Attention deficit hyperactivity disorder (ADHD)-diagnosed individuals are often found to have a number of comorbidities/disorders as well (Barkley, 2006). Kessler et al. (2005) found that adults with ADHD are six times more likely to suffer one or more additional psychiatric disorders in their lifetime than individuals without ADHD. The number is even higher for children drawn from clinics. Around 87% of clinically diagnosed ADHD children may have at least one other disorder and 67% have at least two other disorders (Kadesjö and Gillberg, 2001).

Despite being the most commonly studied and diagnosed psychiatric disorder in children and adolescents, the cause in the majority of cases is unknown. It affects about 6-7% of children when diagnosed by the Diagnostic and statistical manual of mental disorders, 4th ed. (DSM-IV) criteria (Willcutt, 2012) and 1-2% when diagnosed by the ICD-10 criteria (Cowen et al., 2012). The rates are similar between countries and depend mostly on how it is diagnosed (Tsuang et al., 2011). ADHD is diagnosed approximately three times more frequent in boys than in girls (Singh, 2008; Emond et al., 2009). About 30-50% of people diagnosed in childhood continue to have symptoms into adulthood (Bálint et al., 2008) and between 2 and 5% of adults have the condition (Kooij et al., 2010). The condition can be difficult to tell apart from other disorders as well as that of high normal activity (Dulcan and Lake, 2011).