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鈥淲HILE there is infection in disease and sorrow, there is nothing in the world so irresistibly contagious as laughter and good-humour.鈥� So wrote Charles Dickens in A Christmas Carol. He was in London in the 1840s, but these words ring true in any time or place. Laughter is one of humanity鈥檚 few universal traits. Even in the time of covid-19, many people have found that a good chuckle has helped them cope with the stresses, uncertainties and interminable lockdowns.

It is surprising, then, that psychologists and neuroscientists were once reluctant to devote serious attention to laughter, with many believing expressions of mirth to be less important than those of unhappiness or despair. 鈥淧sychology still has a lot of catching up to do to balance out what is known about negative emotions with positive ones,鈥� says .

This has been science鈥檚 loss because recent results reveal that there is far more to laughter than you might think. Beyond the obvious connection with humour, it offers some truly profound insights into the nature of our relationships and the state of our health. The study of infant giggles may even help us understand how we develop our sense of self and the ability to read the minds of others. What鈥檚 more, laughter turns out to be surprisingly common in other species.

Non-human animals aren鈥檛 known for their sharp wits, but many do engage in play, often producing characteristic sounds to signal that their behaviour is friendly rather than aggressive. According to by and , Los Angeles, published this year, scientists have documented these 鈥減lay vocalisations鈥� in 65 species. Most are mammals, but a handful of bird species are also known to signal their harmless intentions in this way. , for example, warble gently as they tussle on the ground or chase each other through the air.

In many animals, including chimpanzees, gorillas, rhesus macaques and dogs, play vocalisations take the form of fast, rhythmic panting. Cotton-top tamarins whistle. Rats emit an ultrasonic squeak beyond human hearing. And elephants make a kind of soft trumpeting noise. Winkler and Bryant speculate that such sounds represent a rudimentary form of proto-laughter that could have formed the evolutionary basis of our own humour. Given its prevalence among primates, he estimates that proto-laughter evolved in one of our direct mammalian ancestors at least 100 million years ago.

Human relationships are much more complicated than those of the average mammal 鈥� and we have much more control over our voice. As a result, human laughter has evolved to be a potent and flexible social tool. According to , . The first is reward: when we laugh together, it shows appreciation of a particular behaviour and reinforces the interaction, so that we are more likely to act in the same way in the future. Spontaneous laughter triggers the , which is probably what creates these rewarding feelings. Laughter鈥檚 second function is to signal connection. These affiliation laughs tend to be voluntary (or 鈥渇ake鈥�) and help to smooth over tension and embarrassment rather than reinforcing a particular behaviour. If you have said something potentially hurtful, for example, a polite chuckle might help to reassure someone that it was just playful teasing. The third purpose of laughter is to signal dominance 鈥� like when your boss laughs dismissively at your outlandish idea. Whereas a direct challenge, such as a cutting put-down, might trigger aggression, laughter indicates disapproval in a more subtle way. 鈥淚t maintains a facade of social harmony,鈥� says Wood.

To provide evidence for this hypothesis, Wood and her colleagues asked , reassuring (a sign of affiliation) or mocking (a sign of dominance). Each type was found to have different acoustic properties. The reward laughs were louder and longer. The affiliation laughs were quieter, shorter and mellower. The dominance laughs, meanwhile, lacked the pleasing melodic features of the others. 鈥淭hey were basically uglier and noisier and had all these acoustic markers of chaos,鈥� says Wood.

The conclusion that laughter is a powerful social signal chimes with findings by Bryant and his colleagues that , giggles and snide little titters. The laughers were all from the US, yet people from Europe, Asia and Africa were just as able to identify the nature of the laughs as fellow Americans were. People鈥檚 capacity to tell whether . Other research has identified , but Bryant鈥檚 results suggest that the core signals remain recognisable across the world.

鈥淟aughter has evolved as a potent and flexible social tool with three key purposes鈥�

Further evidence for laughter鈥檚 universality comes from its early emergence in a child鈥檚 emotional vocabulary. A baby鈥檚 first laugh typically arrives by the age of 4 months 鈥� long before their first words. 鈥淚t鈥檚 the least complicated type of laughter because it鈥檚 purely emotional, bubbling up from the limbic system, and it doesn鈥檛 require control of the vocal track,鈥� says Mireault, who was inspired to study this developmental process by her own baby鈥檚 helpless giggles at her brother sneezing.

As any caregiver knows, people will go to ridiculous lengths to tickle a baby鈥檚 funny bone. In Wood鈥檚 framework, these are reward laughs, reinforcing the loving interactions. Mireault makes a similar argument, and points out that it brings obvious evolutionary benefits. 鈥淟aughter may serve as a kind of 鈥榖onding agent鈥� that keeps caregivers connected 鈥� in the loveliest way 鈥� to an infant,鈥� she says. 鈥淭he infant subsequently benefits from having an engaged caregiver, both in terms of its physical survival and in terms of developing those critical feelings of attachment toward the caregivers.鈥�

that it doesn鈥檛 take long for babies to learn how to use the polite affiliation laugh. A 6-month-old playing peek-a-boo, for instance, may offer a muted giggle as it grows bored of the game. 鈥淚t鈥檚 using an effective social strategy,鈥� she says. By 8 months of age, babies are clowning around to amuse others. They are constrained by their clumsy movements and lack of speech, but their burgeoning humour involves making funny faces, imitating others and deliberate teasing, such as holding out a car key and then snatching it away before the caregiver can reach it.

鈥淭ickling could prime a baby鈥檚 brain to acquire a sense of self鈥�

Mireault thinks such behaviours show the early development of theory of mind 鈥� the ability to understand that others have their own thoughts and perspectives 鈥� which is generally not believed to blossom until several years later in a child鈥檚 development. 鈥淲hen a baby attempts to tease another person, it reveals that the infant understands that it can influence what someone else is thinking,鈥� she says.

If that isn鈥檛 profound enough, some researchers have suggested that tickling 鈥� one of the most common ways to elicit laughs in a baby 鈥� might be one of the first experiences that help . The argument is that we aren鈥檛 ticklish to our own touch, only to someone else鈥檚, so these seemingly superficial interactions could prime a baby鈥檚 brain to recognise itself as a unique body distinct from other things in the world.

Because laughter is so intimately linked with sociality, learning to laugh is a serious business. 鈥淚f you can鈥檛 join in with laughter, or you don鈥檛 want to join in with laughter, or laughter straightforward irritates you 鈥� that will have a really big impact on the interactions that you have with people,鈥� says . She and others have found that . Her team has also shown that , and that this skill is particularly impaired in those with damage to the frontotemporal lobes, which is mostly .

These are early findings, but Scott thinks that the ongoing study of laughter will shed light on the development of these and many other psychiatric and neurological conditions, including depression. She also proposes programming apps to record people鈥檚 laughter through their smartwatches or phones, providing a kind of 鈥渋ndex鈥� of someone鈥檚 mental health that might be useful for healthcare providers.

Like many scientists studying laughter, . She is now more convinced than ever of the profound insights it can offer for understanding the human condition. 鈥淭hings that seem trivial and silly may actually be the most important elements of people鈥檚 lives,鈥� she says. We may think of laughter as just a simple expression of humour, but it really is no joke.