By Emer Maguire

11 March 2015 - 10:52

A kiss causes our brain to create a chemical cocktail that can give us a natural high. Image (c) Walt Stoneburner licensed under CC-BY and adapted from the original (links below).
A kiss causes our brain to create a chemical cocktail that can give us a natural high. Image ©

Walt Stoneburner licensed under CC-BY, adapted from the original.

Is there a scientific reason that explains why humans kiss? Yes, says Emer Maguire, winner of the Northern Irish instalment of the international science communication competition FameLab.

Why do we kiss?

Kissing is wonderful - so wonderful that most of us can recall 90 per cent of the details of our first kiss. Human beings have been preoccupied with kissing for years. It features as the climax of all great Hollywood love stories, and is celebrated by singers and poets alike. In reality, kissing is nothing more than two people putting their faces together and exchanging spit. How on earth did something so gross become so appealing? The act of kissing has developed to become advantageous to humans: if it didn't serve an evolutionary purpose, we simply would not do it.

So what’s in a kiss? More than you might think.

Nature versus nurture

A kiss might seem like a natural thing to do for most of us, but the scientific jury is still out on whether it is a learned or instinctual behaviour. Approximately 90 per cent of cultures kiss, making a strong case for the act being a basic human instinct. I know what you’re thinking...what about the other ten per cent? If kissing was a natural behaviour, surely all cultures would do it? While this small minority doesn’t 'kiss' like the rest of us (due to superstitions and cultural beliefs), they may still engage in kissing-like behaviours, such as rubbing noses together.

If kissing is a natural instinct, why don’t animals kiss?

Many animals actually do engage in kissing-like behaviours to show affection. These behaviours are so diverse, from dogs sniffing and licking potential mates, to elephants putting their trunks in each other’s mouths. However, one animal kisses just like we do: the bonobo ape. This isn't too surprising, considering we share 98.7 per cent of our DNA with this hairy cousin. Bonobos kiss for comfort and to socialise. Sometimes after a fight they even kiss and make up. We humans kiss for the exact same reasons, indicating that kissing might be ingrained deep in our DNA.

How did the kiss evolve?

Many scientists believe that kissing came from the practice of kiss-feeding, where mothers would feed their young mouth-to-mouth. Imagine birds feeding worms to their little chicks. Cute, right? Now imagine someone feeding you your chewed-up breakfast via their mouth. This sounds disgusting to most people, but we humans used to do it all the time! From this passing of food, pressing lips became synonymous with love. Understandable, since the way to most people’s hearts are through their stomachs. Over time, this symbol of affection may have evolved to give us romantic kissing.

So what is the purpose of kissing?

Imagine a kiss being like a job interview for the elusive role of being someone’s significant other. The interviewer is looking for the candidate who best matches the job description. Similarly, when we kiss, we are looking for a mate that best matches our genetic make-up. 'Wait, what do genes have to do with kissing?!' - I hear you scream. Well, we actually have a group of genes called the MHC (major histocompatibility complex) genes that form part of our immune system and give us our natural scent. In a famous experiment, women overwhelmingly preferred the smell of t-shirts worn by men with different MHC genes from their own. This is because when two people with different MHC genes mate, the baby they would produce would have a selection of components from each of their immune systems. A more diverse immune system has a greater ability to fight disease. Therefore, opposites really do attract. This explains why we prefer kissing one person over another. It’s in our genes.

What happens in our brains when we kiss?

The brain goes into overdrive during the all-important kiss. It dedicates a disproportionate amount of space to the sensation of the lips in comparison to much larger body parts. During a kiss, this lip sensitivity causes our brain to create a chemical cocktail that can give us a natural high. This cocktail is made up of three chemicals, all designed to make us feel good and crave more: dopamine, oxytocin, and serotonin. Like any cocktail, this one has an array of side-effects. The combination of these three chemicals work by lighting up the 'pleasure centres' in our brain. The dopamine released during a kiss can stimulate the same area of the brain activated by heroin and cocaine. As a result, we experience feelings of euphoria and addictive behaviour. Oxytocin, otherwise known as the 'love hormone', fosters feelings of affection and attachment. This is the same hormone that is released during childbirth and breastfeeding. Finally, the levels of serotonin present in the brain whilst kissing look a lot like those of someone with Obsessive Compulsive Disorder. No wonder the memory of a good kiss can stay with us for years.

Is there a difference in Platonic and non-Platonic kisses?

Anyone who has ever given their best friend a quick kiss on the cheek will know it feels very different to the rush of sensations experienced when you make out with the smouldering hottie you've been chatting up all night. Non-romantic kissing is very common, but it is much more of a cultural phenomenon than the romantic kiss. Children blow kisses to their parents, some Europeans air-kiss as a greeting, and we kiss friends to say farewell. The closeness inherent in these kisses can create feelings of affection or respect, but not the feelings of euphoria that usually follow a romantic kiss. Platonic kisses are usually brief pecks on the cheek. In contrast, romantic kisses involve intimate, lengthy lip-to-lip contact. Since it is this lip contact that activates the chemical cocktail in the brain, a Platonic kiss just can’t compete.

Like many human behaviours, kissing is fascinating and complex. We have a lot left to learn about kissing, so get out there and research in the name of science! (As if you needed an excuse...)

Follow Emer on Twitter @EmerMofficial.

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