As any parent knows, a laughing baby is a happy one, a crying baby is sad. A baby’s coos, squeals, and growls, however, are the soundtrack of many occasions. They may be bubbly, upset, or feeling just about average.
This production of multi-purpose sounds may be unique to humanity, propose D. Kimbrough Oller and colleagues in a new PNAS Early Edition paper. An ape, for instance, seems to only make sounds that indicate how it feels. A “threat growl” is a threat, end of story. On the other hand, human babies utter the same sounds in several emotional states. As early as three months old, they can emotionally distance themselves from the sounds they make–laying the foundation for learning language.
“That freedom is critical,” says Oller, a professor at the University of Memphis. “In order to ever learn a word, you have to be able to make that vocalization freely. For language to evolve the mind of humanity needed to be able to control the vocal track independent of the emotional state.”
Sounds that have only cultural meaning is essentially the definition of a language, Oller says. There is nothing about a canine that requires us to call it a ‘dog’ after all. Nor does saying ‘dog,’ or any other word, require us to feel a certain way. ‘Okay’ may come across as pleased, depressed, angry, rueful, or ironic, depending on our tone and facial expression.
Prior to this, early speech research has mostly focused on searching for meaning in early infant sounds. Our inborn ability to make “functionally flexible” sounds has been overlooked in the past, the authors write.
“There previously has been no direct comparison illustrating the functional flexibility of protophones [squeals, growls, and vowel-like coos] versus the functional fixedness of cry and laughter. This is a key gap because the demonstration of this difference could illustrate straightforwardly the very early emergence of a vocal capability present in and required for language and not yet reported in other primates.”
Oller noticed this ability decades ago, but says technology just recently made it possible to collected the data he needed. “Before this it was too hard to coordinate audio and video,” he says.
Researchers evaluated nearly 7,000 cries, laughs, coos, squeals, and growls made by nine babies collected during their first year of life, as they interacted with a parent or caretaker. Using just the sound, the vocalizations were coded for type. The accompanying facial expressions, caught on video, were coded separately, without hearing the vocal track. A software package that kept track of the time signatures synched the two data sets.
The results revealed that, as expected, laughter and crying are essentially fixed signals indicating happiness and displeasure. While other sounds, the coos, the squeals, the growls are, for the most part, made in a neutral state; they can indicate positive or negative emotions as well, with positive slightly more common than negative.
This ability hasn’t been seen in non-human primates, so far. “We’ve put out a sort of challenge to primatology in this paper to see if they can come up with anything similar” says Oller. If such an ability is found, it would suggest our capacity of language stretches to the deepest roots of our human tree.
See videos of infant vocalizations used in the study, along with other supplementary material online.