The Centers for Disease Control and Prevention estimate that 1 in 68 people in the United States has autism spectrum disorder (ASD), but just 10 years ago only 1 in 150 was diagnosed with ASD. Researchers have been investigating the reasons behind the sharp rise in ASD prevalence in recent years. A team of researchers at Penn State have discovered a possible cause: reclassification of individuals who were diagnosed with other intellectual disability disorders.
Data for the study came from the United States Individuals with Disabilities Education Act (IDEA). IDEA classifies special education students into 1 of 13 disability categories. Although students may have co-occurring disorders, they can be classified into only one category. Overall, the researchers evaluated data from 11 years of special education students, which comprised 6.2 million students annually.
Although the number of students diagnosed with ASD increased three-fold from 2000 to 2010, there was not an increase in the total number of students enrolled in special education. Approximately 65 percent of the increase in ASD diagnosis can be accounted for by a corresponding decrease in the number of students classified in the intellectual disability category.
Student age and location were also factors in ASD’s increasing prevalence. Among eight-year-old students, approximately 59 percent of the increase in ASD is due to reclassification. The same is true for around 97 percent of 15-year-olds. Some states had no relationship between ASD and intellectual disability, suggesting that state-specific policies could be a factor in ASD 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. Every patient is different and must be treated as such,” explained lead researchers Santhosh Girirajan, assistant professor of biochemistry and molecular biology and of anthropology at Penn State.
This research is published in the American Journal of Medical Genetics.
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