We examine U.S. and Canadian children with symptoms of Attention Deficit Hyperactivity Disorder (ADHD), the most common child mental health problem. ADHD increases the probability of delinquency and grade repetition, reduces future reading and mathematics scores, and increases the probability of special education. The estimated effects are remarkably similar in the two countries, and are robust to many specification changes.Moreover, even moderate symptoms have large negative effects relative

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... the effects of poor physical health. The probability of treatment increases with income in the U.S., but not in Canada. However, in models of outcomes, interactions between income and ADHD scores are statistically insignificant in the U.S. (except for delinquency), while in Canada these interactions indicate that higher income is protective. The U.S. results are consistent with a growing psychological literature which suggests that conventional treatments for ADHD improve behavior, but have inconsistent effects on cognitive performance.