Vocabulary and Syntactic Development in Japanese Children With Autism Spectrum Disorder and Down Syndrome Accompanied by Intellectual Disability

ABSTRACT Background Children with intellectual disability (ID) have significantly delayed morphological and syntactic development. This study aimed to establish the link between vocabulary and syntactic development in children with autism spectrum disorder (ASD) and Down syndrome (DS) compared with children with typical development (TD), controlling for mental age (MA) as defined by the Tanaka–Binet Intelligence Scale V. Methods Participants comprised children with ID ( N = 33), including 14 with ASD and 19 with DS; chronological age (CA) ranged from 9 to 17 years, with an MA of over 4 years. Children with TD ( N = 28) had a CA of 5 years. Participants were assessed on vocabulary comprehension, vocabulary expression, syntactic comprehension and syntactic expression. We examined both group differences and within‐group associations between vocabulary and syntax. Results Although we witnessed no significant differences in vocabulary comprehension or expression, children with ASD and DS performed significantly lower on syntactic comprehension and expression tasks than MA‐matched children with TD. Both groups demonstrated difficulty with grammatical items requiring understanding of grammatical morphemes and grammatical knowledge. Conclusion Both groups exhibited vocabulary development similar to that of children with TD; however, their syntactic development was lower than expected considering their MA and vocabulary development. Building and examining approaches focusing on syntax, particularly grammatical morphology, is important in educational and clinical practice for Japanese children with ASD and DS.