INTRODUCTION Aggression is a universal phenomenon, spanning every historical period and age group. Although preschoolers are the most aggressive human beings with respect to frequency of aggressive behavior (1), aggressive acts perpetrated by older, more powerful individuals receive greater public attention. Much behavioral genetic research has focused on aggression, impulsivity, and related behaviors. Recent reviews of aggression (2-6) and impulsivity (7,8) are available. Most research indicates that aspects of aggression and impulsivity show at least some genetic inﬂuence; however, estimates range from 0% to 94% (2). The picture is complicated by deﬁnitional and/or measurement issues, gender differences, the different types of genetic variance (additive vs. nonadditive-e.g., the high estimates that are reported do not take nonadditive genetic variance into account, so the estimates are greater than the identical twin correlations and the heritability has been overestimated), and age differences or developmental factors. The research also results in the inescapable conclusion that aspects of the environment are important for the development of these behaviors-although whether the en-
vironmental effects are shared (contribute to familial similarity) or nonshared (make family members different from one another) remains equivocal. The purpose of this chapter is to review the recent behavioral genetic research on the etiology of aggression, impulsivity, and related behaviors, to highlight factors that impact estimates of heritability, and to focus on models of the combined effects of genes and environment, speciﬁcally genotype-environment (GE) interaction, diathesis-stress models, GE correlation, and G→E effects.