Racial and ethnic disparities in the co-occurrence of intellectual disability and autism: Impact of incorporating measures of adaptive functioning

Intellectual disability (ID) commonly co-occurs in children with autism. Although diagnostic criteria for ID require impairments in both cognitive and adaptive functioning, most population-based estimates of the frequency of co-occurring ID in children with autism-including studies of racial and ethnic disparities in co-occurring autism and ID-base the definition of ID solely on cognitive scores. The goal of this analysis was to examine the effect of including both cognitive and adaptive behavior criteria on estimates of co-occurring ID in a well-characterized sample of 2- to 5-year-old children with autism. Participants included 3264 children with research or community diagnoses of autism enrolled in the population-based Study to Explore Early Development (SEED) phases 1-3. Based only on Mullen Scales of Early Learning (MSEL) composite cognitive scores, 62.9% (95% confidence interval [CI]: 61.1, 64.7%) of children with autism were estimated to have co-occurring ID. After incorporating Vineland Adaptive Behavior Scales, Second Edition (VABS-II) composite or domains criteria, co-occurring ID estimates were reduced to 38.0% (95% CI: 36.2, 39.8%) and 45.0% (95% CI: 43.1, 46.9%), respectively. The increased odds of meeting ID criteria observed for non-Hispanic (NH) Black and Hispanic children relative to NH White children when only MSEL criteria were used were substantially reduced, though not eliminated, after incorporating VABS-II criteria and adjusting for selected socioeconomic variables. This study provides evidence for the importance of considering adaptive behavior as well as socioeconomic disadvantage when describing racial and ethnic disparities in co-occurring ID in epidemiologic studies of autism. Many studies of children with autism classify children as having intellectual disability (ID) if they have very low scores on intelligence tests, without considering their adaptive functioning, which are the skills needed for activities of daily living, social interaction, and communication. However, impairments in both intellectual and adaptive functioning are required for an ID diagnosis. We found that when both IQ and adaptive functioning are used to define ID, the percent of children with autism classified as having co-occurring ID is reduced from 63% to 38%, and racial and ethnic differences are reduced. This analysis demonstrated the importance of considering adaptive functioning when describing the frequency of ID in children with autism, including for the accurate and equitable characterization of diverse populations.