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, directly contributes to infants’ vocabulary by providing names of objects in infants’ focus. Another possibility is that follow-in speech contributes to vocabulary growth via sustained attention, extending infants’ looking to a specific object of interest and enhancing the encoding of the word-object mapping. Yet another way in which follow-in speech can contribute to vocabulary growth is by elevating infants’ attention to the object and further speech. Our analyses suggest that follow-in speech likely contributes to infants’ vocabulary in the first year via two pathways: directly, by providing the names of objects in infants’ focus and indirectly, via influencing infants’ attention. We also show that the effects of follow-in speech are distinct from the contribution of parents’ verbal responses to infants’ other communicative bids, such as vocalizing.