The ES recruitment strategy included random assignment to one of two samples. Seventy-six percent (1697 total pairs; 855 male and 842 female) of the twin pairs were randomized to the screened sample. In this sample, a parent (usually the mother) was contacted by phone and interviewed regarding symptoms of ADHD and CD as well as indications of academic disengagement (e.g., “Lacks interest in school work”) in each twin. ADHD symptoms and academic disengagement items were assigned weights of one; CD symptoms were assigned weights of three. The entire set of ES screening items is provided in Table 1. Only those pairs in which at least one member met or exceeded a threshold of five were invited to participate in the study (e.g., the threshold could be attained by having 5 symptoms of ADHD or 1 symptom of CD plus several symptoms of ADHD). This threshold was chosen, based on an analysis of data already collected from our existing 11-year-old cohort, to maximize both the sensitivity and specificity for predicting a twin having either a diagnosis of ADHD at age 11 or a diagnosis of CD by the age-14 follow-up.
