Quantitative and molecular genetic studies of attention-deficit hyperactivity disorder in adults

ADHD is a common, highly heritable neurodevelopmental disorder (Asherson, 2004) affecting around 5% of children (Polanczyk et al., 2007) and 2.5% of adults (Fayyad et al., 2007; Simon et al., 2009). The disorder starts in early childhood and is characterized by pervasive inattention, hyperactivity, and impulsivity that are inappropriate to the developmental stage. The fact that the adult outcome of childhood ADHD is not always benign has been known for a long time. An early review of outcome studies of hyperactive children reported that they experience significant academic, social, and conduct difficulties during adolescence and that social, emotional, and impulse problems persist into young adulthood for the majority (Hechtman & Weiss, 1983). The authors concluded that, although some hyperactive children were found to be functioning normally as adults, a troublesome minority were experiencing severe psychiatric or antisocial problems. Despite some reports that ADHD might be a self-limiting condition (Hill & Schoener, 1996) this view has not been supported by more recent evidence, and we now know that ADHD persists into adult life in the majority of cases either as a full-blown condition (around 15% of cases) or in partial remission (around 50% of cases), with persistence of symptoms associated with significant levels of academic, occupational, or social impairment and high levels of psychiatric comorbidity (Faraone, Biederman, & Mick, 2006).