Parents provide children with both genes (nature) and linguistic input (nurture). A growing body of research demonstrates that individual differences in childrens language are correlated with differences in parental speech. Although this suggests a causal link between parental input and the pace of language development, these correlations could reflect effects of shared genes on language, rather than a causal link between input and outcome. We explored effects of maternal input on English vocabulary development in internationally-adopted (IA) childrena population with no genetic confound. IA preschoolers demonstrated some of the same correlations with input as in previous studies; specifically, measures of input quality were significantly correlated with vocabulary. However, IA infants did not demonstrate this pattern. Differences between the age groups may be related to the pace of acquisition; more rapid vocabulary development in the preschoolers suggests that access to, and childrens ability to make use of input, may be a limiting factor for the infants.