity. It also garners more attention in serious relationships than in casual sexual encounters, and scientists are discovering some fascinating reasons why. The warm fuzzy feelings that we attach to kissing probably have a lot to do with the hormones coursing through our bodies as a result.

  AS WE’VE ALREADY LEARNED, kissing someone you care about and crave releases multiple neurotransmitters and endorphins that serve to relieve stress, regulate mood, and lower blood pressure. It’s a natural upper with the power to stimulate feelings of euphoria. But there’s much more going on than that. Hormones are involved too, and while they share many similarities with neurotransmitters, there is an important difference in how and where they are released in the body, and the kinds of effects they can generate.

  Humans produce a long list of hormones, from estrogen and testosterone to insulin and cortisol, which modulate bodily activities relating to growth, development, reproduction, and metabolism. Glands in our endocrine system—including the thyroid, parathyroid, adrenal, pituitary, pancreas, hypothalamus, and most of all the ovaries and testes—produce different hormones and release them into the bloodstream. From there, these molecules circulate in order to act on a target somewhere else in the body. In this way, hormones differ from neurotransmitters, which are sent directly from a nerve cell to a destination in a much quicker (and less sustained) transfer.

  It helps to imagine these two different kinds of chemical agents as being akin to two people communicating by letters. A hormone would put a postage stamp on hers and send it out in the mail. It may be slow, but she’s confident her recipient will eventually read what she wrote. The neurotransmitter, though, is on edge. He has his target’s personal address and walks directly to its door, shoving his message in the mail slot. A neurotransmitter’s delivery is quick and direct, happening in milliseconds. Hormones, on the other hand, can take seconds or even months to have an effect, and that effect lasts much longer. (Complicating matters further, some substances, like oxytocin, can function as both hormone and neurotransmitter, depending on where they’re working in the body.)

  Hormones influence our behaviors and our emotions. They regulate a woman’s body throughout her reproductive years, causing the uncomfortable changes associated with premenstrual syndrome (PMS). Later in life, they are also the reason for hot flashes during menopause.

  Less obviously, hormones can even affect the way we taste to a partner. At the onset of menstruation, a woman’s body sheds special cells in the lining of her mouth, and this can cause the growth of excess bacteria. Similarly, estrogen can lead to higher than normal oral sulfur concentrations. Both situations can cause her to have bad breath.

  Hormones are not exactly easy on men, either. Testosterone can lead to all sorts of embarrassing situations right around the time puberty hits. It deepens men’s voices and causes excess hair growth in some places—and eventually contributes to the loss of it on top of their heads. Hormones can also drive some men to act like aggressive brutes or even sexual predators. In short, they plague both sexes with inconvenient upheavals throughout our lives.

  But aside from the trials and tribulations associated with hormonal vicissitudes, they also maintain our general health and well-being. So while they can be annoying and uncomfortable, hormones are also crucial and involved in many vital activities, ranging from breast milk production to mood regulation. They literally drive our behaviors and are also responsible for nudging us to propagate the human species.

  Humans aren’t slaves to these motivational substances, but an intricate dance takes place between our chemistry and our consciousness. Hormones and neurotransmitters do not “create” our emotions; they merely instruct our brains to produce a series of responses, which in turn motivate us to do things like create art, cook dinner, and even experience the romance of a special kiss. They’re always at work sending out subconscious signals that tell us how to behave and feel in the world we inhabit. So do not think of them as independent molecules. Rather, they’re a fundamental part of who we are.

  KISSING ANOTHER PERSON GREATLY shapes the ebb and flow of hormones through our bodies. The pattern of responses is laid down early; the hormones released in the body of a nursing baby girl, for instance, influence the responses she will have later in life. As an adult, she will experience positive emotions when she is kissed, hugged, massaged, and touched—thanks to the very same hormones once associated with nursing in infancy.

  Hormones may also explain some of the gender divisions around kissing that show up repeatedly, some of which we’ve already encountered—but there are many other similar examples. Social surveys report that men overwhelmingly prefer wet, sloppy, openmouthed kisses. Women, on the other hand, are more likely to opt for less saliva and tongue. There’s every reason to think these tendencies have a great deal to do with the principal male sex hormone, testosterone.

  Men naturally produce greater quantities of testosterone than women, but women are more sensitive to its influence. This amazing little molecule raises a woman’s libido and engorges her clitoris with blood—priming her for sex—so a little extra dose via a partner’s mouth may be very much to his sexual and reproductive advantage. Since a man’s saliva contains his testosterone, swirling his tongue into a woman’s mouth is a way to legally slip her a natural sex stimulant.

  Over weeks and months of kissing, scientists theorize that she may become more interested in getting physical. It’s not something that would happen in one night, but as her suitor persists over time, the influence of added testosterone may have a cumulative effect. This provides ample reason for a man to continue pursuing the woman he’s been kissing, and also explains why men are more likely to view kissing as a prelude to sex, and to report the preference for more tongue. While women may not enjoy a sloppy kissing experience as much, it can give men an added sexual advantage, and has probably been a successful strategy throughout history.

  A second possibility that has been suggested to explain why men prefer deep kissing has to do with the fact that they are less sensitive to smell and taste (technically termed “reduced chemosensory detection”). This could mean that a man requires a much larger saliva sample during a kiss than a woman does in order to assess a partner. Added tongue movement would provide increased exposure to her saliva, and additional time for him to obtain hidden clues about her reproductive status. However, scientists aren’t sure how much can be subconsciously inferred about female fertility through this kind of exchange, so while the hypothesis is interesting, it’s also speculative.

  • • •

  AND INCIDENTALLY, WHEN IT comes to male preferences for tongue-kissing, I couldn’t pass up another opportunity to query my friends to see if survey trends held. When I asked around, they most definitely did. Men frequently added that lots of oral action provided insight into how a particular partner might perform sexually. Conversely, most women complained that “too much tongue” is a turnoff. Once again, my acquaintances do not constitute a scientifically valid sample, but their stated tastes emphasize the way that men’s and women’s kissing preferences have been influenced by strategies that have evolved over millions of years.

  Of course, many hormones beyond testosterone are involved in kissing. And when it comes to pinning down their role—not always an easy task—neuroscientists Wendy Hill and Carey Wilson of Lafayette College have been conducting some fascinating research. Their methodology involves inviting college-aged couples to kiss in a controlled setting, while their team carefully collects information about the changes going on in the volunteers’ bodies. They are most interested in the role of two main hormonal players: oxytocin and cortisol. So before we get into discussing their research, let’s meet these molecules in more detail.

  Often called the “love hormone,” oxytocin is inextricably involved in intimacy, and has been shown to have extremely powerful effects in a laboratory setting. For example, when it is injected into the bra
in of a virgin female rat, this hormone causes her to immediately adopt the babies of another rat as her own. Nobody has tried the above experiment in women (for reasons that are perhaps obvious), but we know oxytocin works similarly in our own species. It is responsible for cementing the connection between parent and child and also serves to trigger lactation in new mothers. It helps to regulate mood and acts as a natural painkiller.

  But here’s the most interesting aspect for our purposes: Oxytocin is very important in developing feelings of attachment, not just to our mothers but to our lovers. Scientists believe it is the substance that keeps love alive in couples that stay together happily over decades, long after the novelty (and the dopamine) has worn off. Thanks to oxytocin, a kiss, a hug, or a warm caress can help maintain a solid and strong attachment. And it’s important to take the physical affection further, too. When a woman has sex, levels of oxytocin can peak up to five times higher than normal, and it is the substance responsible for the pleasurable “jolts” she feels in her pelvis during orgasm. Studies in men have also revealed that the hormone can spike three to five times higher than normal during sexual climax. Oxytocin is a force of nature.

  Furthermore, this hormone may explain why the suggestion to “kiss and make up” works so well. When asked why they kiss, men often report that it helps to resolve a disagreement. Although most women surveyed claim a kiss doesn’t make things better automatically, evolutionary psychologists disagree: It can. Research has demonstrated that a kiss or series of kisses tend to foster a woman’s forgiveness. It sounds formulaic, but when it comes to the rules of engagement, hormones don’t play fair.