current at the time of participant intake assessment. To simplify grouping, DrgD was collapsed into one group that included a substance dependence diagnosis for any of the following psychoactive substance classes: amphetamines, cannabis, cocaine, hallucinogens, inhalants, opioids, phencyclidine, and sedatives. Childhood disruptive disorders (CDDs; ADHD, ODD, CD) were only assessed at the twins’ intake assessment whereby twins and mothers (asked in pertinence to the twins’ behavior) reported symptoms for these disorders in accordance with the Diagnostic Interview for Children and Adolescents (Reich, 2000; Welner et al., 1987) child- and parent-versions, respectively. To ensure diagnostic certainty, a “best-estimate” approach combining twin and mother reports was adopted (Kosten and Rounsaville, 1992; Leckman et al., 1982). All other disorders (AAB, NicD, AlcD, DrgD) were assessed at all four visits. AAB was diagnosed if an individual met DSM-III-R criteria for antisocial personality disorder but did not necessarily qualify the presence of childhood history of CD (cf. Elkins et al., 1997). The presence of SUDs (NicD, AlcD, DrgD) was assessed using the expanded substance abuse module (Robins et al., 1988) as they pertained to lifetime (intake and second visits) and since-last-visit (third and fourth visits) experience. Clinical reports were independently reviewed by at least two
