structured interviews, and children with comorbid ASD, epilepsy, ID (IQ < 70), and other conditions were excluded. As such, the false positive rate for an ADHD diagnosis is likely higher in the iPSYCH and Swedish register-based datasets than in the PGC studies, while the latter is likely underrepresented for individuals with neurodevelopmental comorbidities. Another major difference is that many of the PGC studies used DSM criteria and thus included children with inattentive and hyperactive-impulsive subtypes of ADHD, whereas the Swedish and iPSYCH studies (and some European PGC studies) used the stricter International Classification of Diseases definition. Despite these differences, we find very high genetic correlation between PGC and iPSYCH, suggesting that overall these diagnostic differences have no perceptible impact on the involvement of common risk variants. PGC-only and iPSYCH-only sex-specific analysis results were similar, with the caveat that the PGC study was smaller and results had larger confidence intervals.
