Adolescent aggression was explored in relation to neighborhood and genetic characteristics. Child saturation (the proportion of the population consisting of children under the age of 15), ethnic heterogeneity, poverty, and urbanicity of neighborhoods were examined in relation to adolescent aggression in 12,098 adolescents followed longitudinally for 1 year. Longitudinal analyses indicated that child saturation was positively associated with increases in aggression, with this finding emerging among those living in the same neighborhood at both testing times and those who moved between testing times. In a subsample of males for whom genetic data were available, the relation of child saturation to adolescent aggression was moderated by the monoamine oxidase A (MAOA) gene. The regression of aggression on child saturation was steeper for those with the low activity version of the MAOA allele than among those with the high activity version of the allele. The implications of the results for an understanding of the origins and ontogeny of aggression and personality disorders are discussed.