A familiar puzzle
Hearing someone laugh, even a stranger or a recording, often makes people smile or laugh in return, sometimes without knowing what was funny. This everyday experience is a good window into how our brains handle social signals. Researchers do not have a single complete explanation, but several ideas fit the evidence well.
Laughter is mostly social
It is natural to assume we laugh because something is funny. Studies that record when people laugh in ordinary conversation, notably work by the neuroscientist Robert Provine, suggest that most laughter follows comments that are not especially humorous, such as friendly remarks or small talk. People are also far more likely to laugh when with others than when alone. This points to laughter as a social signal. It can show friendliness, ease tension, mark that something is meant playfully, or signal belonging to a group.
Laughter also appears early. Babies typically begin to laugh when they are a few months old, often in response to play and interaction rather than jokes, which fits with the social-development picture in child development.
Laughter has ancient roots
Laughter-like vocalizations are not unique to people. Great apes make panting sounds during tickling and play, and researchers who studied young rats found that they produce high-pitched ultrasonic calls during play and tickling that appear to be linked to positive states. These calls are not human laughter, and it is uncertain how closely they correspond. The pattern does suggest that play-related vocal signals are old in evolutionary terms and may have developed to keep play friendly and to bond individuals together.
In humans, researchers distinguish roughly between spontaneous, emotional laughter and more controlled, conversational laughter. Brain studies suggest that these draw on partly different pathways, a distinction similar to the one between genuine and polite smiles.
What happens in the brain
When we hear laughter, auditory areas process the sound, and regions involved in preparing and planning movements, including parts of the frontal cortex that help control facial muscles, have been found to respond as well. In one set of brain imaging studies, listening to laughter and other positive sounds activated areas associated with producing facial expressions more strongly than negative sounds did. One interpretation is that the brain partly prepares to mimic what it hears, which could make us more likely to smile or laugh.
This is often linked to the idea of mirror neurons, cells found in monkeys that fire both when performing an action and when watching it. How far such a system operates in humans, and whether it explains social behavior like contagious laughter, is debated. It is safest to say that brain activity is consistent with a mimicry explanation, but the mechanism is not proven. The underlying communication relies on neuron signals, though no single “laughter center” has been identified.
Emotional contagion
More broadly, people tend to unconsciously mimic one another’s expressions, postures and voices, and to take on some of the other person’s mood. This is sometimes called emotional contagion. Mimicking a smile may slightly shift how we feel, an idea with roots in older theories about bodily feedback and emotion; evidence for the strength of this “facial feedback” effect has been mixed. Taken together, the ideas suggest that laughter spreads because it is both heard and, in some sense, felt. The wider study of feelings is covered in human emotions.
Why it may be useful
Several functions have been proposed, and they are not mutually exclusive:
- Bonding. Shared laughter may signal that people see things in a similar way, building trust and closeness.
- Group coordination. In a group, a laugh can quickly show that a situation is safe or playful.
- Tension relief. Laughing after a stressful moment may help release tension, as in the body’s wind-down from the stress response, though evidence for specific health benefits is limited and should not be overstated.
- Social information. Laughter helps people judge relationships; research suggests listeners can often tell whether two laughing people are friends or strangers from the sound alone, though this ability is not perfect.
Laugh tracks and recordings
Television producers have long used recorded laughter, partly because it can make audiences laugh more. Studies suggest that canned laughter tends to make material seem funnier to viewers, consistent with the contagion idea. Reactions vary, and some people find it irritating.
Who is more susceptible?
People differ. Laughter is more contagious among friends than with strangers, and context matters: a laugh at a funeral may produce an urge to giggle that people find hard to suppress, partly because it is socially forbidden. Personality, mood and culture influence how readily people join in. For some people with certain neurological or psychiatric conditions, laughter may be affected, and unusual or uncontrollable laughing is something to bring to a doctor. This article is general education only.
What remains unknown
Scientists continue to study how laughter evolved, which parts of the brain generate and respond to it, how far animal play vocalizations are related to human laughter, and why a particular laugh can be so infectious. The best-supported summary is that laughter is a social signal that our brains are well tuned to catch, and that catching it feels good and brings people together.