Background and aims: Previous studies suggest that syntactic development in children with intellectual disabilities (ID) is positively correlated with verbal short-term memory (VSTM). This study investigated the characteristics of syntactic development and their relationships of VSTM in children with ID based on type. Methods: The participants were children with ID ( N = 34), including 14 children with autism spectrum disorders (ASD), 20 with Down syndrome (DS), with chronological ages from 8 years 10 months to 18 years 4 months and nonverbal mental ages (MA) of over 4 years, and typically developing (TD) children ( N = 21) with chronological ages from 5 years 0 months to 5 years 10 months. They were assessed using VSTM, syntactic comprehension, and expression tasks. Results: The results showed that both the ASD and DS groups performed significantly lower on the syntactic comprehension task and the syntactic expression task than the TD group with the same nonverbal MA in the complex aspect of grammatical structure. In the VSTM task, the ASD group showed significantly lower performance in sentence and story repetition tasks than the TD group of the same nonverbal MA. The DS group showed significantly lower performance in forward digit span, and word, nonword, sentence, and story repetition tasks than the TD group of the same nonverbal MA. Conclusions: These results suggest that children with ASD have difficulty in understanding and remembering linguistic information with complex semantic structures, and children with DS have a small capacity for VSTM, affecting their syntactic development.

It is known that people with intellectual and developmental disorders (IDD) have more deficits in syntactic than other linguistic abilities. However, only a few studies on the syntactic development of people with IDD have been conducted in Japan. As a result, basic studies, accumulated data and the development of assessment methods have been deficient in Japan compared to other countries. This review compared international research findings with studies conducted in Japan. The results indicated that various factors affect the acquisition of syntactic knowledge, including the type of disability and the educational environment, which have not been sufficiently investigated. Based on the results, we have discussed the need to conduct future studies in Japan. It is suggested that future studies need to undertake the following tasks. (1) Developing scales for assessing language development to objectively and comprehensively evaluate syntactic development. (2) Investigating factors affecting syntactic development in people with IDD from perspectives other than the mental age (MA) and intelligence quotient (IQ), including the type of disability, life experience, and cognitive development, among others. (3) Investigate syntactic development in IDD people based on their stage of development, i.e., before or after acquiring grammar.