How does social contingency facilitate vocabulary development?

Previous research shows that infants of parents who are more likely to engage in socially contingent interactions with them tend to have larger vocabularies. An open question is how social contingency facilitates vocabulary growth. One possibility is that parents who speak in response to their infants more often produce larger amount of language input, which accelerates vocabulary growth. Another possibility is that the simplicity of contingent language input is especially suitable to support early word learning. A third possibility is that more evidence of the communicative nature of language, achieved through frequent contingent responses, helps infants build a link between their own words or vocalizations and others' behaviors. This link may lead to a better understanding of the communicative nature of language and further language advances, including vocabulary growth. To distinguish between these hypotheses, we analyzed the relations between parent-infant interactions when infants were 9 months and their vocabulary size at 12 months, using a naturalistic corpus. Our findings show that the frequency of parents' verbal contingent responses predicts receptive vocabulary size at 12 months and this predictive relation is unlikely to be due to the amount of language input or the simplicity of language within socially contingent interactions. RESEARCH HIGHLIGHTS: Infants of parents who respond to their vocalizations more often during the first year of life tend to have larger vocabularies in the second year. It is an open question what drives the predictive relation between parents' responsiveness and infants' vocabulary; we tested three hypotheses that offer competing explanations. More responsive parents might provide (1) more language input, (2) simpler language input, (3) more evidence of the communicative nature of language (via frequent responses). We find support for the third hypothesis; the frequency of parents' responses predicts infants' vocabularies above and beyond the amount and simplicity of language input.