Young infant's laughter found to share features with ape laughter

A team of researchers from Leiden University, University College London and the University of Amsterdam, has found that human infants laugh in ways that are more like chimpanzees than adult humans. In their paper published in the journal Biology Letters, the group describes their study.

Prior research has shown that when adult humans laugh, they do so only while exhaling. Chimpanzees, on the other hand, laugh during both inhaling and exhaling. In this new effort, the researchers have found that human infants laugh like chimpanzees—they do it while both inhaling and exhaling.

To learn more about infant laughter, the researchers asked two groups of adult volunteers to listen to audio of infants laughing. One group of 117 people was asked to count when laughter was heard while the infant was inhaling and while exhaling. The second group, made up of 102 adults, was asked to rate how pleasant and contagious the laughter by the infants sounded to them.

In analyzing their data, the researchers found that infants laughed in ways more similar to chimps than to adult humans. They suggest this makes sense for two reasons. The first is that throat anatomy in infants more closely resembles that of adult chimps than adult humans. The second reason is because humans learn to laugh in different ways as they grow older based on listening to the people around them, typically their parents. The data also showed that the volunteers found laughter during exhaling more pleasant and contagious than laughing during inhaling—a finding that helps to explain why babies cease laughing while inhaling as they grow into toddlers.

Prior research has shown that laughing is common in both apes and monkeys and also in some rodents. It has generally been found to be used as a bonding device by creatures that live in highly social groups.

The researchers suggest that human babies initially laugh as an automatic response to things they find funny in ways that feel natural to them. It is only as they grow older that they change the way they laugh to suit their environment. They next plan to conduct the same sorts of tests with crying infants.

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