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Among boys, about one-third who exhibit severe attention-deficit/hyperactivity disorder (ADHD) in preschool follow a developmental trajectory to early-onset conduct disorder (CD) in later childhood and adolescence. Moreover, the vast majority of adolescent boys with early-onset CD also meet criteria for ADHD. Although trait impulsivity, a predisposing vulnerability to both ADHD and CD, is about 80% heritable, environmental risk factors play an important role in how impulsivity is expressed, including whether young children with ADHD eventually develop CD. In this article, we (a) describe how environmental risk potentiates early-onset conduct problems among trait-impulsive and therefore vulnerable individuals and (b) outline implications for conceptualizations of externalizing comorbidity. Although other pathways to CD exist, we focus on what is likely to be a common developmental trajectory to this costly …