Why We Kiss
Around the time that Thumbs, Toes and Tears (my third book) came out, I wrote an article for Scientific American Mind magazine exploring the evolution of kissing. It’s one of six uniquely human traits.
The article, like the one I wrote for Scientific American about crying, was very popular and republished multiple times. And both were picked up in other mainstream and peer reviewed publications around the world.
Here’s an updated version…
Lips - They Do More Than You Think
When passion takes a grip, a kiss locks two humans together in an exchange of scents, tastes, textures, secrets and emotions like nothing else. We kiss furtively, lasciviously, gently, shyly, hungrily and exuberantly. We kiss in broad daylight and in the dead of night. We give ceremonial kisses, affectionate kisses, Hollywood air kisses, kisses of death and, at least in fairytales, kisses that bring people back to life.
Lips may have evolved first for food (more on that in a bit), but later applied themselves to different kinds of needs. A kiss triggers a blizzard of neural messages and chemicals that transmit tactile sensations, sexual excitement, warmth, motivation and euphoria. They’re so important, they can even convey crucial information about the future of a relationship. According to recent research, if a first kiss goes bad, it can stop an otherwise promising relationship dead in its tracks.
There’s strong evidence that kissing evolved because it facilitates something called mate selection. In a BBC interview, evolutionary psychologist Gordon G. Gallup of the University at Albany said, “Kissing involves a very complicated exchange of information — olfactory information, tactile information and postural types of adjustments that may tap into underlying evolved and unconscious mechanisms that enable people to make determinations … about the degree to which they are genetically incompatible.” That’s a mouthful, but the idea is that kissing is one way to see if you and the kissee are a proper match. Kissing may even reveal the extent to which a partner is willing to commit to raising children!
Satisfying Hunger
Whatever else is going on when we kiss, it goes way back in our evolution. In the 1960s British zoologist and author Desmond Morris proposed that kissing may have evolved when primate mothers chewed food for their young and then fed them mouth-to-mouth, puckering their lips. Chimpanzees feed this way, so our hominid ancestors probably did, too. Pressing outturned lips against lips may have then later developed as a way to comfort hungry children when food was scarce and, in time, to express love and affection in general. The human species might eventually have taken these proto-parental kisses down other roads until we came up with the more passionate versions we have today.
Silent chemical messengers called pheromones could have sped the evolution of the intimate kiss. Many animals and plants use pheromones to communicate with other members of the same species. Insects, in particular, are known to emit pheromones to signal food, alarm, or sexual attraction.
Whether humans can sense pheromones is controversial. Unlike rats and pigs, people are not known to have a specialized pheromone detector, or vomeronasal organ. But biologist Sarah Woodley of Duquesne University suggests that we might be able to sense pheromones with our nose anyhow. And chemical communication could explain curious findings like studies that show the menstrual cycles of women in a family or dormitory often synchronize, or that women have been known to be attracted to the scents of T-shirts worn by men whose immune systems are genetically compatible with theirs. How? Human pheromones could include androstenol, a chemical component of male sweat that may boost sexual arousal in women, and female vaginal hormones called copulins that some researchers have found raise testosterone levels and increase sexual appetite in men.
If pheromones actually play a role in human courtship, then kissing would be an extremely effective way to pass them from one person to another.
We might also have inherited the intimate kiss from our primate ancestors. Bonobos, which are genetically very similar to us (although we are not their direct descendants), are a particularly passionate bunch. Emory University primatologist Frans B. M. de Waal recalls a zookeeper who accepted what he thought would be a friendly kiss from one of the bonobos he worked with, until he felt the ape’s tongue in his mouth!
Good Chemistry
Since human kissing first revealed itself, the act seems to gone contagious. It turns out, it’s a learned behavior. And there are good reasons why lit’s become so popular. Human lips enjoy the slimmest layer of skin on the human body, and are among the most densely populated sensory neurons we have. When we kiss, these neurons, along with those in the tongue and mouth, rocket intense messages to the brain and body, setting off all sorts of delightful mayhem.
Of the 12 or 13 cranial nerves that affect cerebral function, five are at work when we kiss, subconsciously shuttling messages from our lips, tongue, cheeks and nose to a brain that snatches information about the temperature, taste, smell and movements of the entire affair. Some of that information arrives in the somatosensory cortex, a swath of tissue on the surface of the brain that represents tactile information in a map of the body. In that map, the lips loom large because the size of each represented body region is proportional to the density of its nerve endings.
A 2-D Sensory Homunculus — Our brains see our lips, tongues and nose like this.
All of these messages linked to the brain then unleash cocktails of chemicals that activate hormones linked to stress, motivation, social bonding and sexual stimulation. In one study, psychologist Wendy L. Hill and her student Carey A. Wilson of Lafayette College compared the levels of two key hormones in 15 college male-female couples before and after they kissed, and before and after they talked to each other while holding hands. One hormone, oxytocin, is involved in social bonding, and the other, cortisol, plays a role in stress. Hill and Wilson assumed that kissing would boost levels of oxytocin, which also influences male and female orgasm. They expected the effect to be particularly pronounced in females, who reported higher levels of intimacy in their relationships.
But that’s not what happened. Instead, oxytocin levels rose only in the males, but decreased in the females, whether they were kissing or talking while holding hands. But there was good news when it came to stress. Kissing dropped cortisol levels for both sexes no matter the form of intimacy. The clear message: if you want to reduce stress, kiss more.
Since kissing is linked to love, it may also boost brain chemicals associated with pleasure, euphoria and the desire to connect with a certain someone too. In one study, anthropologist Helen Fisher of Rutgers University scanned the brains of 17 candidates as they gazed at pictures of people with they deeply loved. The study found an unusual flurry of activity in two brain regions that govern pleasure, motivation and reward: the right ventral tegmental area and the right caudate nucleus, the same reward centers that cocaine, and other addictive drugs, stimulate.
Kissing has other primal effects tool. Visceral marching orders boost pulse and blood pressure. The pupils dilate, breathing deepens and rational thought retreats (desire suppresses both prudence and self-awareness). Basically, when we’re necking, we’re too enthralled to care much about thinking. Maybe poet e. e. cummings put it best: “Kisses are a better fate / than wisdom.”
Litmus Test
Although a kiss may not be wise, it can be pivotal to a relationship. “One dance,” Alex “Hitch” Hitchens says to his client and friend in the 2005 movie Hitch, “one look, one kiss, that’s all we get … one shot, to make the difference between ‘happily ever after’ and, ‘Oh? He’s just some guy I went to some thing with once.’”
Can a kiss be that powerful? Maybe. According to another study aseembled by Gordon Gallup and his colleagues, 59 percent of 58 men and 66 percent of 122 women admitted there had been times when they were attracted to someone only to find that their interest evaporated after their first kiss. The “bad” kisses had no particular flaws; they simply did not feel right — and they ended the romantic relationship then and there — a kiss of death, so to speak.
The reason a kiss carries this kind of weight, Gallup theorized, is that it conveys subconscious information about the genetic compatibility of a pro-spective mate. That’s consistent with the idea that kissing evolved as a courtship strategy used to help us rate potential partners.
From a Darwinian perspective, sexual selection is the key to passing on your genes and creating more offspring. For us humans, mate choice usually involves falling in love, or at least a version of monogamy. Fisher wrote in her paper that this “attraction mechanism” in humans “evolved to enable individuals to focus their mating energy on specific others … facilitating mate choice.” Which is, of course, precisely what you need if you want to make babies.
According to Gallup’s findings, kissing may play a crucial role in the progression of a partnership, but one that differs depending on whether you’re a man or a woman. Gallup and his colleagues surveyed 1,041 college undergraduates of both sexes and found that for most of the men, a deep kiss was largely a way of advancing to the next level sexually. But women were generally looking to take the relationship to the next stage emotionally, assessing not simply whether the other person would make a first-rate source of DNA, but also whether he would be a good long-term partner. This makes kissing a kind of emotional barometer: the more enthusiastic it is, the healthier the relationship. For child-bearing women this might be even more important. Women invest more energy in producing children, and have a shorter biological window in which to reproduce than men so there’s eveolutionary pressure to be pickier about the partners they choose. That means a passionate kiss may help a woman choose a mate who is not only good at fathering children but also committed enough to stick around and help raise them.
That said, kissing is probably not strictly necessary, at least not in the way air and food are. Most other animals don’t neck but still manage to produce plenty of offspring. Not even all humans kiss. At the turn of the 20th century Danish scientist Kristoffer Nyrop described Finnish tribes whose members bathed together but considered kissing indecent. In 1897 French anthropologist Paul d’Enjoy reported that the Chinese regard mouth-to-mouth kissing to be as horrifying as many people deem cannibalism to be. In Mongolia some fathers do not kiss their sons. (They smell their heads instead.)
In fact, up to 10 percent of humanity does not kiss, according to Fisher in a 1992 study. If that’s still true, it means that somewhere in the neighborhood of 650 million members of the human race do not osculate (the scientific term for kissing); more than the population of any nation on earth except for China and India.
Lopsided Love
But let’s say that like most humans, you do kiss. Then osculation has one more secrets up its long sleeves. Psychologist Onur Güntürkün of the Ruhr-University of Bochum in Germany recently surveyed 124 couples kissing in public places in the U.S., Germany and Turkey and found that they tilted their heads to the right twice as often as to the left. Right-handedness doesn’t explain this tendency, because being right handed is four times more common than the act of kissing on the right. So what explains it?
Studies show that as many as 80 percent of mothers, whether right-handed or left-handed, cradle their infants on their left side which means the infants must turn to the right to nurse or nuzzle. So our tendency to tilt right when kissing may be because we associate warmth and security with turning to the right.
It’s a theory.
But that’s okay. We don’t have to know everything. Maybe kissing, and romance, should give up its mysteries grudgingly. Personally, I kind of like it that way.
Chip Walter is a National Geographic Explorer, screenwriter, documentary filmmaker and former CNN bureau chief. His most recent book is Immortality, Inc. — Renegade Science, Silicon Valley Billions and the Quest to Live Forever. This article was based on Chip’s book Thumbs, Toes and Tears and Other Traits That Make Us Human, and is an updated version of an article originally published in Scientific American Mind. Chip’s books are available online and at bookstores everywhere, but if you’d like to buy a signed copy, please visit his website here and drop him an email on his home page.