In The Politics of Autism, I discuss prevalence and talk of an "autism epidemic."
Is it vaccines? Air pollution? Infections?
Nope. The reason that the latest numbers for autism prevalence among US children have climbed traces largely to a simple change in how interviewers asked a question.
The US Centers for Disease Control and Prevention last conducted the National Health Interview Survey (NHIS) for the years 2011 through 2013. The 2014 survey, though, included a tweak, and that tweak is the reason that autism prevalence climbed from 1.25% to 2.24% in 2014. Not even the most die-hard causation theorist could argue that in a single year or handful of years, something environmental, like vaccines, caused a near-doubling of autism prevalence in children ages 3 to 17 years.
So what underlies the increase?
For the 2011-2013 survey, parents answered a series of three questions. The first asked if their child had intellectual disability. The second asked if their child had any developmental delay. And the third question listed several conditions, from Down syndrome to sickle cell anemia to autism spectrum disorder (ASD), and parents were asked if their child had been diagnosed with any of them.
But 2014 brought some tweaks, and those tweaks made a difference.
At CDC, Benjamin Zablotsky and colleagues have the article: "Estimated Prevalence of Autism and Other Developmental Disabilities Following Questionnaire Changes in the 2014 National Health Interview Survey." The abstract:
Objectives --The developmental disabilities questions in the 2014 National Health Interview Survey (NHIS) were changed from previous years, including question reordering and a new approach to asking about autism spectrum disorder (ASD). This report examines survey-based estimates of the lfetime prevalence of ASD, intellectual disability (ID), and any other developmental delay (other DD) following the inclusion of a standalone ASD question, the inclusion of specific diagnoses in the ASD question, and the ASD question preceding the other DD question, and compares them with estimates from previous years.
Methods—In NHIS, one child is randomly selected from each family to be the subject of detailed questions on health conditions, functional limitations, and health care utilization. Parents are asked if a doctor or health professional had ever told them that their child had each of a series of developmental disabilities. Prevalence estimates of ASD, ID, and other DD for children aged 3–17 years were calculated using data collected in 2011–2014.
Results—The estimated prevalence of ASD based on 2014 data was 2.24%, a significant increase from the estimated annualized prevalence of 1.25% based on 2011–2013 data. In contrast, the prevalence of other DD declined significantly from 4.84% based on 2011–2013 data to 3.57% based on 2014 data. The prevalence of ID did not significantly change from 2011–2013 (1.27%) to 2014 (1.10%). The prevalence of having any of the three conditions was constant across survey years.
Conclusions—The revised question ordering and new approach to asking about developmental disabilities in the 2014 NHIS likely affected the prevalence estimates of these conditions. In previous years, it is likely that some parents of children diagnosed with ASD reported this developmental disability as other DD instead of, or in addition to, ASD. Following these changes, the 2014 ASD estimate was more similar to ASD prevalence estimates from other sources.