Morphosyntax in Down’s Syndrome: Is the Extended Optional Infinitive Hypothesis an Option?

This paper examines the question of whether language impairment in Down’s syndrome (DS) can be characterized in terms of the same linguistic markers that were identified for Specific Language Impairment (SLI). The specific proposal we will assess is whether English-speaking adolescents with DS are more impaired in tense-related grammatical morphemes than in non-tense-related morphemes, a dissociation that has been argued to explain the linguistic profile of individuals with SLI. We tested 8 adolescents with DS and various groups of unimpaired children matched for mental age to the DS participants in four elicitation tasks examining the past tense, noun plurals, and comparative adjectives. We found that non-tense related morphemes are affected in a similar way to tense-related morphemes in DS indicating that the linguistic impairment in DS is broader than in SLI and not restricted to the finiteness cluster.