Babies as young as three months use three types of sounds - squeals, growls and vowel-like utterances - to express a range of emotions, which enables them to learn how to speak, a new study has found.
Infants' flexible manipulation of sounds to signal how they feel lays the groundwork for word learning, scientists said.
Attaching sounds freely to different emotions represents a basic building block of spoken language, said psycholinguist D Kimbrough Oller of the University of Memphis in Tennessee and his colleagues.
"At this point, the conservative conclusion is that the human infant at 3 months is already vocally freer than has been demonstrated for any other primate at any age," Oller said.
Oller's group videotaped infants playing and interacting with their parents in a lab room equipped with toys and furniture, ScienceNews reported.
Acoustic analyses identified nearly 7,000 utterances made by infants up to 1 year of age that qualified as laughs, cries, squeals, growls or vowel-like sounds.
Trained experimenters separately judged whether each sound an infant made, and the facial expression accompanying that sound, was positive, negative or neutral.
Overall, infants produced the flexible trio of emotion sounds much more often than laughs or cries. Babies most frequently uttered vowel-like sounds, which were less distinctive than babbling that starts at around 7 months of age.
Neuroscientists previously reported that monkeys, apes and humans share an ancient brain pathway linked to emotional sounds such as laughing and crying.
In the new study, babies' laughs overwhelmingly expressed positive feelings and cries usually conveyed negative feelings.
Ancient humans must have evolved new neural connections that supported early voluntary control of sounds other than laughing or crying to communicate emotions, said psychologist Michael Owren of Georgia State University in Atlanta.
"This groundbreaking work shows that, from the beginning, human infants have flexible vocal chops that put them on a very different developmental course than found in monkeys and apes," Owren said.
The study was published in the Proceedings of the National Academy of Sciences.
