The objective of this research paper is to present evidence showing that sex differences in aggression are universal, but that within limits the differences are also highly susceptible to experiential modification. Aggression is defined as any behavior that intends to hurt a person or a thing, physically or verbally. Investigation was conducted within the framework of three independent methodologies: (1) holocultural research of enculturation, (2) cross-cultural community research of sexual division of labor, and (3) psychological research of phylogenetics within the United States. Children were examined in each component of the research to show aggression-level differences between males and females. The first methodology shows that sex differences in aggression result from enculturative pressures that differentially encourage greater aggressive behavior in boys than girls. The second research component finds that sex differences in the division of labor produce sex differences in aggression. The third method shows that sex differences in aggression result from a phylogenetically acquired species predisposition for males to behave more aggressively than females, subject to modification by experience and culture learning. Generally, the three theoretically competing methodologies show that males are more aggressive than females on a panspecies level. Research notes and data tables are included in the document. (Author/ND)