Structural complexity and the time course of grammatical development

One traditional view of the time course of language acquisition holds that a child's difficulties in learning her language are due to general processing, memory or conceptual limitations. As the child's cognitive capacities expand, so do her abilities to use her already-acquired grammar or to recognize additional properties of her soon-to-be native language. Recent work in the study of child language, however, has discovered the existence of a number of characteristic stages, transitions between which are best described in the terms of linguistic theory proper. These stages are surprising under a view of language acquisition according to which developmental delay derives from general cognitive limitations, which cannot characterize difficulties explicable only in language-specific terms. At the same time, current linguistic theories so severely restrict the variation among possible human grammars that there remains little reason why there should be any learning problem at all or characteristic developmental stages. In this paper, I propose that these two views can be reconciled. I show that children's difficulties with a wide range of syntactic constructions, which are indeed best defined in linguistic terms, should nonetheless be derived from limitations on the child's ability to deal with processing load and formal representational complexity. I suggest however that this can be done only in the context of a particular view of syntactic representation, one which is articulated in the terms of the formal system of tree adjoining grammar (TAG). I demonstrate how precisely those difficulties that children experience in the acquisition of relative clauses, adjectival modification, control constructions, raising, wh-questions and the obligatoriness of finite inflection can be traced to the complexity associated with one of the TAG combinatory operations, adjoining. This proposal relates this apparently disparate set of constructions in a novel way and provides us with a new type of explanation for the time course of syntactic development in terms of the complexity of formal grammatical devices.