Infants
differ substantially in their rates of language growth, and slow growth
predicts later academic difficulties. In this study, we explored how the
amount of speech directed to infants in Spanish-speaking families low in
socioeconomic status influenced the development of children’s skill in
real-time language processing and vocabulary learning. All-day recordings of
parent-infant interactions at home revealed striking variability among
families in how much speech caregivers addressed to their child. Infants who
experienced more child-directed speech became more efficient in processing
familiar words in real time and had larger expressive vocabularies by the age
of 24 months, although speech simply overheard by the child was unrelated to
vocabulary outcomes. Mediation analyses showed that the effect of
child-directed speech on expressive vocabulary was explained by infants’
language-processing efficiency, which suggests that richer language
experience strengthens processing skills that facilitate language growth.
