Williams syndrome and specific language impairment do not support claims for developmental double dissociations and innate modularity

It has been argued that Williams Syndrome (WS) and Specific Language Impairment (SLI) show developmental double dissociations. This has been put forward as an argument that language is a module which is biologically programmed and develops independently of other cognitive abilities. However there have been very few studies which have directly compared the two populations. In this article we compare individuals with WS with individuals with SLI on a number of verbal and non-verbal standardised tests as well as on their language abilities in a narrative task. The participants with WS performed similarly to the participants with SLI on the verbal standardised tests and in the narrative task (there were no significant differences between the two groups of participants) although they performed significantly lower on the non-verbal standardised tests in comparison to the participants with SLI. The results suggest that although there are dissociations between WS and SLI in the non-verbal domain, the two populations are very similar in the domain of morpho-syntax, hence there is no support for developmental double dissociations between these two populations.