Valian, V., Hoeffner, J., & Aubry, S. (1996). Young children’s imitation of sentence subjects: evidence of processing limitations. Developmental Psychology, 32, 153-164.

Abstract: Elicited imitation was used to determine whether young children’s inconsistent production of sentence subjects was due to limitations in their knowledge of English or in their ability to access and use that knowledge.  Nineteen young children (age range = 1 year 10 months to 2 years 8 months; Mean Length of Utterance [MLU] range = 1.28 to 4.93) repeated sentences that varied in length, structure, and type of subject.  A competence-deficit hypothesis would predict that children below MLU 3 would differentially omit expletive subjects and subjects preceded by a discourse topic more often than children above MLU 3.  That hypothesis was disconfirmed.  A performance-deficit hypothesis would predict that children below MLU 3 would omit more subjects from long sentences than short ones, and that the high-MLU children would not show a length effect.  That hypothesis was confirmed. Processing limitations, rather than a defective grammar, explain very young children’s absent subjects.