Mechanisms of Vocabulary Acquisition Persist Under Variable Language Experience.

Deaf children with hearing caregivers are uniquely at risk for incomplete and/or delayed first language exposure: their spoken language access is often incomplete, and hearing caregivers who choose to learn a sign language usually begin after their child's diagnosis. Limited access to language during early childhood delays language learning. However, it is unclear whether the mechanisms underpinning vocabulary acquisition, reflected in sensitivity to various lexical properties, are also impacted by variability in early language experience. In this study, we compared the composition of early signed vocabularies among deaf children with deaf signing caregivers (N = 115) to those with hearing caregivers (N = 122). Although children with hearing caregivers had smaller vocabularies, their vocabularies were largely similar in composition to those of the group with deaf caregivers along all tested properties (phonological neighborhood density, iconicity, morphosyntactic, and semantic categories) except for frequency and the categories of predicates and action signs. Infrequent signs, predicates, and action signs were underrepresented in the vocabularies of deaf children with hearing caregivers compared to those with deaf caregivers. These findings indicate that when children are provided early and accessible language input, the cognitive tools driving their lexicon building remain robust, even in the face of variability in many aspects of their language experience. SUMMARY: When deaf children learn a sign language alongside their caregivers, their vocabulary acquisition is delayed and proceeds more slowly compared to children learning from caregivers who are already fluent in the language. Children with delayed, inconsistent, and/or limited early sign exposure successfully learn high frequency signs but are slower to learn rare signs. Iconicity, neighborhood density, and morphosyntactic/semantic category were similarly represented in both groups' vocabularies, suggesting learning mechanisms are largely unaffected by highly variable early language environments.