Early childhood aggression is a ubiquitous and developmentally normal behavior; however, different report sources (e.g., child self-report vs. teacher) often yield markedly different interpretations. The current study examined how typical demographic and cognitive factors that have been previously found to explain child behavior (e.g., SES, inhibitory control) differentially explain child aggression and prosociality as a function of the informant. Preschool-age children (N = 121) completed several cognitive tasks (PPVT, Theory of Mind battery, Day/Night Stroop, moral judgment) as well as a dual-puppet self-report behavior interview. Parents reported demographics. Teachers completed the Preschool Proactive and Reactive Aggression-Teacher Report. As expected, teachers’ and children’s reports differed considerably and corresponded with different aspects of child development—teachers’ reports of aggression were largely explained by demographic variables (i.e., fathers’ education level), whereas children’s self-reports of aggression were explained by cognitive maturation (i.e., PPVT and moral understanding). These findings support the continued position that studies of early childhood psychopathology should employ multiple mixed methods of reporting, and considers the strengths and weaknesses of each approach. (author abstract)