Attention-deficit/hyperactivity disorder (ADHD) is a common neuropsychiatric disorder. The prevalence of the disorder in children is estimated at 5.3%, estimates for adults vary from 1 to 4% (Faraone et al. 2000b; Kessler et al. 2006; Kooij et al. 2005; Polanczyk and Rohde 2007). A large number of studies, including family, adoption and twin studies suggest that ADHD is among the most heritable of neuropsychiatric disorders: ADHD was found to segregate in families with different studies finding risk increases of two to eightfold for parents and siblings of affected children (reviewed by Faraone and Doyle 2001). Adoption studies showed that the biological parents of hyperactive children carry a higher risk for ADHD compared to adoptive parents (Cantwell 1975; Morrison and Stewart 1973), and that first-degree adoptive relatives of probands with ADHD have a lower disease risk than the first-degree biological relatives of non-adopted ADHD probands (Sprich et al. 2000). More than 20 twin studies have been published in the last 32 years, most reporting estimates of heritability for ADHD between 60 and nearly 100%, with a mean of 76% (Faraone et al. 2005; Haberstick et al. 2008; Heiser et al. 2006; McLoughlin et al. 2007; Schultz et al. 2006).
