There is considerable epidemiological and neuropsychological evidence that attention deficit hyperactivity disorder (ADHD) is best considered dimensionally, lying at the extreme end of a continuous distribution of symptoms and underlying cognitive impairments. The authors investigated whether cortical brain development in typically developing children with symptoms of hyperactivity and impulsivity resembles that found in the syndrome of ADHD. Specifically, they examined whether a slower rate of cortical thinning during late childhood and adolescence, which they previously found in ADHD, is also linked to the severity of symptoms of hyperactivity and impulsivity in typically developing children.