Parent American Sign Language skills correlate with child-but not toddler-ASL vocabulary size

Most deaf children have hearing parents who do not know a sign language at birth and are at risk of limited language input during early childhood. Studying these children as they learn a sign language has revealed that timing of first-language exposure critically shapes language outcomes. But the input deaf children receive in their first language is not only delayed, it is much more variable than most first language learners, as many learn their first language from parents who are themselves new sign language learners. Much of the research on deaf children learning a sign language has considered the role of parent input using broad strokes, categorizing hearing parents as non-native, poor signers, and deaf parents as native, strong signers. In this study, we deconstruct these categories and examine how variation in sign language skills among hearing parents might affect children's vocabulary acquisition. This study included 44 deaf children between 8- and 60-months-old who were learning American Sign Language (ASL) and had hearing parents who were also learning ASL. We observed an interactive effect of parent ASL proficiency and age, such that parent ASL proficiency was a significant predictor of child ASL vocabulary size, but not among the infants and toddlers. The proficiency of language models can affect acquisition above and beyond age of acquisition, particularly as children grow. At the same time, the most skilled parents in this sample were not as fluent as "native" deaf signers, and yet their children reliably had age-expected ASL vocabularies. Data and reproducible analyses are available at https://osf.io/9ya6h/.