Early onset aggression precipitates a cascade of risk factors, increasing the probability of a range of externalizing and internalizing psychopathological outcomes. Unfortunately, decades of research on the etiological contributions to the manifestation of aggression have failed to yield identification of any risk factors determined to be either necessary or sufficient, likely attributable to etiological heterogeneity within the construct of aggression. Differential pathways of etiological risk are not easily discerned at the behavioral or self-report level, particularly in young children, requiring multilevel analysis of risk pathways. This study focuses on three domains of risk to examine the heterogeneity in 207 urban kindergarten children with high levels of aggression: cognitive processing, socioemotional competence and emotion processing, and family context. The results indicate that 90% of children in the high aggression group could be characterized as either low in verbal ability or high in physiological arousal (resting skin conductance). Children characterized as low verbal, high arousal, or both differed in social and emotional competence, physiological reactivity to emotion, and aspects of family-based contextual risk. The implications of this etiologic heterogeneity of aggression are discussed in terms of assessment and treatment.