A new study, published Monday in the Proceedings of the National Academy of Sciences, suggests that the chuckles we share with our friends sound different from our chuckles with strangers.
That difference enables onlookers to gauge the type of relationship the laughing people have, Dr. Greg Bryant, professor of communication studies at the University of California, Los Angeles, and lead author of the study, told The Huffington Post.
Laughter between friends "is often faster, louder, longer, and has more minor fluctuations in pitch and loudness," he said.
The researchers played 48 short audio clips of two people laughing for 966 volunteers representing 24 different cultural backgrounds, including remote hunter-gatherer communities. Some of the recordings were of two recently acquainted strangers, while the others were of two friends. The pairs could be two men, two women, or a man and a woman.
The volunteers were asked to listen and identify which laughing pairs were friends and which were strangers.
Can you hear the difference?
The volunteers correctly identified whether the people were friends or strangers 61 percent of the time. Their accuracy rate reached 83 percent when the two chortlers were female friends, Bryant said. But it fell to 44 percent when the two women were strangers.
While the researchers had predicted that people in all cultures would be able to distinguish between friends and strangers, he said, "We did not expect specifically that women friends would be reliably judged best in every society, or that people would have a bias to think two women laughing were friends, even when they were not."
The study indicates "how universal the perception of vocal emotions can be," in Bryant's words. The researchers noted that it also highlights one way laughter allows us to communicate social information indirectly.
"Laughter may be a simple behavior, but it's also a powerful tool that provides insight into more complicated and difficult vocalizations, like speech and language," Dr. Robert R. Provine, a neuroscientist at the University of Maryland, Baltimore County, who was not involved in the research, told Smithsonian.com. Provine is the author of Curious Behavior: Yawning, Laughing, Hiccupping, and Beyond.