The roles of language use and vocabulary size in the emergence of word-combining in children with complex neurodevelopmental disabilities

Parent report data on 82 preschool children with complex neurodevelopmental disabilities including Down syndrome, dyspraxia, autism, and global developmental delay suggests communicative language use must reach a threshold level before vocabulary size becomes the best predictor of word combining. Using the Language Use Inventory and the MacArthur-Bates CDI (with sign vocabulary option), statistical modelling using regression trees and random forests suggests that, despite high linear correlations between variables, (1) pragmatic ability, particularly children's emerging ability to talk about things, themselves and others is a significantly better predictor of the earliest word combining than vocabulary size; and (2) vocabulary size becomes a better predictor of later word combining, once this pragmatic base has been established.