ABSTRACT
Attention Deficit Hyperactivity Disorder (ADHD) is considered the most prevalent mental health disorder of childhood. Its prevalence depends on the diagnostic criteria and procedures used for assessment, and variations in classification methods have resulted in a 20-fold difference in reports in the literature. A large cross-national difference in administrative prevalence exists, but when the same criteria and methods are used, similar values for epidemiological prevalence have been reported. Prospective follow-up studies have documented poor outcome in education, social adjustment , and other important areas of functioning in adolescence and adulthood. Biological bases for ADHD have been proposed. One of the earliest and most prominent of these being the dopamine (DA) hypothesis of ADHD. The largest population-based twin cohort studied to date consisted of 1938 families with twin and siblings aged 4 to 12 years. The phenotype of ADHD is very complex and the genetics will probably prove to be likewise.
