Why we fake laughter — and how to tell the difference

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By Brian Alexander

You know how, when the neighbors’ child does that thing where he smacks his hand into your head over and over and the neighbors get hysterical and, to be nice, you laugh, too, though you don’t think the kid is the next incarnation of Jim Carrey?

They probably know you’re faking it, according to an increasing body of research on the joyful and complicated form of human behavior.  While laughter may seem to be about fun and jokes, it's really about "communicating affiliation and trust," Greg Bryant, UCLA associate professor of communication studies, writes in the Washington Post. 

Laughter is an important social signal, Bryant told TODAY last year when his research was first published in the journal Evolution and Human Behavior. It can tip off others that we want to cooperate, we want to be friends, we’re not threatening. That’s why primates, like humans, as well as many other animals, like dogs and even rats, laugh.

So we need to be able to distinguish types of laughter. To test how well we do that, Bryant and colleagues recorded undergraduates laughing spontaneously with roommates, and undergrads laughing when told to laugh by an investigator.

The team then played the recordings to 63 male and female undergrads. The students picked out spontaneous, or “genuine” laughter, about three-quarters of the time and the laugh-on-command, or “fake” laughs, about two-thirds of the time.

In two more experiments, the recorded laughter was slowed down and sped up. Other study participants identified the slowed genuine laughter as being human at no better than chance rates, but they did much better at identifying the faked laughter as human.

Our ability to detect real laughter from fake is rooted in the breathy sounds woven in with the “haha.”JGI/Jamie Grill / Today

At faster-than-normal speeds, they could tell genuine laughter at about the same rate as at normal speeds, but they mistook the faked laughs for genuine much more often.  

That’s because our ability to detect real from fake laughter is rooted in the breathy sounds woven in with the “haha.” Two different vocal systems make the laughs — an emotional system (genuine), and a speech system (fake). Emotional laughter has a bigger proportion of the breathy sounds.

“Fake” laughter isn’t always bad or unappreciated, Bryant said. It can communicate a willingness to cooperate and openness to bonding. He also pointed out that most children can detect deception at about age 5 or 6 and can probably detect faked laughter by then, too.

Which means the neighbor kid is doing that head slapping thing just to annoy you.   

Brian Alexander is a frequent contributor to NBC News and a co-author of “The Chemistry Between Us: Love, Sex, and the Science of Attraction.”

This updated story was originally published in May, 2014