At each assessment, each twin was interviewed individually by a trained clinical interviewer. Because P3AR has been shown to persist through adolescence and into adulthood (Iacono et al., 2002; Yoon et al., 2013) and we wanted to identify all cases of externalizing psychopathology, the present study constructed diagnostic groups based on lifetime occurrence of an externalizing diagnosis (Hamdi and Iacono, 2014). In other words, those who received a disorder diagnosis at any one of our four assessments were given a lifetime diagnosis for that disorder. Externalizing diagnoses of attention-deficit hyperactivity disorder (ADHD), oppositional defiant disorder (ODD), conduct disorder (CD), adult antisocial behavior (AAB), nicotine dependence (NicD), alcohol dependence (AlcD), and illicit drug dependence (DrgD) were based on criteria from the Diagnostic and Statistical Manual of Mental Disorders, third edition, revised (DSM-III-R; American Psychiatric Association, 1987), the diagnostic standard that was 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.
