Objective: To compare the characteristics of children with attention-deficit hyperactivity disorder (ADHD) who have high intelligence quotient (IQ) versus normal and low IQ through long-term follow-up of children with ADHD from a population-based birth cohort.
Methods: Subjects included children with research-identified ADHD (N = 379) from a birth cohort (N = 5718). Full scale IQ scores obtained between ages 6 and 18 years were used to categorize children into 3 groups: Low (IQ < 80), Normal (80 ≤ IQ < 120), and High IQ (IQ ≥ 120). Subjects were retrospectively followed up from birth until emigration, death, or high school graduation/dropout. The groups were compared on demographic characteristics, age at which ADHD case criteria were met, comorbidities, treatment, and school outcomes.
Results: There were no significant differences among children with high (N = 34), normal (N = 276), or low IQ (N = 21) and ADHD in numerous characteristics, including median age at which ADHD criteria were fulfilled (9.5, 9.7, and 9.8 years); rates of comorbid learning disorders (85.3%, 78.3%, and 76.2%), psychiatric disorders (47.1%, 50.4%, and 47.6%), and substance abuse (17.6%, 23.6%, and 19.0%); and rates of stimulant treatment (79%, 75%, and 90%). In comparison to children with normal or low IQ, those with high IQ had mothers with higher educational levels (e.g., college graduation rates 44.1%, 11.6%, and 14.3%), and higher reading achievement (median national percentiles on standardized reading tests 77.0, 42.0, and 29.0, p < 0.001).
Conclusions: These findings suggest that ADHD is similar among children with high, normal, and low IQ, although high IQ may favorably mediate some outcomes such as reading achievement. Diagnosis and treatment of ADHD are important for all children, regardless of cognitive ability.