Longitudinal trajectories across the Command, Modifier, and Syntactic Phenotypes of language comprehension in over 6,000 autistic children

Typically-developing children progress through three distinct language-comprehension phenotypes. 1) The Command Phenotype, emerging by age 2, is characterized by understanding single words and simple commands. 2) The Modifier Phenotype, observed around age 3, is characterized by understanding adjective–noun combinations. 3) The Syntactic Phenotype, reached by age 4, is characterized by understanding stories and complex syntactic structures. This study examined language-comprehension trajectories in autistic children, using parent-submitted longitudinal assessments collected approximately every 4.8 months, with at least four assessments per child across 6,736 participants. The average observation period was 2.2 ± 1.3 years, covering ages 1.5–22 years. Autistic children advanced through the same three phenotypes as neurotypical children but with substantial delays. Increasing autism severity reduced the likelihood of attaining the Modifier and Syntactic Phenotypes and lengthened the time required to reach them. Median ages for acquiring the Modifier Phenotype were 2.5 years for neurotypical children and 3.7, 4.6, and 5.7 years for those with mild, moderate, and severe autism. For the Syntactic Phenotype, median ages were 3.5, 4.8, 5.9, and 6.5 years across the same groups. These findings provide the first large-scale quantification of discrete language-comprehension trajectories in autism and underscore the importance of early, phenotype-specific intervention.