California Autism Prevalence Trends from 1931 to 2014 and Comparison to National ASD Data from IDEA and ADDM

Cynthia Nevison, Mark Blaxill and Walter Zahorodny | Journal of Autism and Developmental Disorders | Published 05 July 2018

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The evidence supporting an increasing rate of autism in the U.K. and the U.S. has gathered strength. Although both the nomenclature and the criteria set used to define autism have changed over the years, these changes are not so great as to prevent comparative analysis and do not explain major differences in reported prevalence over time. The largest stable source of variability in reported autism rates comes from incomplete ascertainment in young age cohorts, which limits the ability to detect an underlying and rising secular trend. Reviews that have downplayed the rising trend have overemphasized unimportant methodological problems, employed flawed meta-analytic methods, and failed to take into account the most relevant biases in survey methodologies. Point prevalence comparisons made within and across surveys conducted in specific geographic areas, using year of birth as a reference for trend assessment, provide the best basis for inferring disease frequency trends from multiple surveys. A comparison of U.K. and U.S. surveys, taking into consideration changing definitions, ascertainment bias, and case-finding methods, provides strong support for a conclusion of rising disease frequency. The rate of autism in the U.S., once reported as 3 per 10,000, has now risen to 30 per 10,000, a 10-fold increase. The rate of autism in the U.K., once reported as 10 per 10,000, has risen to roughly 30 per 10,000. Reported rates for ASDs in both countries have risen from the 5 to 10 per 10,000 range to the 50 to 80 per 10,000 range. This review has found little evidence that systematic changes in survey methods can explain these increases, although better ascertainment may still account for part of the observed changes. A precautionary approach therefore suggests that increased rates of autism and related disorders be accepted as an urgent public health concern

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