Attention-Deficit/Hyperactivity Disorder, Trait Impulsivity, and Externalizing Behavior in a Longitudinal Sample.

Attention-deficit/hyperactivity disorder (ADHD) is highly comorbid with and predictive of externalizing behavior, yet is most often examined categorically, not dimensionally. We tested a recently proposed trait impulsivity model by dimensionally examining measures of childhood inattention and hyperactivity/impulsivity separately as predictors of later externalizing behavior in an all-female longitudinal sample of 228 young women. We also examined influences of parenting and peer relations, given the transactional nature and importance of environmental factors. We analyzed the relative contribution of hyperactive/impulsive (HI) and inattentive (IA) symptoms of girls with and without childhood-diagnosed ADHD (M age = 9.5; 140 ADHD and 88 Comparison) to the development of externalizing behaviors in adolescence (M age = 14.2) and early adulthood (M age = 19.6). Authoritarian parenting was examined as a moderator and adolescent externalizing behavior as a mediator of the relation between childhood HI and later externalizing behavior. Childhood HI symptoms significantly predicted multiple externalizing behaviors in adolescence and early adulthood, after accounting for IA and covariates (ΔR 2 ranged from 2.6 to 7.5 %). Mother's authoritarian parenting moderated this relation. Adolescent externalizing behavior mediated the relation between childhood HI symptoms and early adult externalizing behavior. In no case did childhood IA significantly predict externalizing behavior after accounting for HI symptoms. Findings support a trait impulsivity model, as HI symptoms, but not IA symptoms, significantly predicted later externalizing behavior. Results support the importance of dimensional predictors of developmental trajectories. We discuss implications for assessment, intervention, and future research.