Children's grammars grow more abstract with age--evidence from an automatic procedure for identifying the productive units of language.

We develop an approach to automatically identify the most probable multiword constructions used in children's utterances, given syntactically annotated utterances from the Brown corpus of CHILDES. The found constructions cover many interesting linguistic phenomena from the language acquisition literature and show a progression from very concrete toward abstract constructions. We show quantitatively that for all children of the Brown corpus grammatical abstraction, defined as the relative number of variable slots in the productive units of their grammar, increases globally with age.