Understanding the developmental dynamics of subject omission: the role of processing limitations in learning.

P. Bloom's (1990) data on subject omission are often taken as strong support for the view that child language can be explained in terms of full competence coupled with processing limitations in production. This paper examines whether processing limitations in learning may provide a more parsimonious explanation of the data without the need to assume full competence. We extended P. Bloom's study by using a larger sample (12 children) and measuring subject omission phenomena in three developmental phases. The results revealed a Verb Phrase-length effect consistent with that reported by P. Bloom. However, contrary to the predictions of the processing limitations account, the proportion of overt subjects that were pronominal increased with developmental phase. The data were simulated with MOSAIC, a computational model that learns to produce progressively longer utterances as a function of training. MOSAIC was able to capture all of the effects reported by P. Bloom through a resource-limited distributional analysis of child-directed speech. Since MOSAIC does not have any built-in linguistic knowledge, these results show that the phenomena identified by P. Bloom do not constitute evidence for underlying competence on the part of the child. They also underline the need to develop more empirically grounded models of the way that processing limitations in learning might influence the language acquisition process.