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The science of passion: Why do we kiss?

May 19, 2023

Humans exchange millions of disease-spreading bacteria when we kiss. Scientists believe that kissing tests the compatibility of mates ― that it all comes down to swapping spit. But where does the behavior come from?

Young woman kissing figure of frog prince
Will he turn into a prince?Image: Creasource/picture-alliance

Kisses, storytellers believe, are transformative. A kiss on the lips can transform a frog into a prince or rescue a sleeping beauty from a coma spell. Symbolically, kisses are a pivotal point in a character's arc ― the moment they supposedly develop from a child to an adult, ready for romance and reproduction. A frog to a man.

In relationships, kissing creates physical intimacy that affects us perhaps more than words of love. Kisses are also sacred to many. Prostitutes, films tell us, don't kiss on the lips — they guard that level of intimacy for their true lover.

But kissing can also be non-romantic. A kiss planted on the forehead of a child is tender and caring, whereas a kiss on the cheek can be a greeting. Kissing someone's feet or the ground in front of them is groveling and shows subservience — a way of showing respect and awe to gods and tyrants.

The problem is kissing becomes a bit disgusting when you zoom in on the mouth. A 10-second kiss swaps around 80 million oral bacteria between two mouths. Eew! What's more, kissing is a very common way of spreading diseases.

So why do we do it?

Humanity's first kiss

"The very first evidence we have of lip kissing is [from] around 2,500 BCE," said Troel Arboll, an Assyriologist at University of Copenhagen, Denmark.

"It occurs in a mythological text from Mesopotamia, ancient Iraq. The text describes two gods copulating and kissing. It's definitely a sexual encounter," Arboll told DW.

Arboll and co-author Sophie Rasmussen have written a new perspective on the ancient history of kissing, published in Science on Thursday. They have a new theory that romantic kissing developed in multiple ancient cultures over several millennia. Aside from Mesopotamia, texts also depict sexual kissing in India and Egypt from at least 1,500 BCE onwards.

Arboll argues that sex and kissing began to feature more frequently as writing developed from an administrative tool to a way to tell stories. Kissing, for example, featured dramatically in the Epic of Gilgamesh, one of the oldest written stories dating to around 2,000 BCE. Like in modern interpretations, intimacy is transformative ― Enkidu's sexual encounter with Shamhat transforms him from an animal into a man.

While kissing is seen very early in a kind of erotic literature, Arboll also finds it in everyday documentation of ancient Mesopotamian life.

"For example, a man or woman who are not married are meant to keep apart and not kiss. The societies were trying to regulate the romantic kiss. The fact it was being regulated clearly means it was a common practice [in daily life]," said Arboll.

Ancient Egyptian relief of two men kissing
Two men kiss in this ancient Egyptian reliefImage: Herve Champollion/akg-images/picture-alliance

Is kissing innate?

Arboll proposes that romantic kissing might not be an innate human behavior, instead developing in complex societies as a learned mating behavior.

"Kissing doesn't seem to be universal across all cultures. It coincides with increased complexity of social interactions," he said, referring both to historical texts and more recent data that 46%of human cultures don't kiss in the romantic sense.

But some anthropologists propose that kissing is innate, at least the non-romantic kind. Kissing behaviors like licking and nuzzling are common in mammals like cats, dogs, elephants and apes. Bonobos in particular kiss like humans do, on the lips for comfort and socializing and even after a fight to kiss and make up.

Experts believe romantic kissing may have evolved out of this more mammalian nuzzling-type kissing. It's the same during our development, too. We first experience love from our parents through kisses and hugs before we redirect the behaviors to our adult lovers.

Boris Becker kissing his trophy in Wimbledon, 1985
Former tennis star Boris Becker and his lover having a momentImage: Steve Powell/Getty Images

Kissing disease

New evidence suggests kissing may have played an unintended role in facilitating disease transmission throughout human history. Pathogens like herpes simplex virus 1 (HSV-1) and Epstein-Barr virus (also known as kissing disease, or glandular fever), transmissible through saliva, have been found in human remains.

Arboll points to evidence that HSV-1 lineages shifted in the Bronze Age, possibly due to increasingly common romantic kissing. More recently, too, kissing played a role in COVID-19 transmission, prompting China to ban kissing and France to avoid the kiss-greet.

For Thuy Do, an oral health microbiologist at Leeds University, UK, it's not surprising kissing is a conduit for disease.

"We all have 800-900 different types of microbes living in our mouths," Do told DW. "When we kiss, we exchange a lot of saliva and all kinds of microbes. There's a danger of transmitting diseases if viruses like hepatitis and HSV-1 are exchanged."  

Pleasure and sex

However, it's really not all that disgusting. Do explained that a healthy mouth needs a balanced microbial environment, and kissing may actually be an important way of maintaining healthy microbe diversity in the mouth by exchanging microbes with our partners.

"Some bacteria species like streptococcus salivarius can help to bring down inflammation. People with a high abundance of health-associated species tend to have healthier mouths," said Do.

She thinks that kissing benefits more than just oral health.

"The mouth is a gate to the whole body — it links to our gut microbiome and skin. So when you're kissing it may have a positive impact on the microbiome in your whole body, even affecting our brain and mood," said Do.

Scientists believe that kissing is a way to test out potential mates. Kissing allows us to assess the partner's genetic computability or general health by picking up biological cues from the saliva.

And if the saliva matches, sex can follow. Tactile inputs from the tongue and lips trigger an ancient body-wide response to the prospect of sex in many people. The pleasure and reward centers of the brain are engaged, followed by releases of hormones like oxytocin, dopamine, and serotonin. Your skin flushes, your heart-rate increases and your pupils dilate all doe-eyed and glassy. Time to close the bedroom door.

Edited by: Carla Bleiker

DW journalist Fred Schwaller wears a white T-shirt and jeans.
Fred Schwaller Science writer fascinated by the brain and the mind, and how science influences society@schwallerfred