Two forms of aggression are present in both experimental animal and human models. Throughout the animal literature, predatory aggression has been differentiated from defensive aggression (Scarpa and Raine, 1997). Similarly, human studies often discriminate between instrumental/proactive aggression and hostile/reactive or impulsive aggression (Crick and Dodge, 1996; Nelson and Trainor, 2007). Reactive aggression is characterized as aggressive behavior occurring in the context of anger and high emotionality. It is often more impulsive, less controlled, and occurring in reaction to a provocation or frustration (Scarpa and Raine, 1997). Instrumental aggression is qualified as being more goal-oriented and relatively nonemotional. Studies suggest that this latter type of aggression is regulated by higher cortical systems rather than brain regions (i.e., limbic systems) associated with impulsiveness (Nelson and Trainor, 2007).
Of note, instrumental aggression is characteristic of psychopathy, a subtype of aggression often associated with particularly low levels of physiological arousal (Raine, 2002a). While psychopathological disorders are outside the scope of this chapter, understanding that individuals may display different types of aggressive behavior is essential to conceptualizing physiological research findings and the development of violence, in general.