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Why Do Humans Kiss? What Science and History Tells Us
The biological and cultural origin story of our chosen unit of affection.
Sean Kernan, 07/24/24·
Ryan’s mother walked to the front door and abruptly turned to his sister, Julie, “No boys, tonight. You got it?” She was our babysitter that night.
“Of course not!” Julie said with a wry smile. Her mother stared back, hard, to emphasize the point. Julie was 16 and quite beautiful, with big eyes and long curly brown hair. You had to get in line to have a crush on her.
Ryan’s parents left and just 15 minutes later, Julie’s boyfriend seemingly rappelled in from the ceiling. They were instantly making out in the pool. Ryan and I stared out the window in disgust at them. They were like two suckerfish attached at the face.
I vividly remember thinking, “Why do people kiss? It’s so weird.” My confused curiosity was at least partially driven by my lack of male hormones. But even today— it still feels like an unusual practice. What gives? Why do we like lip-on-lip contact so much? There are a few reasons.
The origin story
Evolutionary psychologists argue that what we know today as “kissing” may have come from “kiss-feeding” which was an exchange of pre-chewed food from mother to baby. It’s an ancient practice, often used during the weaning process. The mother helped the baby swallow and digest food through the use of saliva and chewing. The process also looks a heck of a lot like french kissing.
There are other factors at work too. A baby can only see 8–10 inches in front of its face. So from an early age, we are primed to recognize faces with spectacular accuracy relative to other animals.
The whites of our eyes are also evolved to help distinguish the direction of gaze from another person, so much so that special neurons fire when you recognize someone looking at you (they turn off when the person looks away). Faces are highly important to us and are a key component of social cohesion.
In addition, our wiring causes us to mimic behavior as a form of social bonding (such as someone kissing you back). When you combine this with our drive to seek out warmth and comfort, and the maternal upbringing with kiss-feeding, you have a strong case for the origins of kissing — which goes back, very, very far.
The first documented instances of romantic kissing are from the Bronze Age, circa 2500–3500 BCE. One tablet was inscribed in 1900 BCE, “My upper lip becomes moist, while my lower lip trembles! I shall embrace him, I shall kiss him.” Notably, I read this quote to my partner, who is a Bronze Age Archaeology professor, and the first thing she said was, “That may well have been a male who wrote that — as there weren’t many females writing in cuneiform back then.”
From here, kissing became so culturally ingrained that it eventually served as a way to seal marital contracts in Ancient Rome. Today, a kiss will certainly contract you something, but it doesn’t serve as a legal contract. Which brings us to the next reason.
A mouth-to-mouth sensory test
Years ago, and without getting into the gory details, I kissed a woman I’d been dating and it didn’t go well. She wasn’t a bad kisser nor were there any hygiene issues. I was even quite attracted to her. However, something just felt off and the experience didn’t pass the vibe check. I didn’t understand it at the time.
It’s quite possible my aversion was related to a genetic compatibility issue. One theory suggests our olfactory senses (smell) can detect immunological compatibility with a mate. People with heterozygous MHC genes (in short, different genes) tend to produce healthier offspring. Dr. Maria Bicalho, professor of immunology at the University of Paraná in Brazil, said, “It may be tempting to think that humans choose their partners because of their similarities. But our research has shown clearly that it is differences that make for successful reproduction, and that the subconscious drive to have healthy children is important when choosing a mate.”
In their study of 90 married couples, they found far greater genetic dissimilarity compared to a randomized pairing of 152 couples.
Biological forensics at work
The kissing theory I’m most fascinated by is related to pathogen exposure. Some scientists believe humans kiss to intentionally pass germs between them.
The idea is that, by kissing, a woman exposes herself to a man’s germs to develop antibodies which are passed down to the child, increasing its odds of survival. Which makes sense if you think about it: The moment you begin kissing someone, the odds of you two sleeping together rises dramatically. Which is why some scientists argue that kissing has a strong genetic and instinctual component.
The counter argument to this is that there are still cultures where kissing doesn’t exist (it’s practiced in 90% of cultures). Kissing wasn’t present in many Africans tribes, nor in the indigenous people of Australia until settlers arrived.
We often designate mouth kissing as an expression of love. There’s an element of choosing, and selectivity. Your lips and tongue are among the most sensitive parts of your body, and wired to trigger pleasure and reduce stress. Kissing also heightens bonds, particularly in moments of uncertainty.
Our bodies know intrinsically that you are exposing yourself to germs, and in some cases, physical risk by being intimate with someone. And yet — by design, when you kiss the right person, it still feels so darn good.
The conclusion on kissing
We haven’t had the luxury of interviewing our creator to ask him why we kiss. But the theories behind kissing remain substantive. It may have evolved from of kiss feeding, a biological drive to detect chemistry via hormone and smell, to boost antibodies for a baby — or simply to enhance our bonds with loved ones.
We aren’t the only species that kisses romantically (the mighty bonobos, and a few others, join us in this journey). Yet we do it in such a way that is quite unique and of personal importance.
After all, those who have had their first kiss and were reasonably sober when it happened — can generally remember the story quite well. For better or worse, kisses are hard to forget.
Sean Kernan
I'm a former financial analyst turned writer out of sunny Tampa, Florida. I began writing eight years ago on the side and fell in love with the craft. My goal is to provide non-fiction story-driven content to help us live better and maximize our potential.
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