A common assumption is that a violent environment produces violent youth; this project interrogated this assertion in two ways examining if exposure to intergroup antisocial behavior increases youth aggression, and in turn, if changes in general youth aggression are related to participation in intergroup conflict. Improving on past work, the current study utilized four waves of a prospective, longitudinal dataset of mother/child dyads (N=820; 51% female; ages 10 to 20 years old) in Northern Ireland. Hierarchical linear modeling (HLM) addressed new questions about inter-individual differences in intra-individual change in a setting of protracted political conflict. Although boys were higher than girls in initial aggression, there were no significant gender differences in the average trajectory or linear change in aggression from 10 to 20 years old. As a risk factor, experience with sectarian antisocial behavior predicted greater aggression problems; however, that effect weakened with age and was buffered by a cohesive family environment. Regarding the continuation of intergroup conflict, being female and having a more cohesive family negatively predicted youth participation in sectarian acts, whereas the trajectory of general aggression (i.e., intercepts and linear slopes) predicted significantly more youth engagement in out-group antisocial behavior. On an individual level, the findings identify ways the family environment serves to protect youth from greater aggression and from engaging in out-group hostility; at a societal level, the project suggests multiple ways to decrease the potential for youth mobilization in protracted conflict.