This shows autism identification in schools, not a clinical diagnosis rate. Each value is the number of students aged 3–21 served under the Individuals with Disabilities Education Act (IDEA) whose primary special-education category is autism — the only measure that covers all 50 states every year for 20 years (CDC ADDM covers only a few sites; global modelled estimates aren’t openly available per state). The count tracks the dramatic rise but is dominated by state size; per 1,000 special-education students is population-independent and comparable across states. The rise reflects expanded diagnostic criteria, greater awareness, and ‘diagnostic substitution’ (children once labelled intellectual disability or learning disability now identified as autistic) as much as any true change in occurrence, and states differ in eligibility practices — so cross-state differences are partly policy, not prevalence. Autism became an IDEA reporting category in 1991–92.