Attention Deficit Hyperactivity Disorder (ADHD) is a common childhood neurobehavioural disorder defined by symptoms of developmentally inappropriate inattention, impulsivity and hyperactivity. A recent meta-analysis estimated the worldwide prevalence of ADHD to be 5.29% 1, making it the most prevalent psychiatric disorder of childhood. The social and economic costs of childhood ADHD are considerable 2, and difficulties often persist into adulthood. Children with ADHD are at high risk for developing adjustment problems, antisocial behaviour, substance abuse, other psychiatric disorders, and difficulties in education and work 3. Although the exact aetiology of ADHD has not been determined, the related factors include familial and hereditary factors, prenatal or perinatal factors, chemotoxic factors, sociopsychological stress, structural and functional abnormalities of the brain, and developmental neurobiological factors in the regions of the brain related to ADHD 4. Family, twin, and adoption studies provide overwhelming evidence for an inherited contribution to the pathogenesis of ADHD, with heritability estimated to be about 70% 5.