Valian, V. (1991). Syntactic subjects in the early speech of American and Italian children. Cognition, 40, 21-81.

Abstract: Why do young children leave out sentential subjects? Two competence-deficit hypotheses and a performance-limitation account are evaluated in the present set of studies.  American children appear to understand that English requires subjects before mean length of utterance (MLU) 2.0.  On balance, performance factors account for the data best.  Natural conversations between 21 American children (ranging in age from 1;10 to 2;8 and in MLU from 1.53 to 4.38) and their mothers were taped, transcribed, and analyzed to determine when American children understand that English requires subjects.  We measured the frequency of subjects (Study 1); types of pronominal subjects, including expletives (Study 2); frequency of modals and semi-auxiliaries (Study 3); frequency of infinitival to, past tense, third person singular, and subordinate clauses (Study 4); length of verb phrase, frequency of different types of verbs, and frequency of direct objects (Study 5).  For Studies 1 and 3 we also used , for comparative purposes, transcripts of 5 Italian children, taped monthly for a year.  Even our lowest-MLU American group (5 children between 1.5 and 1.99) used subjects and pronominal subjects more than twice as often as the Italian children, and correctly case-marked their subjects.  The American children also produced examples of all the sentence elements measured.