Constraints on the computational component vs. grammar in the lexicon: a discussion of Bates & Goodman.

In The emergence of language, it is argued that much of what generative linguistics has characterized as ‘rules’ in fact can be derived from domaingeneral cognitive mechanisms. One example of this perspective is Bates & Goodman’s contribution ‘On the emergence of grammar in the lexicon’, which Sabbagh & Gelman say offers ‘...a series of compelling arguments detailing how development and early acquisition shape the subsequent acquisition of new information.’ The thrust of the argument presented by Bates & Goodman is that there is no need to posit the existence of an independent grammar domain, because grammar can be reduced to the lexicon, which in Bates & Goodman’s account, can be acquired using general purpose mechanisms. One of several major arguments in favour of this position presented by the authors is that grammar and vocabulary grow at the same rates in child speech. Thus, the authors argue that the development of the lexicon and grammar correlate because there is no grammar outside of the lexicon. Bates & Goodman, indeed, go on to show a developmental correlation between vocabulary size and particular aspects of grammar. However, defining ‘grammar’ so that it can be correlated with growth in vocabulary size is extremely problematic. The two principle metrics for grammatical development used are MLU (mean length of utterance) and inflectional morphology. Using these metrics to measure the growth of grammar is a problem, which the authors recognize, because they include many of the elements used to measure grammar (e.g. modal and auxiliary verbs, definite articles, etc.) in their ‘ lexicon’ category, too. Thus, the initial curves the authors show could simply show a correlation between the growth of grammatical elements in the ‘ lexicon’ category with the growth of grammatical elements in the ‘grammar’ category (i.e. correlating something with itself). To solve this problem, the authors then show that even when closedclass elements are removed from what is counted as the lexicon, there is still a correlation between the growth of grammar and the lexicon. All of this is to refute the contention by generative linguists that word learning and grammatical development proceed in a different way and at a different pace