Definitions versus categorisation: Assessing the development of lexico-semantic knowledge in Williams syndrome

Williams syndrome (WS) is associated with relatively strong language abilities despite mild to moderate intellectual disability, particularly when language is indexed by vocabulary. The aim of the study was twofold: (1) to investigate whether reported lexical anomalies in WS can be explained with reference to anomalous semantic development; (2) to assess whether receptive vocabulary skills in WS, a relative strength, are underpinned by commensurate semantic knowledge. The development of lexical-semantic knowledge was investigated in 45 typically developing (TD) individuals (CA range: 5-10, MA range: 5-13) and 15 individuals with WS (CA range: 12-50, MA range: 4-17), by means of (a) a categorisation task and (b) a definitions task, which was expected to make additional metacognitive demands. At younger ages, the performance level of TD individuals and individuals with WS did not differ on the definitions task. However, the WS group’s ability to define words fell away from the level predicted by the TD group at older ages, as more sophisticated definitions were expected. The results of the categorisation task indicated that individuals with WS had less lexical-semantic knowledge than expected given their level of receptive vocabulary, although from this lower level, the knowledge then developed at a similar rate to that found in typical development. We conclude first that conventional vocabulary measures may overestimate lexical-semantic knowledge in WS, and second concerns about the metacognitive demands of the definitions task when used with atypical populations may be well founded (cf. Benelli et al., 1988). Abbreviations: Williams syndrome (WS), typically developing (TD), chronological age (CA), mental age (MA), verbal mental age (VMA), British Picture Vocabulary Scale (BPVS)