Logic and felicity in the face of intellectual disability: linguistic scales and Williams syndrome

Quantifiers (some, all), numerals (two, three), and logical connectors (and, or) belong to the class of logical expressions that convey abstract and complex meaning, denoting properties of sets or relations between sets. In this paper, we show that individuals with Williams syndrome (WS), who display severe deficits in spatial and numerical cognition, and mild-to-moderate intellectual disability, can nevertheless carry out the semantic computations necessary for understanding these expressions. We report data from two experiments testing knowledge of (a) the set-theoretic properties of basic English quantifiers, numerals, and connectors and (b) the logical consequences of these properties (i.e. entailment relations). We compared the performance of 32 adults with WS and 32 neurotypical adult speakers of English. In Experiment 1, people with WS performed significantly above chance (and close to ceiling) on the tests that evaluate the truth-conditional meaning of logical expressions. Moreover, in trials that permit both a strictly logical interpretation and a pragmatically enriched interpretation involving an implicature, speakers with WS and neurotypical adults displayed similar interpretive preferences. In Experiment 2, the WS group scored correctly on more than 90% of trials that evaluated knowledge of the felicity conditions associated with each expression. We interpret these findings as evidence of deep semantic and pragmatic representations in people with WS, notwithstanding their other limitations. We maintain that the relatively small differences in performance between neurotypical adults and individuals with WS are a matter of degree, not of kind. This conclusion allows us to move beyond the question of whether language development in WS is “typical” and instead focus on the underlying computational mechanisms that support language use and understanding in adults with WS. We discuss our results in the context of questions about the independence of language from other cognitive systems, and more generally, the modularity debate.