incipient science is beginning to attend far more to the importance of emotions. Emotions are central to determining who among us ultimately acts violently and who deals with life’s challenges without resort to violence.

Emotions are subjective feelings in response to either internal or external stimuli. They derive both from innate, inborn, and probably genitivally and epigenetically determined personality traits and from many fetal, childhood, and later developmental and environmental influences. When one of three boys in a family joins a gang and the others do not, it is insufficient to point to political structural factors, economic factors, or even parenting as the trigger. The gang joiner is different. He or she probably exhibits different types and degrees of emotional and physiological responsiveness to loss, to perceived threat, to perceived injustice, to out-group exposure, and to the rewards of peer acceptance. I predict that empirical research will identify both emotional and biological traits that distinguish gang participants from the nonparticipant siblings.

Discoveries in this domain potentially have implications for the prevention of violence. If, for example, early childhood depression or traumatic brain injury were shown to account for a significant amount of the variance in gang participation, the community could redouble its efforts to provide comprehensive maternal health care, and to protect young brains from injury (e.g., by evidence-based revisions in return to play guidelines in youth sports).

Cause(s) of Violence

Based on the foregoing discussion, it becomes clear why any unidisciplinary approach to the terrible dilemma of nonsanctioned human violence will founder. The metaphor of contagion (which shares a great deal of its empirical authority with findings from studies of network theory), may help to account for some aspects of trends in violence over time. Yet, rather than searching for “the” cause of a violent event, it may be useful to consider the interaction of multiple causes.

The epidemiology of primary injury prevention also offers a potentially useful way to conceptualize this kind of causality: the Haddon Matrix. Haddon matrixes were originally devised by physician William Haddon as a new way to analyze the causes of injuries and the multiple potential avenues for prevention. This framework alerts policy makers that factors influencing injuries—including violent injuries—are subject to the influence of three overlapping tiers of potential intervention: individual behavioral, environmental, and public policy. Since Haddon’s introduction of this conceptual framework, it has been applied to diverse forms of injury, including

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