Compounding and inflection in language impairment: evidence from Williams Syndrome (and SLI)

We have examined potential dissociations between lexical and grammatical knowledge in language impairment by investigating noun plurals and plural formation inside compounds in Williams Syndrome (WS) subjects. For comparison, we also report results from studies investigating the same linguistic phenomena in children with Specific Language Impairment (SLI). We found that nouns taking regular plurals were always correctly inflected by the WS subjects, whereas they performed much worse on nouns that take irregular plurals. The WS subjects heavily overregularized the regular −s plural, and they even used the plural −s as non-head elements inside compounds, i.e. in circumstances in which unimpaired children (and adults) would typically not use regular plurals. We argue that the excessive use of the regular −s plural in WS results from an impairment of the lexical system and/or its access mechanisms. The dissociation between lexical and grammatical phenomena in WS supports the theoretical distinction between listed lexical entries and a (rule-based) computational system for language.