A cackle and a giggle can mean different things.
Scientists say there are two types of laughter: the kind that comes from pure glee, and the kind that's meant to send a social message. New research suggests autistic children don't often express the latter type, a finding that could reveal more about the nature of human laughter.
From the beginning
Laughter probably predates human speech by millions of years, scientists think. It likely evolved as an early form of communication to help people negotiate group dynamics and establish hierarchy, said William Hudenko, a psychologist at Ithaca College who led the new study.
Babies usually learn to laugh before they learn to speak.
"We think it's so hardwired that even infants start to use laughter in order to promote affiliation and bonding with caregivers," Hudenko told LiveScience.
People are about 30 times more likely to laugh in the presence of others than alone, reinforcing the idea that laughter is a social phenomenon.
And though we associate laughter with humor, a large proportion of laughs aren't in response to anything remotely funny. Rather, they are often just affirmations, communications, or expressions of joy.
We laugh "to kind of grease the social wheels," Hudenko said.
Laughter mainly comes in two types, researchers think: voiced, and unvoiced.
"We need more research to be done to understand the function of voice versus unvoiced laughter, but our best hypothesis is that unvoiced laughs are probably used more to negotiate social interactions, and voiced might be more linked to a positive internal state," Hudenko said.
So voiced laughter — the prototypical, belly-laugh type, that sounds more like sing-song — is usually spontaneous. We create the sound with our vocal chords (hence the name), and usually laugh this way naturally and spontaneously.
Unvoiced laughter, however, is more of a conscious expression. We make these panting, grunting, snorting noises when we are trying on purpose to laugh, usually for a social purpose, such as to ease conversation or make friends.
For adults, each type of laughter represents about 50 percent of the total. Young children may express more voiced than unvoiced laughter, as they haven't yet learned to purposely laugh.
But strikingly, Hudenko and team found that autistic children almost never produce unvoiced laughs. They monitored kids between ages 8 and 10 playing in a lab, and set up situations that would elicit laughter, such as falling block towers and squeaky balloons.
The test group of autistic children laughed just about as often as the non-autistic kids, but the autistic children's laughter was 98 percent voiced, while non-autistic children produced both types.
"We take this as some preliminary evidence that children with autism might not be using laughter the same way," Hudenko said. "Our hypothesis is that typically-developing children, through the course of development, learn a large repertoire of laugh sounds in order to negotiate social circumstances. We suspect the children with autism are not attuned to the same social subtleties."
The finding helps underscore the different purposes of the two kinds of laughter. It could also help researchers design better ways to help kids with autism navigate social situations.
"The idea is that we might be able to help them to express laughter more readily during social interactions," Hudenko said. "This shows that these children with autism have a really fantastic skill that might help them to promote relationships with individuals."
In fact, research shows that most people prefer to listen to voiced laughter more than unvoiced. Call it our suspicious nature: We react better to laughter that is spontaneously produced, rather than laughter that is calculated to send a message.
This fact could prove beneficial to autistic children, who tend to produce the more popular variety of laughter.
"We at least know they're producing sounds that other people like to hear," Hudenko said. "We think this is a powerful first start to look at ways we might be able to use emotional capacities these children already have instead of trying to teach them some new skill."
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