The two types of human laugh
Summary
One is caused by tickling; the other by everything else
ANGLOPHONE NOVELISTS describing amusement are laughing all the way to the bank. Depending on context, characters can chortle, chuckle, titter, hoot, giggle, snigger, howl or guffaw. This richness of language may suggest to some that laughter, itself, is a phenomenon of infinite variety, one that lends itself to endless subcategorisation. The joke would be on them.
New work led by Roza Kamiloglu, a psychologist at the Free University of Amsterdam, provides evidence that there are just two primary types of laughter: one generated when people find something funny and one that can be induced only through the physical act of tickling.
The work started with the serious business of laugh collection. Dr Kamiloglu instructed research assistants to search YouTube, a video platform, for footage featuring spontaneous laughter. They collected a total of 887 videos that were then categorised based on the inciting comic incident, ranging from tickle attacks to Schadenfreude and verbal jokes.
Roughly 70% of these videos were then used to train a laughter-categorising machine-learning algorithm to connect different forms of laughter with the activities that caused them. The algorithm was then asked to deliver its verdict on the remaining 30%. After a quick listen, Dr Kamiloglu and her colleagues thought the different laughs would be too varied for any connections to be made. The algorithm disagreed.
Based on acoustic traits like loudness, rhythm and changes in frequency brought about by vocal-cord vibrations, the algorithm was able to correctly identify laughter produced by tickling 62.5% of the time. All other forms of laughter, whether they came from viewing stand-up comedy or watching someone pour salt into their tea instead of sugar, were nowhere near as easy to tell apart. This suggested there was something unique about the post-tickling laugh. When Dr Kamiloglu ran the experiment again, this time asking human observers to categorise the laughs, a similar phenomenon presented itself: the observers correctly identified tickling laughter 61.2% of the time.
The findings, published in Biology Letters, are more than light entertainment. They could, instead, point scientists towards the evolutionary roots of laughter. After all, many mammals including dogs, squirrel monkeys, Barbary macaques and chimpanzees produce vocalisations during play that sound remarkably like laughter. One of the first things that infants do early in life is laugh. Even babies born deaf spontaneously produce laughter. Humans are not the only animals that tickle either. Macaques and chimpanzees both engage in the activity too.
All this suggests that laughter from tickling evolved over 10m years ago with the common ancestor that humans shared with these other primates. Dr Kamiloglu suspects that this early sort of laughter probably evolved to help primates build friendly relations, especially during play. With this in mind, she is now keen to study how infectious different sorts of laughs are. If the tickling laugh is one that truly evolved to bring primates together, it ought to be particularly infectious—but nobody has yet tested if it is.
As for all the other forms of laughter that only people produce, these probably evolved millions of years after tickling came along, when the human brain became complex enough to understand irony, slapstick and puns. But he who laughs last, it would seem, laughs longest.
