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 from 6,736 participants, with a mean observation period of 2.2 {+/-} 1.3 years, spanning ages 1.5-22 years. Autistic children advanced through the same three phenotypes as neurotypical children but showed systematic differences. Increasing autism severity both reduced the likelihood of attaining higher-level phenotypes and lengthened 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 for those with mild, moderate, and severe autism. For the Syntactic Phenotype, median ages were 4.8, 5.9, and 6.5 years across the same groups. These findings provide the first large-scale quantification of language-comprehension trajectories in autism and underscore the importance of early intervention.