Why Is Laughter So Contagious?

Laughter is contagious because hearing it triggers the premotor cortex of the brain, priming our facial muscles to laugh in return. Positive sounds like laughter produce a notably stronger response in this region than negative sounds do, which is why a giggle from one person so easily sets off a room. This mirroring response evolved as a social bonding signal in our primate ancestors.

“Laugh and the world will laugh with you.” That legendary line by Ella Wheeler Wilcox still holds true.

Just like yawning, laughing is also highly contagious, but laughing is one contagious disease that we love and gladly spread it to others. In fact, even the sound of someone laughing can make you let out a gut-busting chuckle.

How many times has it happened that you laughed your heart out at a movie theatre, but when you watched the movie alone again on TV, you didn’t find it that funny, or at least didn’t laugh nearly as loud?

It’s because you were watching the movie by yourself, but that isn’t the same thing that happens when we experience humor in larger groups. So what makes laughing so contagious? Why do we end up laughing with other people even if we don’t find the joke all that funny?

Why Do We Laugh?

People may laugh for various reasons, such as if they like a joke, if they see their friend take an awkward tumble, or if they experience anything else that might tickle their funny bone.

Studies have shown that laughing has been passed on to humans from our ancient primate ancestor. For our ancestors, laughter was a process of strengthening the bond between them. People laugh when they feel free and are comfortable with one another, so laughter definitely increases bonding between people, just as it did for our ancestors.

Why Is It Contagious?

The way a person reacts to a sound depends on the brain’s premotor cortical region. This region is responsible for how the muscles in the face react to the corresponding sound. In a 2006 fMRI study led by neuroscientist Sophie Scott at University College London (UCL), researchers played a mix of sounds to volunteers and measured how their brains responded.

Responses were stronger for positive sounds, such as laughter or shouts of triumph, and weaker for negative ones like screaming or retching. The premotor response tracked the pleasantness of the sound, running clearly higher for the positive vocalizations than for the negative ones, suggesting that humans are far more susceptible to the sound of laughter than to negative sounds. This explains the involuntary smile that creeps in when we see other people laughing.

Part of this mirroring effect has been linked to the brain’s mirror neuron system. These neurons fire both when we perform an action ourselves and when we watch someone else do it, so seeing a friend crack up activates roughly the same circuits that fire when we laugh. The result: their laughter pulls our own to the surface almost before we know what was funny.

There is a famous case of the contagious nature of laughing known as the Tanganyika laughter epidemic. On January 30, 1962, three girls at a mission-run boarding school in the village of Kashasha, in what is now Tanzania, began to laugh uncontrollably, and the fits soon spread to the rest of the school. Eventually, 95 of the 159 pupils (aged 12 to 18) were affected, individual episodes lasting anywhere from a few hours to 16 days. The school was forced to close on March 18, 1962. Sending the students home only spread the problem further: over the following months, the laughter rippled through neighboring villages, ultimately shutting 14 schools and affecting roughly 1,000 people before the outbreak burned itself out about 18 months later. Researchers today classify the event as a case of mass psychogenic illness, a stress-driven response that spreads through observation. However, don’t get any ideas about trying the same trick to get out of going to school!

This natural response is also the reason pre-recorded laugh tracks are still used in sitcoms; canned chuckles nudge the audience at home to laugh along. Laughter therapy and laughter yoga lean on the same principle: surround yourself with people laughing and you involuntarily end up laughing too, even when nothing genuinely funny just happened.

What Does It Mean When Someone’s Laugh Is Contagious?

When we say a laugh is contagious (or infectious), we are borrowing the language of disease for a much friendlier idea. A contagious laugh is simply one that makes other people start laughing too, often before they even know what the joke was. So if a friend tells you, “your laugh is so contagious,” they don’t mean you are spreading germs. They mean that the moment you start laughing, it sets them off as well. It is a compliment about how easily your laughter pulls everyone else along with it.

The “contagious” part isn’t just a figure of speech, though. As we saw above, hearing a real laugh nudges your brain’s premotor cortex to ready your own face for laughing, and your mirror neurons quietly rehearse the same action you are watching. That is why a single giggle can ripple through a room in seconds. Some people simply have a louder, more open and more frequent laugh, so theirs tends to trigger this chain reaction more reliably, which is exactly what we mean when we call a particular laugh contagious.

It is worth noting that this works best with genuine, warm laughter. We are far less likely to “catch” a forced or sarcastic laugh, because our brains are surprisingly good at telling a real, spontaneous laugh from a put-on one (there are, after all, several different kinds of laughter), and it is the real thing that spreads most easily.

Is Catching a Laugh the Same as Catching a Yawn?

You have almost certainly yawned just because someone near you did, or even because you read the word “yawn” a moment ago. Catching a laugh and catching a yawn are close cousins: both are forms of emotional contagion, the automatic tendency to mirror the expressions and states of the people around us. Neither is something you decide to do; both happen well below the level of conscious choice.

The link to other people runs deep. A naturalistic study of 109 adults, published in PLOS One, found that the closer two people were emotionally, the more readily one caught the other’s yawn: contagion climbed steadily from strangers to acquaintances to friends to family, and social bonding predicted it better than anything else. Laughter behaves in much the same way. You are far more likely to dissolve into helpless giggles with close friends than with people you have just met, which is one reason both behaviors are thought to help bind social groups together.

There is one important exception worth knowing about. Sometimes laughter is neither contagious nor genuinely felt. In a neurological condition called pseudobulbar affect, people have sudden, uncontrollable bouts of laughing or crying that don’t match how they actually feel. According to the Cleveland Clinic, it stems from damage along the brain pathways that keep emotional expression in step with the situation, and it can appear after a stroke or a traumatic brain injury, or alongside conditions such as multiple sclerosis, ALS, Parkinson’s disease or Alzheimer’s disease. That kind of laughter is a medical symptom rather than the warm, infectious laugh that travels around a dinner table.

It is often said that laughter is the best medicine, as it has many benefits for human health. In that case, I just have one thing to say: laugh and make the whole world laugh with you.