Infants can laugh like some monkeys during their first few months of life

Babies can laugh like some monkeys a few months after birth before switching to giggling more like human adults, according to a new study.

Laughter connects humans to the great apes, our evolutionary parents (NS: 6/4/09). Adult humans tend to laugh when exhaling (NS: 6/10/15), but chimpanzees and bonobos mainly laugh in two ways. One is like a gasp, with a sound produced on both inhalation and exhalation, and the other has explosions occurring on exhalation, like human adults.

Less is known about how human babies laugh. Mariska Kret, a cognitive psychologist at Leiden University in the Netherlands, and her colleagues scoured the internet for videos of laughter of children aged 3-18 months and asked 15 sound-of-speech specialists and hundreds of novices to judge the laughs of babies.

After reviewing dozens of short audio clips, experts and a hundred non-experts both found that younger infants laughed during inhaling and exhaling, while older infants laughed more when exhaling. . Finding suggests that laughter in infants becomes less ape-like with age, researchers report in September issue Biology letters.

Humans start laughing around 3 months of age, but very early on, “he hasn’t reached his full potential,” Kret says. According to the researchers, the maturing vocal tracts of both babies and their social interactions may influence the development of sounds.

A second try of the new study with different audio clips and a new group of 100 novices also found that older infants seemed to laugh mostly when exhaling. And participants in both trials reported that more adult laughs were more pleasant to hear and contagious. This second finding suggests that the change in laughter as infants get older may in part occur due to unconscious assertions by the babies’ parents, Kret explains. Laughter during the exhale is clearer and louder than during the inhale, she says, sending a stronger signal during interactions, which may be better for bonding.

The idea that social interactions shape the laughter of human babies matches observations of chimpanzees, says Marina Davila Ross, a comparative psychologist at the University of Portsmouth in England who was not part of the new study. Davila Ross discovered that among chimpanzees of different social groups, laughter can have somewhat different sounds and social functions. And humans and chimps adjust their laughter based on feedback from their peers, she says.

Still, the number of audio clips analyzed in the new study is low, making it difficult to spot trends, says D. Kimbrough Oller, a theoretical biologist at the University of Memphis in Tennessee who was not involved in the research. . In total, listeners to the study’s trials heard 108 clips of laughing infants, each clip lasting four to seven seconds.

And while people may think babies laugh a lot, says Oller, recordings throughout the day suggest babies rarely laugh. Most often, they make other sounds before speech: “babbling, screaming, growling, raspberries – babies make them all day”. The intense examples of laughter used in this study that were filmed are probably not so representative, he says, so scientists should listen throughout the day to better understand the range of early laughs.

There’s also a lot more to laughing at than breathing, says Davila Ross. Laughter can take on greater complexity with the vibration of the vocal cords and sound more melodious if vowels are heard. It’s not clear how these aspects of laughter in infants and human monkeys compare, she says, and further analysis would examine the structure of laughter sound waves.

Reading in the pleasure of laughter could also be problematic, explains Carolyn McGettigan, psychologist and cognitive neuroscientist at University College London. What people find enjoyable can be related to how they perceive the ages of children. People may, for example, enjoy laughter from older infants more if they think toddlers are more fun than younger infants.

Still, the research provides a good place to start, she says. “Studying these types of childhood vocal behaviors gives us a window into the evolution of what we can do with our voices.”

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