We created a composite measure of externalizing behaviors using the first principal component extracted from a principal components analysis. In COGA, we used DSM-IV clinical criterion counts (American Psychiatric Association, 1994) for five externalizing disorders: (1) alcohol dependence, (2) alcohol abuse, (3) illicit drug dependence (cocaine, marijuana, sedatives, stimulants, opioids, and other drugs), (4) illicit drug abuse, and (5) antisocial behavior measured as either DSM-IV antisocial personality disorder or DSM-III-R conduct disorder criteria (American Psychiatric Association, 1987) depending on age. For those younger than 18, we used conduct disorder (CD) criteria as our index of antisocial behavior, as ASPD is only assessed in those 18 years and older. We omitted other disorders often included on the externalizing spectrum, including nicotine dependence and attention-deficit hyperactivity disorder (ADHD) because these have weaker genetic overlap with the above phenotypes (Young et al., 2002). In S4S, we used both clinical and non-clinical phenotypes meant to match the phenotypes used in COGA. Indicators included: (1) alcohol dependence, (2) alcohol abuse, (3) high school antisocial behavior, (4) college antisocial behavior, and (5) a count of illicit
