Do face-to-face interactions support 6-month-olds' understanding of the communicative function of speech?

Infants by 6 months recognize that speech communicates information between third parties. We investigated whether 6-month-olds always expect speech to communicate or whether they also consider social features of communication, like how interlocutors engage with one another. A small sample of infants watched an actor (the Speaker) choose one of two objects to play with (the target). When the Speaker could no longer reach her target object, she turned to a new actor (the Listener) and said a nonsense word. During speech, the actors were either face-to-face, the Speaker was facing away from the Listener, or the reverse. When the actors had been face-to-face, infants looked longer when the Listener selected the non-target object compared to the target. Infants looked equally regardless of what the Listener chose when either actor had been disengaged. Area-of-interest gaze coding suggests that infants were similarly interested in the interaction across conditions, but their pattern of attention to Speaker and Listener differed when the Listener was disengaged during speech. Although these experiments should be replicated with a larger sample, the findings provide initial evidence that 6-month-olds do not expect speech alone to communicate, but also attend to the social context in which speech is produced.

Acoustic regularities in infant-directed speech and song across cultures.

When interacting with infants, humans often alter their speech and song in ways thought to support communication. Theories of human child-rearing, informed by data on vocal signalling across species, predict that such alterations should appear globally. Here, we show acoustic differences between infant-directed and adult-directed vocalizations across cultures. We collected 1,615 recordings of infant- and adult-directed speech and song produced by 410 people in 21 urban, rural and small-scale societies. Infant-directedness was reliably classified from acoustic features only, with acoustic profiles of infant-directedness differing across language and music but in consistent fashions. We then studied listener sensitivity to these acoustic features. We played the recordings to 51,065 people from 187 countries, recruited via an English-language website, who guessed whether each vocalization was infant-directed. Their intuitions were more accurate than chance, predictable in part by common sets of acoustic features and robust to the effects of linguistic relatedness between vocalizer and listener. These findings inform hypotheses of the psychological functions and evolution of human communication.