Bumping into someone you know but would rather not speak to while in the subway or on the street happens more often than it should in big cities. The random encounter is usually filled with small talk, cheap jokes, and fake laughter — in that order, too. But lo and behold, if you’re the one forcing a laugh about a joke that’s not funny, the other person will know, according to a new study.

It turns out our fake laughs aren’t as convincing as we’d expect, according to the researchers. “Quite a few fake laughs sound pretty good, but listeners seem to pay attention to certain acoustic features that are really hard to fake,” Greg Bryant, an associate professor of communication studies at the University of California, Los Angeles, said in a press release.

The researchers found that laughs could essentially be broken up into two different vocalization systems: speech and emotion. Although both kinds of laughs are vocalized as “ha-ha-ha,” the intermittent breaths a person took in-between the sounds were what gave away a real laugh from a fake one. After analyzing the acoustics of the laugh, Bryant found that real laughs had a higher proportion of breathy parts in-between — or more efficient control of opening and closing the windpipe. If you think about every time you’ve forced a laugh, you have to actually say the “ha-ha-ha,” and probably never considered how you breathed while doing it.

The team first recorded random conversations from college dormitories, and then asked a group of undergrad students to laugh on command. Once they had 18 real and 18 fake laughs, they played the recordings to three different groups of college participants. Thirty-seven percent of the participants were fooled by the fake laughs. Then, when they sped the recordings up, about 50 percent of students believed the fake laughs.

It was the slowed-down recordings that helped the researchers understand the difference between the two. For the last round, they asked participants to tell them only if they believed the recordings were from a human or an animal. Though they couldn’t tell the origin of the real laugh, they were able to tell that the fake laugh came from a human.

“Genuine laughs are produced by an emotional vocal system that humans share with all primates, whereas fake laughs are produced by a speech system that is unique to humans,” Bryant said in the release. “Across the animal kingdom, laughter signals, ‘we’re in play mode.’ In fact, laughter is thought to have evolved from labored breathing during physical play. In this way, genuine laughter reveals our animal nature.”

Indeed animals such as chimpanzees, gorillas, and orangutans have been seen laughing. But laughter extends further than them. Anyone who owns a dog has most likely seen them pant once or twice, quickly, knowing that they’re not overheated. In her book Inside of a Dog: What Dogs See, Smell, and Know, Alexandria Horowitz talks about this so-called laughter — it’s a form of communication between one playful dog and the next. Similar behaviors can be seen among rats, too, which chirp while they play. And it’s likely this penguin is laughing, too.

Source: Bryant G, Aktipis C. The animal nature of spontaneous human laughter. Evolution & Human Behavior. 2014.