This study was approved by Wales and northwest England multicentre research ethics committees. 410 children thought to have ADHD were consecutively recruited from 90 community child psychiatry and paediatric outpatient clinics across the UK between 2001 and 2009. All were of white UK origin (including parents and grandparents), aged 5–17 years, and met Diagnostic and Statistical Manual of Mental Disorders (DSM)-IV or International Classification of Diseases (ICD)-10 criteria for ADHD or hyperkinetic disorder. Children with schizophrenia, Tourette's syndrome, autistic spectrum disorder, or a neurological disorder including epilepsy were excluded. Diagnoses were confirmed with the Child and Adolescent Psychiatric Assessment,13 a research diagnostic interview. Interviews were undertaken by trained psychologists who were supervised weekly by a child psychiatrist. Inter-rater reliability for a diagnosis of ADHD was perfect (κ=1·0). Symptoms and impairment at school are a diagnostic requirement of the disorder, and we confirmed the presence of these criteria by interviewing the children's teachers using the Child ADHD Teacher Telephone Interview.16 Of the 410 children recruited to the study, 375 were single cases, and 35 were known to have an affected sibling
