The average adult laughs about 15 times a day, but not all laughs are created equal. According to a recent study, laughter specifically shared between close friends can signal relationship status to third-party listeners. The finding not only shows the important role laughter plays in a relationship, but also suggests how these sudden outbursts influenced the evolution of human social behavior.

For the study, now published online in PNAS, lead researcher Dr. Gregory Bryant from UCLA used 48 audio clips of laughter taken from 24 recordings which Bryant had recorded many years ago for unrelated research. In the clips, volunteers either had a short conversation with a friend or with a stranger that they had just met. The average length of the conversation was around 13 minutes and volunteers were not told anything about why they were being recorded. Bryant told Medical Daily that volunteers were asked to start a conversation about roommates, but in reality could talk about anything they wanted.

The clips were played to a total of 966 participants from 24 societies around the world. After listening to the clips, the participants were then asked two questions: if they thought the people laughing were friends or strangers, and how much they thought the people liked one another. The responses were used to gauge what the listeners gleaned about the pair's relationship hearing nothing more than their brief laughter.

“I have been looking at colaughter for some time, and I developed the idea that colaughter might be a group-produced signal that gives information to over-hearers about the affiliation status of the group,” Bryant told Medical Daily. “All theories about laughter until then were about the functions within groups.”

Results showed that most participants were between 53 percent and 67 percent accurate in judging the pairs, and most accurate at judging pairs of female friends. Clips that were most often associated as laughter shared between friends were characterized by acoustic features such as high speaker arousal and spontaneous, genuine emotions.

Everyone loves a good laugh and laughter’s ability to spread happiness and joy transcends all cultural, linguistic, and geographical barriers. Still, the function of laughter in modern humans still is not well understood. According to Bryant, the fact that people can make such a quick judgement about friends or strangers could suggest that laughter is a particularly good sign of social affiliations. This finding could even give insight as to why humans evolved the capacity to laugh, and suggests that laughter is a universal indicator of relationship status to outside parties.

“It can be quite important in social life to know who is allied and who is not, so given the long evolutionary history of laughter in nonhuman primates, it stands to reason that it played a role in social behavior in early humans and before,” said Bryant.

Source: Bryant GA, Fessler DM, Fusaroli R, et al. Detecting affiliation in colaughter across 24 societies. PNAS . 2016