What are the origins of vocal communication in human infants?  In our new paper in Developmental Science, we find that a crucial building block of communicative development is learning that one’s voice has the power to influence others.

We show that communicative development originates in adults’ responses to babbling. As infants (2- and 5-month-olds) realize that their early sounds have social potency, they develop expectations about the efficacy of their voice. Infant behaviors, particularly vocal behaviors, change how people immediate around them behave. 

To measure infants’ social expectations, we used a “still-face” paradigm. In this paradigm an experimenter interacts, face-to-face with the infant, normally for 1 minute. Then experimenters assume a neutral face expression for 2 minutes. If infants have figured out the social potency of their vocalizations, what will they do when their social partner stops responding?

The effect is similar to what you would do if you put money into a vending machine but didn’t receive anything.  When the machine doesn’t give you your soda, your arousal and exploratory behaviors increase – you bang on the buttons, maybe you shake the machine, etc.  Five-month-old infants, but not two-month olds, used their babbling in a similar way.  They increased their babbling when their social partner stopped responding.

What explains the developmental change in this behavior?  Both 2- and 5-month-olds decreased their smiling over the course of the still-face which suggested they do notice the withdrawal of social responsiveness. But only the 5-month-olds showed a burst in vocalizations after the still-face begins. Our social expectancy hypothesis posits that the 5-month-olds, over the course of everyday interactions with people, have incrementally built an expectation about the social efficacy of their vocalizations.

Consistent with our social expectancy hypothesis, we found that the extent to which caregivers were responsive to their infants’ vocalizations during free play positively predicted the strength of 5-month-olds’ expectation for social responses in the still-face.  The more that their caregivers responded to their babbles during play, the bigger the increase in vocalizing when the experimenter stopped responding.

Thus infants’ knowledge of the social efficacy of their vocalizations gradually emerges over the first 5 months of life. Our results point to vocal interactions with caregivers as an important source of early individual differences in learning the social efficacy of their sounds.