What's the Deal with Autism Rates?

Summary

One of the major priorities of the new Secretary of Health and Human Services (HHS), Robert F. Kennedy Jr. (RFK Jr.), is to figure out why rates of autism have been increasing over time.But is this a good priority? To determine why autism rates have increased, we first have to know if autism rates have increased. There’s some evidence that speaks to that. For example, diagnosed rates of autism have clearly increased over time across every major data collection effort. If we look at recent snapshot data from one such effort conducted by the California Department of Developmental Services (CDDS), we can see that autism diagnoses have stunningly increased since the 1930s, from applying to 0.001% of those in 1931 to 1.2% of five-year-olds in 2021.Despite the striking and shocking nature of the data on this graph, interpreting it as evidence that autism has increased over time is hard.For starters, the ages in the most recent cohorts are young—they’re children—but in the oldest cohorts, we’re talking about adults whose data wasn’t gathered by the CDDS when they were kids, but instead much later, long after these people had made it to adulthood, when diagnosis is frequently neglected. Autism is also associated with shorter lifespans, which are even shorter for the people with the most severe manifestations of the condition. Accordingly, many of the oldest autistics would have already died off by the time the CDDS got around to surveying. These biases deflate the rate of autism the further back you go, exaggerating the subsequent increase.The other, more pressing issue is that the constellation of traits categorized as “autism” wasn’t named until Leo Kanner’s descriptions of “abnormal behaviour” in the 1940s, and it would be another nearly forty years before American psychiatry provided criteria for autism diagnoses in the third edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-III). Before the DSM-III came out in 1980, autism diagnoses were usually...