Participating families were selected from a population-representative sample identified through birth records of the state of Missouri, for a genetic epidemiological study of the prevalence and heritability of ADHD. The original sample included 812 complete male and female twin pairs and six individual twins ages 7 to 19 years at the time of interview identified from the Missouri Family Registry from 1996 to 2002. As detailed in previous reports, 7, 8 families were invited into the study if at least one child exhibited three or more inattentive symptoms on a brief screening interview. Parents reported on their children and themselves, and the youths on themselves, using the Missouri Assessment of Genetics Interview for Children (MAGIC). 9 The MAGIC is a semi-structured psychiatric interview allowing DSM-III, DSM-IV, and ICD-10 diagnoses, and exhibited excellent reliability and 1-year stability of diagnoses for both parent report and child self-report of the major DSM-IV diagnostic categories pertinent to youths. Adolescents and parents were interviewed in person or by telephone about half the time. All interviews with children 12 years and younger were in person. DSM-IV
