ADHD may be thought of as a group of conditions with similar behavioral symptoms consequent to different etiologies.2 ADHD is defined as a persistent pattern of behavior including hyperactivity and impulsivity, and/or inattention that is more severe and frequent than is usually seen in an individual at the same stage of development. The Diagnostic and Statistical Manual of Mental Disorders, 4th Edition (DSM-IV) defines three subtypes, ie, a combined type which includes symptoms of inattention and hyperactivity or impulsivity, a predominantly inattentive type, in which hyperactive and impulsive symptoms are minor, and a predominantly hyperactive-impulsive type, in which inattentive symptoms are minor.3 The subtypes may be discontinued in DSM-5.4 The mean intelligence quotient (IQ) score and IQ distribution in the ADHD population are similar to that in the general population.2,5 ADHD is a neurobiologic disorder with genetic and environmental origins. Family, adoption, and twin studies demonstrate a strong genetic contribution to ADHD, which involves polymorphisms in a number of genes, such as those which code for dopamine transporters. Environmental risk factors for ADHD include antenatal exposure to toxins, such as
