Stability and Change in the Cognitive and Adaptive Behaviour Scores of Preschoolers with Autism Spectrum Disorder.

We examined the stability of cognitive and adaptive behaviour standard scores in children with autism spectrum disorder (ASD) between diagnosis and school entry approximately age 6. IQ increased 18 points in 2-year-olds, 12 points in 3-year-olds, and 9 points in 4-year-olds (N = 281). Adaptive behaviour scores increased 4 points across age groups (N = 289). At school entry, 24 % of children met criteria for intellectual disability (cognitive and adaptive behaviour scores <70). No children with both scores ≥70 at diagnosis later met criteria for intellectual disability. Outcomes were more variable for children with initial delays in both areas (in 57 %, both scores remained <70). Findings are relevant to clinical decision-making, including specification of intellectual disability in young children with ASD.