The researchers found that between 2000 and 2010, the number of children in the autism category more than tripled from 93,624 in 2000 to 419,647 a decade later. Yet nearly 65 percent of this increase was significantly associated with a corresponding decline in the number of students classified under the intellectual disability category in the IDEA data. The researchers estimate that, for 8 year-olds, approximately 59 percent of the observed increase in autism is accounted for by reclassification, but by age 15, reclassification accounts for as much as 97 percent of the increase in autism. Likewise, the researchers found the category of specific learning disability as another potential contributor to diagnostic reclassification. They also note that the relationship between autism cases and disability categories varied state-by-state, suggesting that state-specific health policy may be a significant factor in estimates of autism prevalence.
"Because features of neurodevelopmental disorders co-occur at such a high rate and there is so much individual variation in autism, diagnosis is greatly complicated, which affects the perceived prevalence of autism and related disorders," said Santhosh Girirajan, assistant professor of biochemistry and molecular biology and of anthropology at Penn State and the leader of the research team. "For quite some time, researchers have been struggling to sort disorders into categories based on observable clinical features, but it gets complicated with autism because every individual can show a different combination of features. The tricky part is how to deal with individuals who have multiple diagnoses, because the set of features that define autism is commonly found in individuals with other cognitive or neurological deficits."
Polyak A, Kubina RM, Girirajan S. 2015. Comorbidity of intellectual disability confounds ascertainment of autism: implications for genetic diagnosis. Am J Med Genet Part B 9999:1–9.
Lee A. Wilkinson, PhD, NCSP is a licensed and nationally certified school psychologist, registered psychologist, and certified cognitive-behavioral therapist. He provides consultation services and best practice guidance to school systems, agencies, advocacy groups, and professionals on a wide variety of topics related to children and youth with autism spectrum disorders. Dr. Wilkinson is author of the award-winning books, A Best Practice Guide to Assessment and Intervention for Autism and Asperger Syndrome in Schools and Overcoming Anxiety and Depression on the Autism Spectrum: A Self-Help Guide Using CBT. He is also editor of a best-selling text in the APA School Psychology Book Series, Autism Spectrum Disorder in Children and Adolescents: Evidence-Based Assessment and Intervention in Schools. His latest book is A Best Practice Guide to Assessment and Intervention for Autism Spectrum Disorder in Schools (2nd Edition).