Valian V. & Eisenberg, Z. (1996). The development of syntactic subjects in Portuguese-speaking children. Journal of Child Language, 23, 103-128.

Abstract: In order to separate competence and performance factors in acquisition of knowledge of syntactic subjects, we audiotaped and analyzed the spontaneous speech of 20 Portuguese-speaking two-year-olds in natural conversation with Portuguese-speaking adults.  We separated the children into three groups based on Mean Length of Utterance in Words: 1.5-1.99; 2.0-2.99; 3.0-4.99.  Our cross-sectional data demonstrated that Portuguese-speaking children increased their use of subjects from 28% in the lowest-MLUW group to 57% in the highest-MLUW group.  The children in the highest-MLUW group almost perfectly matched the adult speakers in the study on every measure.  The increase in the children’s use of subjects was primarily due to an increase in the use of pronominal subjects.  A comparison between Portuguese- and English-speaking children suggests that adult competence about the status of subjects is present at the onset of combinatorial speech, as shown by differential production of subjects.  Each group also experiences performance limitations, as shown by the increase in subject use as development proceeds.