Understanding stimulus poverty arguments1 JANET DEAN FODOR AND CARRIE CROWTHER

The argument from the poverty of the stimulus as Pullum and Scholz deÞne it (their APS) is undeniably true, given that all language learners acquire the ability to generate more sentences of the target language than they have heard. Uniformity across learners with respect to the additional sentences they project suggests that grammar induction is guided by general principles, which must be innate. What remains to be established is exactly which sentences can be projected on the basis of which others. The details of this are important to linguistic theory and to the psycho-computational modelling of natural language acquisition. They are not of great signiÞcance to the generic issue of nativism versus empiricism, except that they may clarify the extent to which the innate knowledge in question is speciÞc to language. The argument for linguistic nativism appears to be solidly supported by the distinctive patterns of generalization that learners adopt in the absence of systematic negative evidence (a limitation that Pullum and Scholz exclude from APS). We argue that innate knowledge of how to represent natural language facts is necessary in order for learners to extract from their input the information that it does contain. Pullum and Scholz themselves rely on Universal Grammar in just this role when they make speciÞc suggestions as to how learners arrive at the right generalizations.