Author's personal copy Genetics of Attention Deficit Hyperactivity Disorder

Attention deficit hyperactivity disorder (ADHD) is among the most com-mon childhood onset psychiatric disorders. The worldwide prevalence of ADHD in children is 8 % to 12 % [1], and the prevalence of ADHD in adults in the United States was estimated to be 4 % in the National Comorbidity Survey [2]. Early studies found the risk of ADHD among parents of children with ADHD to be increased by between twofold and eightfold, with simi-larly elevated risk among the siblings of ADHD subjects (for a review of this literature see [3]). Faraone and colleagues [4] recently extended these findings to families ascertained via adult probands meeting criteria for either full ADHD, as defined by the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM-IV), or late-onset ADHD. However, adoption and twin studies are necessary to disentangle genetic from environmental sources of transmission observed in family studies. Three studies found that biologic relatives of ADHD [5] or hyperactive chil-dren [6,7] were more likely to have hyperactivity than adoptive relatives. A more direct method of examining the heritability of ADHD is to study twins: the extent to which monozygotic twins are more concordant for ADHD than dizygotic twins can be used to compute the degree to whic