The role of follow-in naming and sustained attention in vocabulary growth

Recent research suggests that caregivers’ speech in response to their infants’ attentional shifts (“follow-in speech”) is correlated with vocabulary size. It is not yet clear, however, what possible developmental pathways underlie this correlation. One possibility is that follow-in speech, and follow-in naming in particular, predicts infants’ vocabulary because it provides names of objects in infants’ focus. Another possibility is that follow-in speech predicts vocabulary via sustained attention, extending infants’ looking to a specific object of interest and enhancing the encoding of the word-object mapping. We show that despite extending the duration of infants’ attention to objects, the predictive effect of follow-in speech on infants is attributable to parents providing the names of objects, regardless of the duration of infants’ attention to the named objects. We also show that the effects of follow-in naming in response to infants’ gaze shifts are distinct from the effects of parents’ responses to other communicative bids, such as vocalizing.