The squeals, vowel-like sounds and growls made by infants are critical to the development of human language, according a recent paper coauthored by a cognitive scientist at the University of California, Merced.
The paper , published online by the prestigious Proceedings of the National Academy of Sciences, shows infants at 3 to 4 months old are able to use sounds to express how they feel about something, whether it’s positive, neutral or negative. Called functional flexibility, this ability allows sounds to be distinct from their meanings and is a defining characteristic of language.
For example, an infant’s growl could show that they’re happy or angry. In contrast, crying and laughter tend to always represent the same emotional states.
Cognitive science Professor Anne S. Warlaumont  said the appearance of functional flexibility early in the first year of human life is not only a critical step in developing vocal language, but it may have been a critical step in the evolution of human language.
This behavior hasn’t been reported in any nonhuman primates.
“Our findings demonstrate that some of the earliest sounds made by human infants have the characteristic of being very flexible with regard to the emotional states in which they’re used,“ Warlaumont said. ”For example, the same infant might one morning produce a squeal while happy and then later the same day produce a similar squeal while upset.
In a room with toys and furniture, the researchers videotaped infants playing and interacting with their parents. Researchers studied nearly 7,000 sounds and faces the infants made to judge whether the expressions were positive, neutral or negative.
Warlaumont conducted the research as a doctoral student at the University of Memphis with first author Professor D. Kimbrough Oller. The paper, “Functional Flexibility of Infant Vocalization and the Emergence of Language,” was co-authored by researchers at Idaho State University, the Milwaukee School of Engineering and Georgia State University.