A new study by the University of California, Los Angeles (UCLA), researchers reveals that other people's laughter, even for just one second, can determine whether these people are your true friends or not.

In addition, the team claims that the findings hold true all around the world, and when two women are heard laughing together, most people assume they are friends regardless of whether they really are.

The team, consisting of UCLA communications professor Greg Bryant and 32 collaborators from all over the world, played 48 short audio clips of two people laughing together for 966 listeners from 24 different societies, including college students, hunter-gatherers and working-class urban groups.

Each of the laughter tracks stemmed from conversations between undergraduate students at UC Santa Cruz, some of whom were friends and others that were only recently acquainted strangers. The tracks captured the laughter of the following pairs: two women, two men and a woman and a man.

The results revealed that the listeners in the study from every society could identify whether people in the laughter clips were friends or strangers 61 percent of the time. Furthermore, when listening to two women laughing, the listeners were accurate more than 80 percent of the time.

Bryant believes that people can tell the difference between friends by determining spontaneous laughter from "fake" laughter. Previous research by Bryant suggests that these two laughs are created by different vocal systems, and the current study suggests that laughter with friends is more spontaneous and people can pick up on this.

One of the more surprising results of the study was the consistency with which participants, regardless of culture, determined that laughter between women meant that they were friends.

"Obviously, there is an assumption about female relationships at work," Bryant said. "People from around the world assume that when two females are laughing together that they are friends. This is consistent with other research showing that women take longer than men to develop friendships that result in genuine co-laughter."

Bryant believes that the findings reveal the nature of our evolutionary development and how laughter might have bolstered the evolution of cooperation.

"In a highly cooperative species such as ours, it is important for individuals to correctly identify the social alliances of others," he said. "If laughter helps people accomplish that, it has likely played a role in social communication leading to cooperative interactions."

The findings were published in the April 11 issue of the Proceedings of the National Academy of Sciences.