The definition change was predicted to shrink the count

In 2013 DSM-5 merged four previously separate diagnoses - autistic disorder, Asperger's disorder, childhood disintegrative disorder, and pervasive developmental disorder not otherwise specified - into a single autism spectrum disorder, with two symptom domains and three severity levels. The expectation in the literature ran in one direction, and that direction was downward.

Significant findings

What was predicted: a 2014 systematic review and meta-analysis pooled the pre-publication studies and projected DSM-5 would decrease ASD diagnoses by 31% (CI 20-44, p=0.006), with the PDD-NOS subgroup down 70%. Projected onto surveillance records the same year, applying DSM-5 criteria to CDC's abstracted case files suggested prevalence would fall from 11.3 to 10.0 per 1,000, with 81.2% of DSM-IV-TR surveillance cases still meeting the new criteria.

What was measured: the same group's five-year follow-up, covering 33 studies, found a pooled decrease of 20.8% (16.0-26.7, p<0.001), smaller than predicted. Autistic disorder fell 10.1% and Asperger's 23.3%, while the PDD-NOS decrease was not statistically significant. In clinic samples, DSM-5 criteria identified 91% of children who already carried DSM-IV diagnoses. That retention, together with DSM-5's own instruction that well-established DSM-IV diagnoses be carried across, is part of why the shrinkage stayed small.

And the surveillance series itself kept rising through 2013 with no downward step. The predicted drop never appeared in the counts. Two claims that record supports: DSM-5 merged four diagnoses into one, and the predicted shrinkage failed to materialize in surveillance data. One claim it does not support is that DSM-5 caused the rise - the rise long predates 2013, and DSM-5's measured fingerprint in this literature is a decrease, smaller than predicted but still a decrease. The annotated timeline sits at www.resolv.social/topics/how-autism-got-counted.

Worth asking

When a diagnostic definition changes, ask what the change was predicted to do before assuming what it did. Ask whether anyone measured it afterwards, and whether the measured effect points the same direction as the trend it is being used to explain. Here it points the opposite way, which is exactly why the prediction is the interesting part - and why a definition change is a poor explanation for a rise that began decades before it.