INTRODUCTION Despite a century of published scientiﬁc research on human aggression, deﬁnitive answers about the causes and control of human aggressive behavior remain elusive. The ability of mental health professionals to predict which individuals will engage in violent behavior remains poor (1), and the relative stability of violence rates suggests that little progress has been made toward the prevention of aggressive acts. Indeed, data from the U.S. Department of Justice Bureau of Justice Statistics reveal that the homicide rates over the last hundred years have ﬂuctuated from 5 to 10 murders per 100,000, with peaks and troughs interspersed (2). These data, along with evidence from epidemiological studies showing that physical aggression in everyday relationships is a frequent occurrence (3), provide little reason for optimism that effective interventions for reducing violence in the community will be developed in the near future. On the positive side, over the
past century a rich literature has accumulated that supports the notion that reliable correlates of aggressive behavior can be identiﬁed in the ﬁeld and laboratory (4,5). For example, biological (e.g., central serotonin activity, endogenous testosterone activity, alcohol consumption), psychological (e.g., perceived threat, aversive mood states, frustration), and social (e.g., economic ﬂuctuations, normative inﬂuences, political instability) factors have been associated with aggressive behavior. However, no single variable or theoretical model has been able to satisfactorily explain aggressive behavior in the general case. This has had the indirect beneﬁt of allowing researchers to reach a consensus on what may be the guiding principle for aggression research in the next century; speciﬁcally, that the causes of aggressive behavior are complex and multi-factorial.