Early language development relates to a child’s later language ability, cognitive development, and academic achievement. Parent input has long been studied as a predictor of infant language acquisition, and consequently, as a predictor of the differences in early language attainment associated with the documented ‘word gap’ between children from high and low Socio-Economic Status (SES) backgrounds. This dissertation sought to investigate the specific mechanisms of early parent-infant interactions that facilitate infant language learning, and whether SES differences are evident in those mechanisms at 10 months. Specifically, cooperative communication: the conversation-like back and forth between parents and infants, and parents’ contextualized responsiveness: use of responses that contingently elaborate on the infant’s attentional focus, were examined. These relationships were also examined in terms of infant language outcomes at 18 months.

Controlling for infant communication and parent input, SES was significantly correlated with parents’ responses to infant vocalizations, in particular when they were paired with a gesture or other behavior, and to parents’ use of object labels in their interactions with their infants. These noted differences suggest that input differences associated with the ‘word gap’ are evident in development as early as 10 months old. 

Children who were exposed to more contextualized responses overall and in particular those that contained a question, a label, or that responded to infant behavioral communication had better language performance at 18 months, even controlling for child communication and total parent input. Importantly, these components of contextualized responsiveness had a larger effect than SES on language outcomes, such as child word types and scores on the standardized Mullen Scales of Early Learning. This suggests that while SES is related to some positive components of responsiveness (labels and responses to vocalizations), the relationship between SES and language outcomes is often mediated, in full or in part, by parents’ use of specific contextualized responses. 	Together, the findings present contextualized responsiveness as a promising foundation for interventions aimed at diminishing and preventing the word gap and that, for parents from all SES backgrounds, their use of specific contextualized responses elaborating on their infant’s attentional focus facilitates optimal infant language learning.