Several weeks ago I argued that much of the the observed increase in autistic disorder over time can be explained by three phenomenon: (1) Diagnostic criteria have changed over some part of the period during which increases have been observed. The diagnostic criteria for autistic disorder were broadened over time. (2) The average age of diagnosis for autistic disorder became younger. (3) The efficiency of ascertainment (the probability that a true case is identified) has increased with greater awareness of the condition, introduction of new treatments and new resources, advocacy, broadening of diagnostic experience, and changes in diagnostic practices.
In another post in May I described a small study from England that “adds to arguments against the view that incidence of autism has increased over recent decades, and suggests that changes in diagnostic criteria are the most likely reason for the rise in the number of cases diagnosed.” I pointed out, however, that this small study was only a first step and we need more studies with larger sample sizes.
In the July 2008 issue of the the Journal of Autism and Developmental Disorders a much larger and more elegant study has been published. I would love to give you a detailed description, but I’m about to go on vacation. So quoting the abstract will have to suffice for now.
By Helen Coo and Hélène Ouellette-Kuntz of the Department of Community Health and Epidemiology, Queens University; Jennifer E. V. Lloyd of the Human Early Learning Partnership (HELP); and three other authors.
SUMMARY: The authors examined trends in assignment of special education codes to British Columbia (BC) school children who had an autism code in at least 1 year between 1996 and 2004, inclusive. The proportion of children with an autism code increased from 12.3/10,000 in 1996 to 43.1/10,000 in 2004; 51.9% of this increase was attributable to children switching from another special education classification to autism (16.0/10,000). Taking into account the reverse situation (children with an autism code switching to another special education category (5.9/10.000)), diagnostic substitution accounted for at least one-third of the increase in autism prevalence over the study period.