A central goal of every human community is the mitigation of aggression between its members. The seventeenth century English philosophers who articulated today’s Western concepts of liberty and democracy considered the restraint of unsanctioned aggressive behavior the only justification for the state to intrude on personal freedom (1,2). Hobbes wrote about the necessity of a “common power to keep them all in awe.” That power (i.e., the state) exists to constrain the antagonisms that flow from three drives: 1) to acquire resources, 2) to protect those resources and personal safety, and 3) to enhance and defend one’s prestige. He memorably portrayed the downside of the perpetual conflict that would otherwise result (1):

When Charles Darwin considered these issues within a biological framework some 200 years later, these predecessors who depicted life as a struggle for existence influenced his work (3). Natural selection involves competition for the resources that are essential to survival and successful reproduction. Scarcity of these resources means that some creatures inevitably deprive others within their species of access to them, often through aggressive behavior. However, Darwin also devoted much attention to the adaptive benefits of the “social instincts,” such as sympathy, cooperation, altruism, and the desire to maintain the approval of one’s group, which exert an equally natural countervailing force on intraspecific aggression. After all, when danger is at hand humans tend to seek safety in one another’s company. Potential procreative partners may also favor these prosocial characteristics, which would hasten their proliferation through the process of sexual selection (4).

Since then, behavioral research has supported the overall view that a combination of affective, cognitive, and social factors disincline the majority of people from harming others most of the time (6,7). We also know a fair amount about when these factors could lose their potency to inhibit aggressive behavior. For instance, aggressive behavior becomes more likely when one perceives that a potential target belongs to a different social grouping; when one believes that another person unjustifiably threatens his or her prerogatives and well being; when one believes that others will approve of or encourage aggressive acts; and when one believes that the benefits of aggression will exceed its probable cost.

Under normal circumstances, then, human interaction has a default value of relative congeniality. Numerous affective, social-cognitive and experiential factors, however, can tip the balance toward disharmony. In effect, then, most peacekeeping and law enforcement activities do not depend entirely on the external displays of might that Hobbes wrote about, but rather seem to take place chiefly within the human skull.

Consequently, many psychiatric conditions have aggressive behavior as a major complication. High negative emotionality may predispose to a low threshold for anger or frustration, so that one reacts forcefully to situations others would find only mildly bothersome. Distorted cognitive processes may lead to unwarranted alarm about environmental threats, to feeling impelled by some force to hurt others, or to erroneous beliefs about entitlement to impose one’s will on others. High anxiety may trigger avoidance or escape behaviors that can injure others who get in the way. Inadequate impulse control can disrupt response selection so that aggression has precedence over alternatives. Abnormal development may impair the acquisition of coping behaviors and self-regulatory capabilities that ordinarily suppress dyscontrolled outbursts. In addition, some highly prevalent diagnoses have aggressive behavior as a cardinal feature, such as conduct disorder, antisocial personality disorder, or intermittent explosive disorder.

Certain experiential factors can contribute to persistent aggression and therefore have psychiatric significance. Early severe maltreatment may disrupt the development of empathy. Socialization that promotes violence and threats as vehicles for self-preservation may lead to aggression that persists even in new social contexts that disapprove of such behavior.

Persistent aggressive behavior most often originates in childhood. In particular, aggressive behavior among school-age children confers high risk for unfavorable outcomes not just during youth but also throughout later life (8,9,10). Aggressive dyscontrol is also the concern that most often motivates families to obtain mental health care for their preadolescent children. Nevertheless, aggressive behavior still eludes consistently effective intervention. The combined force of troubling outcomes, adverse community impact, high prevalence, and uncertain treatment prospects propels childhood-onset aggression to the forefront of challenges in mental health today.