Prevalence of Autism Spectrum Disorders in Children: A Review of International Population-Based Studies

Background: Prevalence estimates from well-designed epidemiologic studies help inform public health officials and policy makers as to the necessary allocation of resources for treatment and intervention programs for families affected by autism spectrum disorders (ASDs).  In addition, secular trends in autism prevalence across nations with distinct racial/ethnic and environmental backgrounds can provide an alternative descriptive epidemiologic framework for formulating testable questions about heritable and environmental risk factors.  Measuring ASD incidence can be problematic because the actual onset of an ASD is not necessarily related to the time of identification, so changes in identification patterns would influence trends in age-specific incidence rates.  Prevalence estimates are based on new and existing cases up to a defined age.  Therefore, looking at prevalence of ASDs in older children could potentially control for some of the challenges related to only focusing on incidence.  For this reason, this review focuses on prevalence rather than incidence.  By doing so, we hope to diminish any confounding that might be due to timing of identification in order to provide comparable numbers across studies worldwide to inform the ASD distribution.

Objectives:  This study aims to review the most up-to-date reports on prevalence estimates of autistic disorder (AD) and ASDs in children from population-based studies in different continents and regions of the globe.

Methods: We reviewed 45 international population-based studies published from 1997 to 2011 on the prevalence of AD and ASDs in children.  To minimize the diversities in diagnostic criteria and case ascertainment, only population-based studies that adopted DSM-IV, ICD-10, and/or comparable diagnostic systems (e.g., ADI-R and ADOS) were included.  Point estimates and 95% CIs of prevalence rates were calculated and graphed by continent, ordered by median birth year.

Results: Overall, the medians of reported AD prevalence estimates, per 1,000, in the reviewed studies are 1.6 and 1.8 in North America, 1.4 in Scandinavia, 1.7 in Europe, 3.0 in Asia, and 3.2 in Australia.  Median ASD prevalence per 1,000 are 6.6 in North America, 4.0 in Scandinavia, 4.4 and 5.7 in Europe, 6.3 and 8.6 in Asia, and 4.1 in Australia.  For both AD and ASDs, higher prevalence was reported in more recent birth cohorts, and in older children.

Conclusions: Diagnostic criteria for AD are considered to be more akin to “classic” autism than are the criteria for other ASDs and are similar to the diagnostic criteria of earlier versions of the DSM and ICD.  This suggests that the increase of AD, and likely ASD, prevalence internationally over time may reflect the influence of factors in addition to the change of autism diagnosis, such as development of available services, professional and public awareness, diagnostic practice, and early identification.

The findings and conclusions in this report are those of the authors and do not necessarily represent the official position of the CDC.