The Science of Kissing

By Sheril Kirshenbaum | February 7, 2008 10:50 am

Ah, February… that very special time of year for celebrating the one we love (or bemoaning the greeting card industry). While flipping through the month’s Scientific American, I came upon this article about ‘Why We Kiss‘ and immediately found myself intrigued.

Now I expect most of us hope to experience the ‘ever-elusive, out-of-the-ballpark-home-run, earth-shattering, perfect kiss,’ but what exactly is it? How does it happen? Wait a sec, this is ScienceBlogs for goodness sake, so let’s dissect this one carefully and get down to exploring the science of kissing…

Why do we kiss? It’s one of the most intimate expressions between two people, inspiring all forms of art from music to painting to literature. It’s arguably shaped history and legend.

And sure, kissing feels completely natural, but is it instinctive? Given up to ten percent of humanity doesn’t even touch lips, should we accept it’s actually a cultural phenomenon? I’m not convinced. You see, kissing undoubtedly allows us to find out all sorts of information about our partner. We’re exchanging pheromones. In fact, when we’re engaged, our bodies release a cocktail of chemicals related to social bonding, stress level, motivation, and sexual stimulation. We become, in effect, ‘under the influence.’ It’s powerful.

The right kiss boosts feelings of euphoria stimulating pleasure centers in the brain leading me to suspect there’s something to kissing that goes beyond social mores. While it may have evolved from primates feeding their babies mouth-to-mouth (I know, how terribly unromantic!), other scientists suggest it’s crucial to the evolutionary process of mate selection.

2373666220097948545iqWeaL_ph.jpgEver notice the way a bad first kiss can stop a relationship cold? It may very well be a subconscious cue that a pair is not well suited to produce offspring.

Still, for anyone who’s experienced the right chemistry… well… you know. That special and rare kind of kiss makes you weak in the knees and sends your heart racing. And once in a while, if you’re very lucky, there’s that magical kiss that makes the rest of the world fade away…

So as the science goes, I don’t think we’ll ever quite figure out the rationale behind the perfect kiss. And thing is, we don’t need to. Call me a romantic, but I have to admit I like that when you experience one such ephemeral moment, the feeling defies explanation.


Comments (20)

  1. Rebecca

    so true! when its right, you just know. no data or control required.

  2. One of our new SciBlings, Kate, wrote about this on her old blog.

  3. Carol

    GREAT post! This is the style of Sheril I like best! I think it mostly have to with subconcious cues through pheremones. Right?

  4. uncle noel

    The romantics will hate me for this, but…
    We kiss to exchange germs. We are in a constant struggle with disease organisms which are constantly evolving and challenging our immune systems. Exchanging these germs keeps our immune systems on their toes and – this is particularly unromantic – weeds out the weeklings. (One has to accept group evolution for this to make sense.)

  5. I refer you to this study of kissing… why do people turn their heads to the right more than the left when doing it?

    Onur Güntürkün, Human behaviour: Adult persistence of head-turning asymmetry, Nature 421, 711 (13 February 2003) doi:10.1038/421711a

  6. uncle noel wrote:
    “We kiss to exchange germs.”

    Completely wrong. Kissing is sensual stimulation that encourages procreation which ultimately leads to sex and offspring. It’s innate.

    The social more aspect is more probably in the odd instances that some cultures do not kiss.

  7. Charles

    When I was a youngster, I remember being overly nervous about kissing a girl. I didn’t think I knew how. I wanted to impress the woman of my dreams-whenever I found her and whoever she was. I am not ashamed to admit (albeit anonymously) I even practiced on my hand. It was a stressful possibility. What if I did it wrong? What if she knew I didn’t know how to kiss?

    Finally it happened. All that thinking and figuring out was for no reason. We simply know what to do. It’s the most natural thing in the world. Like sex.

    Kissing is innate.

  8. Karley

    Happy Valentines Day Chris and Sheril :->

  9. uncle noel

    Patterson: No disagreement about what kissing is. My theory is about why it might have evolved. Disease organisms cause greater evolutionary pressure than most people realize. They might be responsible for sex evolving in the first place (so adaptive genes could be preserved even when they weren’t necessary, i.e. between epidemics).
    So maybe it’s not coincidental that we have sexual behaviors that encourage germs to spread.
    BTW my name is Patterson, too. Funny to see it in a comment on my post.

  10. Khalil A.

    Just a small note here; isn’t that article in Scientific American Mind instead? If I’m not mistaken, I read it in there instead of Scientific American itself.

    I may be wrong though.

  11. Linda

    The right kiss from the right person makes everything worthwhile…
    This is an everyone can relate post, and I agree with Carol,
    always the most interesting and fun.

  12. That “Cupid Must Die” link is the most unromantic thing I’ve ever read — they try to cloak their burning envy of lovers by suggesting that they’re so beloved and passionate, that they don’t rely on a single day to express it. Ha! “Rebel against society” — or, “whine about being alone.”

    Anyhow, the germ hypothesis is pretty neat — but why not be weirder? Maybe kissing is caused by germs, the better to facilitate their spread from one host to the next. Nah, neither of these ideas are any good, because we don’t take the battle against pathogens lightly — anything that helps us, we do constantly. Both germ hypotheses predict that we’d lock lips with any old person, and that’s wrong.

    The idea that it serves to signal whether a guy will be a good provider and good dad — baloney. Are the best kissers known to be the good monogamous dad types? Or more likely to be serial boyfriends, sexy charmer types, rockstars and models, etc.? And conversely, how good are the good dad types at kissing? This is testable, but we know what the results would be.

    In general, things that activate your sympathetic nervous system — when you get butterflies in the stomach, your pupils dilate, heart races, etc. — are not the things that signal good dad potential. Your heart doesn’t race, and you don’t get weak in the knees, when you see a guy throwing a football with the neighborhood kids, even if you’d say, “Aw, how nice of him!”

    What makes your heart race, etc., is when the person has smoldering good looks, appears a bit dangerous or exciting or unpredictable, is displaying some rare talent like singing or dancing well, and so on.

    This supports the “good genes” idea — if a guy kisses well, which requires coordination, rhythm, and passion as much as dancing does, then he must have good genes that would benefit your kids… whether he sticks around or not. Question: are the women for whom a bad kiss is a deal-breaker more likely to be interested in safe, monogamous good dads, or exciting and sexy guys?

    Again, easy to test by surveys, but we know what they’d say: women who are looking for a good dad would say, “Yeah, he’s a bad kisser, but so what, as long as he’ll stick around and provide for my kids, who cares how good he is at kissing or in bed?”

  13. Perhaps that’s one reason why exciting pretty boys have fuller lips than the average male (examples are easy enough to think of: Steven Tyler, Billie Joe from Green Day, Jared Leto, etc….). If you have fuller lips, and the density of sensory receptors stays the same (or at least isn’t less), then you’ll feel much more of a rush when you kiss a girl than the average guy would feel.

    This would compel pretty boys, more than the average guy, to seek out kissing. Since they’re likely to possess the traits that make girls weak in the knees, this bias would allow them to showcase their good genes as soon and as much as possible. This speed matters if the guy pursues a serial dating strategy (the faster, the more numerous), but not if he’s only going to pick and stay with one female (no rush).

  14. Two other falsifications of the “kissing to find good dads” hypothesis:

    1) Plot the importance of “good kissing ability” to females as a function of their age. Where is it high and when low? High from puberty through early-mid 20s, low after 30-35. Now, plot the importance of “good dad ability” as a function of age.

    The two graphs are reflections of each other. Fiery, impetuous youths are concerned about kissing, good looks, excitement factor, etc., and hardly at all about how good of a dad he’d make — compare who gets the most attention from girls in high school.

    2) Plot the importance of “good kissing ability” to females as a function of time in the relationship. If it is related to providing ability, seeing if he’ll stick around, etc., females should still care very much about it for at least the first 4 years after they have a child, and likely until the kid reaches puberty. As a result, the female should initiate passionate make-out sessions with her husband for this entire time, to keep track of whether he’s going to stick around and provide when it would make the most difference.

    In reality, the importance of kissing declines from the initial head-over-heels honeymoon phase, and the mother will be too busy raising the kid to make-out for awhile with her husband — even if he started it, she’d now be more likely to say, “Oh get off of me, I’m busy!”

    Both of these observations support the “good genes” hypothesis, though. Youngsters care more about traits that signal good genes, and if a female has already gotten a male’s good genes and had a baby by him, she won’t care how good of a kisser he is anymore. She got what she needed from him.

  15. Andrea

    I think all this talk about dating and Don Juans is kind of funny. I personally don’t believe in humans starting out in ape-like clans. But if I did, I don’t know how I’d ever be able to connect the concept to high school type dating relationships. In ape cultures, the biggest alpha male gets ALL the females. The females don’t get much of a choice about it anyway.

    In my experience, the level of intimacy, trust, and vulnerability displayed in a kiss is more of a seal or bond between the couple than a litmus test to find out more information. We can smell the pheremones and see the make up and build of a person long before we get close enough to kiss them. We don’t REALLY kiss someone to see how great a parent they would be or even what great genes they would pass on to our offspring (and neither do animals nor did our ancestors). A kiss often symbolizes a promise, a bond, a togetherness that makes two people a couple between the two of them and in the eyes of those who see them. I’m not just being romantic. I think this informal promise is extremely important in the cooperation needed to produce human infants and raise them to maturity safely.

  16. Janet

    Article on this topic in the Washington Post today:

    The Differences in Gender — Sealed With a Kiss

    By Rob Stein, Washington Post Staff Writer
    Monday, February 11, 2008; A05

    A kiss, it turns out, is definitely not always just a kiss.

    As Valentine’s Day approaches, research has begun shedding light on that most basic of all human expressions of love — the smooch — which has received surprisingly little scientific scrutiny.

    “You’d think there would be a lot of research on kissing behavior. It’s so common,” said Susan M. Hughes, an assistant professor of psychology at Albright College in Pennsylvania, whose recent study is one of the first to probe snogging in depth. “But there isn’t. It’s really been ignored.”

    In fact, much about love and attraction remains mysterious.

    “This is a seminal paper,” said Helen Fisher, a Rutgers University anthropologist who studies love. “It’s remarkable that we don’t know more about these things. But love has not really been well studied until recently.”

    In people, kissing to express affection is almost universal. About 90 percent of human cultures do it.

    One traditional view is that kissing, known scientifically as osculation, evolved from women chewing food for their children and giving it to them mouth-to-mouth, Fisher said.

    But, she said, “I’ve never believed that,” adding that similar behavior is found in many species. Birds tap beaks. Elephants shove their trunks in each other’s mouths. Primates called bonobos practice their own version of French kissing.

    Fisher believes kissing is all about choosing the right mate.

    “There’s so much information exchanged when you kiss someone that I just thought it must play a vital role in mate choice, and this paper is elegantly showing that,” Fisher said.

    A disproportionate amount of the brain, she noted, is geared toward interpreting signals from the mouth.

    “When you look at the brain regions associated with picking up data from the body, a huge amount of the brain is devoted to picking up information from the lips and tongue,” she said. “Very little of the brain is built to pick up what happens to, say, your back. There have been case reports of people being stabbed in the back without even knowing it. But even the lightest brush of a feather on your lips and you feel it intensely.”

    Hughes and her colleagues set out to probe some of the mysteries of lip-smacking by conducting a series of three in-depth interviews with 1,041 students at the University at Albany.

    “This was a fishing expedition,” Hughes admitted. “We didn’t know what to expect.”

    But Hughes and her colleagues had three hypotheses:

    “People may use kissing as a sort of mate assessment,” she said. “You can tell a lot of information about a person by being in close proximity — from their breath, the taste of their saliva, things like that.”

    Their second hypothesis was that kissing promotes bonding.

    “If you are accepting a kiss you are putting yourself at risk of contracting an illness. And we suspect it raises levels of a hormone called oxytocin, which is related to interpersonal bonding,” Hughes said.

    The third hypothesis was that kissing is simply a way of inducing sexual arousal, increasing the chance of having sex.

    “Men might use this more to seduce their partners more than women do,” she said.

    The researchers found support for all three theories, Hughes and her colleagues reported in the October issue of the journal Evolutionary Psychology, as well as provocative differences between men and women.

    Women place more emphasis on the taste and smell of the person they kiss than men do, the researchers found.

    “That clues us in that females may be using it more to make mate assessments than men,” she said.

    Women were also more likely to refuse to have sex with a partner unless they kissed first. More than half of the men said they would have sex without kissing first, but fewer than 15 percent of the women said the same.

    Moreover, kissing is clearly a much bigger potential deal-breaker for women than for men. Women were much more likely to say they would refuse to have sex with a bad kisser.

    “Women are definitely using kissing to make an assessment about the male. If he’s a bad kisser, then she’s not going to want to have sex with him. She’s getting a lot of information from that kiss,” she said.

    Men were also more likely to expect kissing to lead to sex. Men assumed that would be the case about half the time; women only about one-third of the time. And it made no difference to men if they were in a short- or long-term relationship.

    “Men tend to think kissing should lead to sex no matter what,” Hughes said.

    That fits with other research, said Beverly Palmer of California State University, that has found that men and women often interpret nonverbal cues differently.

    “When the woman is first kissing the man, she’s not necessarily sending the signal, ‘Let’s go to the next stage’ — but the man is reading it that way,” Palmer said. “So both can get themselves into difficulties if they don’t verbalize their true intentions.”

    Men were also much more likely to want to exchange more saliva during a kiss.

    “Males like the very moist, wet open-mouth kisses,” Hughes said. “We didn’t expect that.”

    Men tend to have less acute senses of taste and smell than women, which could explain that finding, she said.

    “Perhaps males need more saliva to make subtle mate assessments,” she said, noting that previous research has suggested that a woman’s breath changes across the menstrual cycle. “He may be subconsciously detecting whether she’s fertile or not.”

    Fisher was intrigued that men also were more likely than women to think a kiss could end a fight. “I didn’t expect that. Maybe it’s because they know women find kissing more intimate, so they are doing something not for themselves but to win women over,” Fisher said.

    Another recent study that measured oxytocin levels of kissing couples found kissing only caused the bonding hormone to rise in men. But those researchers speculated that might have been because the experiment took place in the decidedly unromantic setting of a college health center.

    “Our working hypothesis was males are less affected by the ambiance, if you will,” said Wendy L. Hill of Lafayette College in Pennsylvania. The study did find that kissing lowered levels of the stress hormone cortisol in both sexes.

    Women in Hughes’s study were more likely to say kissing was important before, during and after sex, as well as throughout a relationship. “That supports the idea that females are using kissing to create a bond,” Hughes said.

    “No wonder we remember our first kiss,” Fisher said. “No wonder we’re nervous about our first kiss. We haven’t known why. This helps explain it.”


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About Sheril Kirshenbaum

Sheril Kirshenbaum is a research scientist with the Webber Energy Group at the University of Texas at Austin's Center for International Energy and Environmental Policy where she works on projects to enhance public understanding of energy issues as they relate to food, oceans, and culture. She is involved in conservation initiatives across levels of government, working to improve communication between scientists, policymakers, and the public. Sheril is the author of The Science of Kissing, which explores one of humanity's fondest pastimes. She also co-authored Unscientific America: How Scientific Illiteracy Threatens Our Future with Chris Mooney, chosen by Library Journal as one of the Best Sci-Tech Books of 2009 and named by President Obama's science advisor John Holdren as his top recommended read. Sheril contributes to popular publications including Newsweek, The Washington Post, Discover Magazine, and The Nation, frequently covering topics that bridge science and society from climate change to genetically modified foods. Her writing is featured in the anthology The Best American Science Writing 2010. In 2006 Sheril served as a legislative Knauss science fellow on Capitol Hill with Senator Bill Nelson (D-FL) where she was involved in energy, climate, and ocean policy. She also has experience working on pop radio and her work has been published in Science, Fisheries Bulletin, Oecologia, and Issues in Science and Technology. In 2007, she helped to found Science Debate; an initiative encouraging candidates to debate science research and innovation issues on the campaign trail. Previously, Sheril was a research associate at Duke University's Nicholas School of the Environment and has served as a Fellow with the Center for Biodiversity and Conservation at the American Museum of Natural History and as a Howard Hughes Research Fellow. She has contributed reports to The Nature Conservancy and provided assistance on international protected area projects. Sheril serves as a science advisor to NPR's Science Friday and its nonprofit partner, Science Friday Initiative. She also serves on the program committee for the annual meeting of the American Association for the Advancement of Science (AAAS). She speaks regularly around the country to audiences at universities, federal agencies, and museums and has been a guest on such programs as The Today Show and The Daily Rundown on MSNBC. Sheril is a graduate of Tufts University and holds two masters of science degrees in marine biology and marine policy from the University of Maine. She co-hosts The Intersection on Discover blogs with Chris Mooney and has contributed to DeSmogBlog, Talking Science, Wired Science and Seed. She was born in Suffern, New York and is also a musician. Sheril lives in Austin, Texas with her husband David Lowry.Interested in booking Sheril Kirshenbaum to speak at your next event? Contact Hachette Speakers Bureau 866.376.6591 info@hachettespeakersbureau.comFor more information, visit her website or email Sheril at


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