Trends in the Prevalence of Intellectual Disability and Autism Spectrum Disorder

Background: Changes in the diagnosis and treatment of just one psychiatric condition can have an important effect on the diagnosis and treatment of others. This presentation provides a historical review of the epidemiology of intellectual disability (ID) and autism (ASD) in relation to each other.

Objectives: We discuss possible explanations for declines in ID prevalence include the expansion of proven interventions for the prevention of ID, such as newborn screening, early interventions to boost intellectual functioning, and numerous other public health interventions. Declines in the prevalence of ID could also be related to gradual, population-level improvements in intellectual functioning over time.

Methods: Evidence from around the globe, primarily from developed countries, suggests that the recent rises in the prevalence of autism spectrum disorder (ASD) have corresponded with declines in the prevalence of intellectual disability (ID), and with declines in the proportion of ASD cases with co-occurring ID.

Results: Possible explanations for declines in ID prevalence include the expansion of proven interventions for the prevention of ID, such as newborn screening, early interventions to boost intellectual functioning, and numerous other public health interventions. Declines in the prevalence of ID could also be related to gradual, population-level improvements in intellectual functioning over time. Other developments that could contribute to both the decline in ID and rise in ASD prevalence include: improvements in the precision of diagnostic assessment tools, training of professionals and general awareness of ASD; the expansion of diagnostic criteria for ASD; less stigma and better access to services associated with ASD relative to ID; the effectiveness of early autism interventions in raising IQ, but without necessarily eliminating ASD; and possible selective effects of the information age and technology on brain development and behavior. In sum, it is difficult to understand either one of these conditions without understanding the other.  An evaluation of trends in the prevalence of ID and ASD challenges the exceptionality of ASD as a condition isolated from the broader history of childhood onset disabilities and suggests a number of important implications for future research and policies related to intellectual development and ASD across the lifespan and the globe.

Conclusions: An evaluation of trends in the prevalence of ID and ASD challenges the exceptionality of ASD as a condition isolated from the broader history of childhood onset disabilities and suggests a number of important implications for future research and policies related to intellectual development and ASD across the lifespan and the globe.