Although the focus of this paper is on possible genetic sources of influence on ADHD sex bias, other factors, such as ascertainment and diagnostic biases, may play an important role. Female individuals are more likely to be diagnosed with the predominantly inattentive subtype of ADHD, whereas male individuals are more likely to be diagnosed with the combined subtype of ADHD and present with hyperactive-impulsive and disruptive behavioral problems 46, 47, 48, 49. Relative prevalence rates also vary by diagnostic instrument used and case ascertainment. For example, the ratio of male:female cases in the Swedish population was 2:1, somewhat lower than that in the iPSYCH Danish population (2.8:1) and PGC clinical data (3.5:1). Individuals with moderate to severe ID (IQ < 50) were excluded from iPSYCH. ADHD cases in the PGC studies were primarily ascertained from clinics; ADHD was confirmed with 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
