"This study is the first to measure the incidence -- the occurrence of new cases -- of autism by applying consistent, contemporary criteria for autism to a specific population over a long period of time," says William Barbaresi, M.D., a Mayo Clinic developmental pediatrician and one of the study authors. "In doing so, the study accounts for improvements in the diagnostic criteria for autism, the medical community's improved understanding of this disease and changes in federal special education laws."
The study found that the increase in the incidence of autism in Olmsted County, Minn. coincided with broadening of the diagnostic criteria for autism and new federal special education laws including autism as a disability category. Both events occurred many years after immunizations were mandated for school entry. Broader, more precise diagnostic criteria for autism were introduced in 1987. Prior to these new criteria, children with autism may have been given less precise diagnoses such as "developmental delay" or "mental retardation," and children with milder symptoms of autism may not have been identified at all. The 1991 federal special education laws improved the availability of educational services for children with autism.
The study used data from the Rochester Epidemiology Project, a database of all inpatient and outpatient records in Olmsted County, Minn. The database diagnoses are indexed for computerized retrieval, allowing researchers to identify subjects with any developmental disorder. Researchers found
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