equivalent score 86.5 (52 – 128) 91.4 (37 – 156) 96.9 (46 –124) 64.5 (33 – 100)

Language is considered to be a relative strength in Williams syndrome (WS) but results are dependent on the language measures used and the comparison groups employed On tests of grammatical competence and phonological skills, children with WS consistently outperform mental-age-matched controls with Down syndrome (DS) but perform no better than typically developing (TD) controls Conversely, on tests of receptive vocabulary knowledge, children with WS outperform TD controls1 but studies comparing receptive vocabulary in WS and DS have failed to find significant group differences2 Relative strengths in receptive vocabulary knowledge have been noted in Down syndrome3, although the comparison is usually with other language measures Klein & Mervis2 argued that strengths in vocabulary are not specific to WS and it is possible that individuals with low IQ generally perform relatively well on vocabulary tests simply because they are older than TD controls and therefore have greater world knowledge. In this study, we first mapped out the developmental relationship between receptive vocabulary knowledge (BPVS-II4), age and nonverbal ability (RCPM5) using data from TD children and a group of children with non-specific learning disabilities (LD) We then used this data to standardize the vocabulary knowledge of children with WS and DS, controlling for age and nonverbal abilit