Certain studies have found that people with ADHD tend to have lower scores on intelligence quotient (IQ) tests. The significance of this is controversial due to the differences between people with ADHD and the difficulty determining the influence of symptoms, such as distractibility, on lower scores rather than intellectual capacity. In studies of ADHD, higher IQs may be over-represented because many studies exclude individuals who have lower IQs despite those with ADHD scoring on average nine points lower on standardised intelligence measures. In individuals with high intelligence, there is increased risk of a missed ADHD diagnosis, possibly because of compensatory strategies in highly intelligent individuals.

Studies of adults suggest that negative differences in intelligence are not meaningful and may be explained by associated health problems.