To enrich our sample with twins at high risk for the development of substance abuse, we began, in 2000, to recruit ES twin families in which at least one member of the pair was likely to be diagnosed with a childhood disruptive disorder. This sample was ascertained from Minnesota birth records of 2717 like-sex twin pairs born between 1988 and 1994. Of these, 82% (2226 total pairs; 1073 male and 1153 female) were successfully located and, in the year they became 11 years old, were recruited to participate in our study. The overall sampling plan was intended to maximize the number of high-risk twins while achieving our target sample of 500 families from the relevant birth years.
