What underlies inflectional error patterns in genetic dysphasia?

We compare linguistic and non-linguistic explanations which attempt to account for the difficulties that individuals with genetic dysphasia experience with inflectional morphology. The non-linguistic explanations that we focus on are low performance IQ and performance deficits which are manifested at the level of auditory processing or at the mapping between representation and articulatory output. We argue that none of these hypotheses can account for the patterns observed; the impairment lies in the construction of the grammar itself We hypothesise that the dysphasic grammar lacks sub-lexical (morphological) features which mark inflectional information in languages. Consequently, the grammar does not contain the morphological rules which introduce these features and, following from this, there is no structure internal to 'inflected' words. We summarise the experimental evidence which supports this hypothesis. (C) 1997 Elsevier Science Ltd.