The effects of maternal input on language in the absence of genetic confounds: Vocabulary development in internationally adopted children

Previous studies have found correlations between parent input and child language outcomes, providing prima facie evidence for a causal relation. However, this could also reflect the effects of shared genes. The present study removed this genetic confound by measuring English vocabulary growth in 29 preschool‐aged children (21 girls) aged 31–73 months and 17 infants (all girls) aged 15–32 months adopted from China and Eastern Europe and comparing it to speech produced by their adoptive mothers. Vocabulary growth in both groups was correlated with maternal input features; in infants with mean‐length of maternal utterance, and in preschoolers with both mean‐length of utterance and lexical diversity. Thus, input effects on language outcomes persist even in the absence of genetic confounds.