A quarter to two-thirds of the rise is changed counting

"How much of the rise is real?" has a peer-reviewed literature, and it almost never gets quoted with its confidence intervals attached. Two studies carry the numbers cited most often, and they measure different mechanisms in different countries.

Significant findings

Hansen and colleagues used Danish national registries and a population-based birth-cohort design - every child born in Denmark from 1980 through 1991, n=677,915, followed until diagnosis, death, emigration or the end of 2011 - and modelled two specific reporting changes as time-dependent covariates: the 1994 change in diagnostic criteria and the 1995 inclusion of outpatient contacts. The criteria change alone explained 33% of the increase in reported prevalence (CI 0-70), outpatient inclusion alone 42% (CI 14-69), and the two together 60% (CI 33-87). The authors call that "most" of the increase, and conclude the apparent rise is "in large part attributable to changes in reporting practices."

The lower anchor comes from California. King and Bearman examined 7,003 case records from the state developmental-services system, born before 1987 and enrolled between 1992 and 2005, and found that 26.4% (CI 16.25-36.48) of the increased caseload was attributable to diagnostic change through a single pathway - children previously diagnosed with intellectual disability who later acquired an autism diagnosis. Because it traces one pathway, it is a floor for diagnostic-change effects rather than a total.

What the range does not license is either confident story. No peer-reviewed decomposition attributes the entire rise to ascertainment. The honest summary is that every major quantified decomposition leaves an unexplained residual; the artifactual share runs from roughly a quarter to roughly two-thirds depending on country and mechanism; and the one study that came closest to explaining the whole increase by changed detection stated explicitly that a true increase could not be ruled out. The full table, with each study's scope beside its number, sits at www.resolv.social/topics/how-autism-got-counted.

Worth asking

Ask any percentage in this area three questions: what population, what mechanism, and what confidence interval. A figure quoted bare - "60% of autism is just better diagnosis" - has usually lost the country, the birth cohorts, the two specific reporting changes and the interval running from 33 to 87. And notice that both of the confident positions fail against this record: neither "it is all counting" nor "the rise is real and unexplained" survives contact with the studies that actually measured it.