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Rates of autism spectrum disorder (ASD) have been increasing for reasons that are not fully understood. To examine this increase, investigators used data from the Danish Psychiatric Registry.
This registry is considered highly reliable because only child psychiatry specialists can diagnose ASD in Denmark. Also, the investigators were able to examine the effects of two changes in the registry: the change from the 8th edition to the 10th edition of the International Classification of Diseases in 1994 and the addition of outpatient diagnoses in 1995. Participants included birth cohorts between 1980 and 1991 (N=677,915); follow-up continued through December 2011 (3956 participants with ASD diagnoses).
To estimate prevalence changes, the researche…