Evidence for maturational constraints on syntactic comprehension from longitudinal data on over 6,000 autistic children

Understanding the phenotypic complexity of autism is central to elucidating its biology, development, and clinical heterogeneity. Leveraging data-driven clustering applied to a novel syntactic assessment in a longitudinal cohort of 6,736 autistic individuals aged 1.5–22 years, we identified three distinct comprehension phenotypes—(1) Command, limited to understanding single words and simple commands; (2) Modifier, extending to the integration of nouns with adjectives but lacking full syntactic processing; and (3) Syntactic, supporting the integration of nouns with spatial prepositions and complex syntactic structures—and characterized their developmental trajectories. Progression through these phenotypes differed systematically between autistic and neurotypical children. Increasing autism severity both reduced the likelihood of attaining higher-level phenotypes and prolonged the time required to reach them. The Command Phenotype was retained by 11%, 19%, and 39% of individuals with mild, moderate, and severe autism. Among individuals who advanced, median ages for acquiring the Modifier Phenotype were 3.7, 4.6, and 5.7 years across the same groups. For the Syntactic Phenotype, median ages were 4.8, 5.9, and 6.5 years. The absence of upward phenotype transitions after 10 years-of-age suggests a firm maturational cutoff that, together with earlier critical-slowing-down of learning-rate in ASD, helps explain constraints observed in syntactic development.