Mae West once said, “A man’s kiss is his signature.” Surely, if you were blindfolded and kissed three or four people you could still recognize which one was your significant other. There is an art to the kiss, and there are certainly different styles of kissing. My girlfriends and I once theorized that everyone has their own style. How compatible their style is with yours is what makes them a good or bad kisser. This hypothesis came up shortly after discovering that we had all kissed the same guy and each of us had a different reaction.

I’m a scientist and prone to over analyzing, so I thought: “What’s the science in a kiss?” At the risk of ruining a good thing, here it goes: The scientific term for kissing is “osculation,” while the science of studying kissing is “philematology.” Osculologists (these are the scientists who study kissing) tell us that we use no less than 34 of our facial muscles, and perhaps up to 146 total body muscles, when we kiss. Most important is the orbicularis oris, the muscle that helps us pucker up.

Kissing and kissing-type behavior is seen among many species including chimps and bonobos, but humans are the only ones who kiss with everted or “pursed outward” lips. Humans, chimps, and bonobos are all primates on the same branch of the evolutionary tree, a fact that supports a rather interesting and widely accepted theory: The biological reason why women draw attention to the lips by wearing red lipstick, or with the use of lip plumpers, is a phenomenon derived from the occurrence of plump red hindquarters in primates. During ovulation, increased blood flow results in female primates showing swollen red hindquarters, signaling they are ready to mate.

It doesn’t seem like the most flattering of origins, but there’s probably a reason that kissing behaviors have thrived among primates, and why us humans have valued kisses so much throughout the centuries in literature and music. The very act of osculation engages all the senses: touch, sight, hearing, smell, and taste. When you kiss, you are literally in someone’s face so you can really get a good sampling of their scent (both of them and their breath). Did you know we have musk glands under our eyes? We are animals after all and it seems as though smell can help us judge the compatibility of the potential mates’ DNA.

Studies by biologist Claus Wedekind have shown that women are often attracted to men whose smell reflects an immune system very different from theirs. Women often seek men whose DNA code offers a contrasting major histocompatibility complex or MHC (molecules controlling a major part of the human immune system). This makes evolutionary sense. Their offspring would have the best immunity possible against infection (the best mixture of both parents) and therefore a better chance for survival. Interestingly, for women taking birth control pills, this attraction to opposite immune systems is not the case.

We always like to think of cute couples as having the right chemistry — even if they’re only a couple on the silver screen. But what we’re referring to as chemistry really is chemistry. Brain chemistry. Thanks to the release of epinephrine and norepinephrine, our blood vessels and pupils dilate. In fact, the effect of our pupils dilating has been theorized as the reason we close our eyes when we kiss. Adrenaline makes our hearts race and we get more oxygen delivered to our brain. It may be why we sometimes get that “weak in the knees” feeling.

Kissing serves a purpose of creating a more intimate bond with another person. It’s correlated with feelings of attachment and comfort. There is a higher concentration of nerve endings in the lips compared to any other part of the body. Kissing involves five of our 12 cranial nerves and when the nerves in the lips are stimulated they send an amazing amount of stimulation to the brain. Consequently, the brain’s somatosensory cortex is more stimulated by the lips than by any other organ (yes, even more than by the genitals). In fact, the legendary sex researcher Alfred Kinsey noted that some women can have an orgasm just from deep kissing. That’s a kiss you’ll remember! Actually, a Butler University study showed that more people remember the details of their first kiss than they do when trying to recall the details of their first sexual experience.

Dopamine, the pleasure hormone, is released in the prefrontal cortex, making us crave more. It affects the brain in the same way as addictive drugs or sugar. Dopamine is why, in the early blissful stages of a relationship, we’re totally preoccupied with thoughts of that other person, sometimes even to the point of losing sleep or our appetite. It’s the time when you can finally stick to that diet because you’re no longer stress eating and your head is in the clouds — oh, the beginnings of love!

If you’re lucky enough to find yourself in a deep passionate kiss, you’ll get a rush of serotonin. That’s when you really start to think about that person constantly. Researchers have found that the serotonin levels of individuals newly in the throes of a budding, passionate romance look a lot like the serotonin levels of patients who suffer from obsessive-compulsive disorder!

Eventually, after regular kissing for a few weeks, your body begins to produce less of the stress hormone cortisol. This is perhaps one reason why people in long-term relationships have a slightly longer lifespan. On the flip side, if you have high cortisol levels, from stress or being in a setting where the kiss is uncomfortable, you are less likely to deem the kiss as good. Cortisol and kissing seem to just not mix. And if that first kiss is bad? According to evolutionary psychologist Gordon Gallup, 59 percent of men and 66 percent of women quickly end the relationship.

Another interesting tidbit to consider before your next kiss is that two-thirds of people, regardless of whether they are left or right handed, tilt their head to the right when kissing. One theory is that most women breast-feed holding the baby in the left hand thus causing the child to tilt its head to the right. Kissing simply brings back those feelings of bonding, security, and comfort. The same neural pathways are involved and the same release of oxytocin, the bonding hormone. The release makes you feel more relaxed and less stressed. At once, secure and happy. If you think about it, the pursed pucker you make to kiss is similar to that of a nursing baby. Some research suggests oxytocin is one of the key ingredients involved in keeping relationships going for the long term. In fact, a German study suggested that it was oxytocin that was responsible for promoting fidelity in monogamous relationships.

Women, in general, put a higher emphasis on kissing than men. Perhaps this is due to their evolutionary role as mate selector, being the one pursued as opposed to having to woo. Susan Hughes, from Albright College in Pennsylvania, noted that 90 percent of women would not have sex with a man without first kissing him, whereas only 50 percent of men kept the same rule. While in the ovulation phase of their menstrual cycle women have been shown to be more selective, preferring men with a higher degree of genetic compatibility, facial symmetry, and masculine features. Both males and females who thought of themselves as more attractive than average and those who also tended to have more short-term relationships also highly emphasized the importance of kissing.

Kissing is a very personal experience, and at the same time, a realm through which we gain a greater understanding of human biology. It’s both an art and a science, yet much of it has still not been broken down into strictly scientific terms. Nor can one underestimate its importance to life. It evidently plays a role in determining the genetic compatibility of a mate, increases our arousal as a prelude to sex, and it helps to maintain and nurture long-term relationships.

Interpersonal relationships may indeed be the last great frontier we have yet to fully dissect and explore as scientists. They are surely as complicated as the brain or the universe. One thing is for certain: Kissing is a good thing and we should all be doing more of it. So, if you don’t think of your mother’s breasts or a baboon’s butt next time you kiss, perhaps I didn’t ruin it for you after all!

This article was originally published in the Winter 2015 issue of Brain World Magazine.

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