From these recordings, he collected 18 spontaneous laughs, which he considered to be genuine. He then enlisted a different group of co-eds to laugh on command. From this exercise, he recorded 18 fake laughs of the same length as the real ones.
With Athena Aktipis, a research scientist at UC San Francisco, he then played the recordings to three groups of UCLA undergraduates. In the first round, the participants were asked to determine whether the laughs were real or fake, and the students could usually tell the difference. But they were fooled by 37 percent of the fake laughs.
In the second round, the researchers sped up the recordings and played them to a different group of college students. Speeding up the laughter significantly increased the likelihood that both kinds of laughs were judged as genuine, the researchers found. When sped up, the fake laughs fooled the study subjects half the time.
In the final round, Bryant and Aktipis dramatically slowed down the recordings and played them to yet another group of participants. Instead of asking whether the laughs were real or fake, the researchers asked the students to figure out whether the sounds were made by humans or nonhuman animals. As it turns out, the students couldn't tell whether genuine laughs were human or not, but they could tell that the fake laughs were made by people.
Bryant believes that the research illustrates that the two types of laughs - real and fake - are made by two separate vocalization systems.
Many animals laugh, but only humans know how to fake it.
The study has been published online ion the journal Evolution and Human Behavior.