Definition of Attention deficit hyperactivity disorder (ADHD):

ADHD is a problem with inattentiveness, over-activity, impulsivity, or a combination. For these problems to be diagnosed as ADHD, they must be out of the normal range for a child's age and development.

Causes, incidence, and risk factors:

ADHD is the most commonly diagnosed behavioral disorder of childhood. It affects about 3 - 5% of school aged children. ADHD is diagnosed much more often in boys than in girls.

ADHD may run in families, but it is not clear exactly what causes it. Whatever the cause may be, it seems to be set in motion early in life as the brain is developing. Imaging studies suggest that the brains of children with ADHD are different from those of other children.

Depression, lack of sleep, learning disabilities, tic disorders, and behavior problems may be confused with, or appear with, ADHD. Every child suspected of having ADHD should be carefully examined by a doctor to rule out possible other conditions or reasons for the behavior.

Most children with ADHD also have at least one other developmental or behavioral problem. They may also have a psychiatric problem, such as depression or bipolar disorder.