Expressive vocabulary ability of toddlers with Williams syndrome or Down syndrome: A comparison

School-aged children and adults with Williams syndrome have repeatedly been found to evidence an expressive vocabulary advantage relative to same-aged individuals with Down syndrome. However, Singer Harris, Bellugi, Bates, Jo nes, and Rossen (1997) argued that this advantage is reversed during the in itial period of language acquisition; during this time, children with Down syndrome have larger expressive vocabularies than children with Williams sy ndrome. This result may have been due to methodological problems, however. This study uses a different design to reconsider the question of whether to ddlers with Williams syndrome show an expressive vocabulary advantage over same-aged toddlers with Down syndrome. Parents of twenty-four 2-year-olds w ith Williams syndrome and twenty-eight 2-year-olds with Down syndrome compl eted the vocabulary checklist from the MacArthur Communicative Development Inventory: Words and Sentences. The 2 groups were carefully matched for chr onological age (CA). Results indicated that the toddlers with Williams synd rome had substantially and significantly larger expressive vocabulary sizes than did the CA-matched children with Down syndrome. Additional analyses o f children for whom data were available between the ages of 2 years 0 month s and 2 years 3 months indicated that the expressive vocabulary advantage f or children with Williams syndrome was present even at this very young age when none of the children had begun to produce word combinations. The Discu ssion section that follows addresses the discrepancy between these findings and those of Singer Harris et al. and considers the variability present wi thin both the Williams syndrome and Down syndrome samples. Also discussed i s the continuity across the lifespan in both the expressive vocabulary adva ntage shown by individuals with Williams syndrome relative to same-aged ind ividuals with Down syndrome and the expressive vocabulary variability withi n each syndrome.