鈥淚t is a fair, even-handed, noble adjustment of things, that while there is infection in disease and sorrow, there is nothing in the world so irresistibly contagious as laughter and good-humour,鈥� wrote Charles Dickens in A Christmas Carol. More than 160 years later, we still don鈥檛 know exactly why laughter is so catching, though a recent study offers some tentative clues.
at University College London measured the brain activity of 20 volunteers in a functional MRI scanner while she played them laughter, squeals of triumph and moans of fear and disgust. She also played a neutral, artificial sound that would have no specific meaning to the subjects.
The result? All the emotive sounds triggered a response in the brain鈥檚 premotor cortical, the area that controls the movement of facial muscles. Inside the brain scanners, though, the subjects were not actually using these muscles. To Scott, that indicates the brain is wired with 鈥渕irror circuits鈥� that prime us to copy another鈥檚 behaviour when we recognise their emotions. The brain response was more pronounced for the sounds of laughter and triumph than the vocalisations of negative emotions, suggesting that the urge to copy is greatest when we hear another鈥檚 delight or amusement ().
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That may explain how laughter is contagious, but why should it be so? One explanation stems from its evolutionary origins in rough-and-tumble play, where laughter sends out a clear message that the fighting is not for real (see 鈥淲hat are you laughing at?鈥�). 鈥淚t might be important to have the whole group safely signal this so that a play fight does not turn ugly because someone 鈥榙idn鈥檛 get the memo鈥�,鈥� says at Purdue University in West Lafayette, Indiana. Scott has an alternative suggestion. She believes that mirroring another鈥檚 emotional state might ease communication and interaction. Laughing at the same joke would help us to show affiliation with others, and this may be why it is especially contagious. 鈥淟aughter is an incredibly binding thing,鈥� she says.
There is another type of contagious laughter that is not so pleasant or playful. In 1962 in what is now Tanzania, a 鈥渓aughing epidemic鈥� broke out in a girls鈥� school, spreading across the whole country for several months. On closer inspection, though, it looks as if the laughter was a single symptom of a more complex disorder known as 鈥渕ass psychogenic illness鈥� or mass hysteria, that emerged as a result of building political and social pressures in the region () 鈥� so it probably had little to do with your everyday infectious laughter.
Read more: The secrets of laughter