Fathers' repetition of words is coupled with children's vocabularies.

Differences in vocabulary size among children can be explained in part by differences in parents' language input, but features of caregivers' input can be more or less beneficial depending on children's language abilities. The current study focused on a specific feature of infant-directed speech: parents' repetition of words across utterances. Although previous work with infants showed a positive relation between repetition and children's vocabulary, we predicted that this would not be the case later in development. Instead, parents may use less repetition as their children become increasingly proficient language learners. In the current study, we examined the extent to which low-income fathers of 24-month-olds (N=41) repeat words to their children using three indices: type-token ratio, automated repetition index, and partial repetition of open-class words. The same finding emerged across all measures of repetition: Fathers whose children had larger vocabularies at 24months repeated wordslessoften, suggesting a developmental coupling of fathers' input and children's language proficiency.