California Autism Prevalence Trends from 1931 to 2014 and Comparison to National ASD Data from IDEA and ADDM.
Cynthia Nevison, Mark Blaxill, Walter Zahorodny
PMID 29974300WHAT IT FOUND
California autism service records show prevalence rising from 0.001% in 1931 to 1.18% in 2012.
Two independent statistical methods agree that roughly 87% of this recent increase represents a true rise in cases, not just better diagnosis or expanded criteria.
Key findings
01California Department of Developmental Services (CDDS) autism prevalence increased from approximately 0.001% in the 1931 birth cohort to 1.18% in the 2012 birth cohort.
02Comparing two statistical methods (age-resolved snapshot and constant-age tracking) yields a slope ratio of 0.87, suggesting that about 87% of the recent increase in autism prevalence is due to a true rise in the condition rather than diagnostic changes.
03Prevalence estimates vary significantly by data source, with CDDS (which requires severe functional disability) showing lower rates than IDEA and ADDM networks, which include milder cases.
STILL TO COME
How it was doneWhat they found
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What it does not show
The study relies entirely on administrative data, which may contain errors or inconsistencies in how diagnoses were recorded over time. CDDS data only include individuals with severe functional disabilities, so the findings may not apply to the broader autism spectrum. Changes in diagnostic criteria (DSM-IV to DSM-5) and coding definitions (Code 1 changes) complicate the interpretation of trends, especially in recent years. The analysis assumes that prevalence trends are linear over the study periods, which may not be true given the accelerating rate of increase.
Declared interests
No conflicts of interest or funding sources are declared in the provided text.
The easy way to misread this
Do not interpret the 87% 'true rise' estimate as a precise biological fact. This figure is derived from a statistical comparison of two modeling methods applied to administrative data, and it assumes that diagnostic changes affect all birth cohorts equally. The actual proportion of the increase due to true incidence versus diagnostic expansion remains uncertain and varies by region and data source.