construct at age 17 (λ = .47) than at age 12 (λ = .15). We note that lifetime symptom counts and substance use measures were used at both time points, dictating that individual scores either remained unchanged or increased over time. Although this may have influenced the factor structure to some degree, the lifetime framework was chosen for two important reasons. First, measures of current or recent (e.g., past year) behavior do not accommodate the developmental fluctuations inherent in these behavioral syndromes (i.e., the fact that certain aspects of externalizing disorders are more salient and prevalent at certain ages). Lifetime scores, in contrast, provide a cumulative record of all symptoms manifest to that point in time and, hence, a more comprehensive and reliable measure of the underlying vulnerability. Second, for individuals who have previously received or are currently undergoing treatment for externalizing problems (e.g., psychoactive medications for ADHD symptoms), a lifetime score captures symptom information that predates treatment or other interventions such as juvenile detention.
