Can you tell if two people are friends just by hearing them laugh? Researchers recorded two people laughing together. Some pairs were already friends, and some had just met.

Then 966 people from 24 societies around the world listened. They guessed "friends" or "strangers" better than by chance. Friends laughed in a more natural way. Laughing together seems to be a sign of friendship everywhere.

If you hear two people laughing in the next room, you might guess whether they are close. A 2016 study tested this ability across cultures. Researchers took short clips of pairs laughing together. Some were already friends and others had just met.

Then 966 listeners in 24 societies, many of whom did not speak English, judged each pair. In every society, listeners identified friends better than chance, with accuracy of about 53 to 67 percent. Friends' laughter sounded more spontaneous.

Another study observed 1,200 natural cases of laughter and found that most came after ordinary comments, not jokes. In a study of 71 couples, partners who spent more time laughing together were more satisfied with their relationship.

Of 1,200 laughs recorded in everyday life, most followed ordinary remarks rather than jokes. Laughter also almost always came at pauses in speech. Observations like these suggest that laughter works mainly as a social signal, and only partly as a response to humor.

In a study of 71 couples talking about how they met, the share of time spent laughing together predicted greater relationship satisfaction and closeness, even after individual laughter was taken into account. That study was correlational, though, so close couples may simply laugh together more. A 2016 study tested the social-signal idea across cultures. The team took short clips of co-laughter, moments when two people laughed at the same time, from conversations between American university students.

Some pairs were already friends, and others had just met. The clips were played to 966 listeners in 24 societies, including small-scale communities far from large cities, and many of the listeners did not speak English. In every society, listeners told friends from strangers at better than chance, with accuracy ranging from about 53 to 67 percent. Friends' laughter had more of the acoustic features of spontaneous laughter. Laughter between strangers may have been more deliberate.