Laughter is, evolutionarily speaking, pretty old news. It’s a behavior we humans share with many of our primate relatives, who make similar vocalizations during play. In fact, based on the species that display this behavior, the precursor to all modern primate forms of laughter is thought to have emerged around 20 million years ago.

But humans differ from our cousins in one very important way: we’ve learned to voluntarily control an imitation of laughter. Much like crying, yawning, or screaming, laughter is at its core an involuntary emotional reaction for most primates. But humans have developed such control over our breathing and vocal apparatus that we can imitate these vocalizations.

The thing is, our imitations aren't exactly like the real thing. Real laughter has different acoustic features from imitation laughter and has more in common with the laughter of other primates—it’s louder and higher in pitch, among other things. Gregory Bryant, a UCLA researcher who studies the evolution of vocal communication, came up with a way to test whether these acoustic differences can actually be perceived by people.

Dr. Bryant teamed up with a huge group of researchers to test laughter perception in 24 different societies throughout Europe, Asia, Oceania, Africa, and the Americas. Many of the societies are industrial, some more rural or agricultural, and a couple are mostly herders or hunter-gatherers. All 966 participants in the experiment were played short snippets of “colaughter,” which is simultaneous laughter produced by a pair of people.

The recordings, which were taken from recordings produced for a different experiment, were only around one second long and had no vocalizations other than laughter. Half of them were from pairs of people who were friends, and half were from strangers. Bryant and the other researchers discovered that people from all around the globe were reliably able to choose which pairs of people were friends and which pairs were strangers.

Participants were correct, on average, 61 percent of the time, which is significantly above chance. The genders of the laughing people made a big difference to accuracy rates: listeners were really good at spotting pairs of female friends (as high as a 95 percent accuracy rate in the US), but worse, sometimes below chance, at spotting pairs of female strangers. The results from male laughing pairs were often the opposite: participants were more likely to spot male strangers than male friends. This could be due to people assuming that women are only comfortable enough to laugh together if they are already friends, the researchers suggest.

The researchers went looking for the acoustic cues that their participants might have been using to make their judgments. They found that a number of features were related to the results: people were more likely to think that pairs of friends produced laughter that was shorter, less regular, and lacked a regular rhythm.

Because they found the effect across very different societies, the researchers suggest that this ability is universal to all humans, which suggests that it’s something useful to us. Many of our primate cousins use their equivalents of laughter as a reliable way to tell whether a relationship is close and strong. Because humans are such a highly social species, the authors write, it’s possible that we use laughter in a similar way, to gauge quickly how solid a relationship between people is.

That said, the results from all societies weren’t identical. The differences between them could turn up some interesting relationships between social structure and laughing behaviors. For instance, people in the US were better at recognizing male friend pairs than male stranger pairs, but in Korea, this pattern was reversed.

A host of new questions spring from this study. For instance, is it actually possible to produce a more convincing fake laugh? And just how close does a relationship need to be before its laughter is identifiably friendly?

To figure out how much of the ability is learned, we could also look at how young children fare in a test like this. Perhaps we need some solid experience of social relationships before we can make these discriminations. Or perhaps newborns are already wincing at the fake laughter set off by dad jokes.

PNAS, 2016. DOI: 10.1073/pnas.1524993113 (About DOIs).