Hear it in Rio, Kathmandu or Timbuktu — it doesn't matter. A hearty, belly laugh means the same thing on every continent: joy.

But when we laugh with someone else, our chuckles may divulge more than we realize.

Scientists have found that people around the world can tell whether folks are friends or strangers by listening to them laughing together. And the ability transcends culture and language.

The study, published Monday in the Proceedings of the National Academy of Sciences, used a simple experiment. Psychologist Gregory Bryant recorded pairs of college students having conversations. Some were friends. Some hardly knew each other. He then isolated out just the parts in which the two people were laughing. Each cut was only about one second long.

Then Bryant and his colleagues at the University of California, Los Angeles, had volunteers listen to the clips of laughter and guess whether the people were friends or strangers. They ran the experiment in 24 societies around the globe, including indigenous tribes in New Guinea, tiny villages in Peru and cities in India and China.

People weren't perfect at the task. They were good at telling whether women were friends. But for other pairs — like two men laughing — it was harder. On average, listeners guessed correctly only about 60 percent of the time. That accuracy slightly better than simply tossing a coin (which would give you a 50 percent accuracy).

But the results were consistent across all the societies studied. That's a big deal, says Robert Provine, a psychologist and neuroscience at the University of Maryland, Baltimore County, who wasn't involved with the study. "That suggest we're dealing with a very basic aspect of human nature," he says.

For instance, a Hadza hunter-gatherer in Tanzania could tell two college girls in California were friends by listening to only one second of laughter.

"Laughter seems to be done by all people and all cultures," Provine says, "but details about what it means require cross-cultural studies. Such research is hard to do and is rarely done."

Neuroscientist Carolyn McGettigan at the Royal Holloway University of London agrees with Provine. "This study is really impressive," she says. "The scale of it is an achievement." But also, she says, it suggest that, even in the most remote places on Earth, a laugh among friends is a special sound.

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