Reference: Kret, M.E., Venneker, D., Evans, B., Samara, I., & Sauter, D. (2021). The ontogeny of human laughter. Biology Letters 17, 20210319.

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For many of us, laughter is fundamental to expressing ourselves and connecting with others. Long before babies learn to talk, they laugh. But a baby’s laughter is different than that of an older child or adult. As scientists recently discovered, the laughter produced by young babies actually sounds more like a chimpanzee’s laughter than an adult human’s.

Human laughter is unique among apes, and a challenge for young babies

While humans over the age of two only laugh while exhaling air, non-human apes like chimpanzees often laugh during both exhalation and inhalation. Scientists have proposed that human laughter evolved from this ape-like type of laughter that mixes exhalation and inhalation. But as our ancestors’ social behaviors and communication evolved, so did their laughter. Nowadays, adult human laughter occurs only during exhalation, making it a unique and recognizable social signal.

But for very young humans, exhalation-only laughter might not even be possible. This is because babies are born with a vocal tract that is shaped quite differently from that of an older child or adult. In fact, the vocal tract of a baby human resembles that of a non-human ape. So while babies can famously cry immediately after birth, the ability to laugh in a characteristically human-like way may take a bit longer to develop.

Does laughter develop along with a baby’s vocal anatomy?

Recently, a team of scientists led by Mariska Kret and Dianne Venneker at Leiden University in the Netherlands decided to investigate how a baby’s laughter changes during the first 18 months of life. They were particularly interested in how much babies were exhaling or inhaling while laughing. If the vocal tract of young babies starts off looking anatomically similar to a chimp’s, then the researchers figured that maybe their laughter resembles a chimp’s too.

To investigate this idea, Kret and Venneker’s team recruited adult participants and trained them to tell the difference between the sounds of exhalations and inhalations when listening to short audio clips. The researchers then collected audio recordings of laughing adults as well as laughing babies of ages ranging from 3-18 months old. For each recording, the participants were asked to estimate the proportion of exhaled laughter and the proportion of inhaled laughter on a five-point scale.

Older babies produce more of the human-specific type of laughter

As expected, the adults’ laughter had more exhalations and fewer inhalations than the babies’ laughter. And even among the babies, exhaled laughter increased as the babies got older. At only three months old, the youngest babies laughed when inhaling and exhaling, just like non-human apes do. But older babies with more developed vocal tracts laughed more often while exhaling than while inhaling.

Social cues might also affect how babies laugh

In addition to the physical anatomy of the vocal tract, the researchers also wondered if social interactions may affect the type of laughter that babies produce. After all, babies, like most people, do not typically laugh when they are alone. If a baby’s caregiver smiles happily whenever the baby laughs on an exhale, but not on an inhale, the baby may quickly learn that exhale-only laughter is more socially beneficial.

This idea that babies actually learn to produce more human-like laughter was supported by the second main result of the study. In addition to asking participants to monitor the exhalations and inhalations in the audio clips, Kret and Venneker’s team also asked them to rate the overall pleasantness of the laughter. The participants found laughter with more exhalation more enjoyable. Not only were the exhale-heavy laughs rated as more pleasant, they were also reported to be more contagious, or more likely to cause the listener to laugh in response.

What can we conclude about laughter development?

Overall, it is not clear whether the tendency for older babies to laugh in a more human-like manner is due to the babies’ vocal tract development, to positive feedback from their caregivers, or both. More research is needed to tease apart these effects and reveal exactly what drives babies to laugh the way they do. In the meantime, anyone interacting with young babies should know that their way of communicating happiness may look and sound just a bit different than ours.