The most widely accepted definitions of aggression define it as behavior that is intended to injure another individual, physically or psychologically. Within this definition, a number of important subtypes are identified. First, a distinction is made between hostile or reactive aggression, which is aggression arising from feelings of anger or hostility, and instrumental aggression, which is aggression that arises in service of goal attainment (e.g., armed robbery).
Second, physical aggression and relational aggression can be differentiated. Relational aggression is characterized by the use of verbal acts (including gossip and taunts) and social exclusion to cause psychological harm. Females engage in relational aggression at higher rates than they engage in physical aggression. However, the overall gender difference in relational aggression is small. Many males also exhibit relational aggression.
A third distinction can be made between interpersonal aggression and institutional aggression (e.g., war, genocide). Although this is an interesting and potentially useful distinction, most aggression research focuses on interpersonal aggression.
Given this heterogeneity in defining aggression, summarizing rates of aggression is a difficult task. The most readily available data are for criminal-level violence. According to the United Nations Statistics Division, the worldwide homicide rates per 100,000 inhabitants in 1996 ranged from 0.3 in Guyana to 80 in Colombia. Other violent criminal offenses can also be considered. In 2005, the U.S. Bureau of Justice Statistics recorded 15,495 occurrences of murder and nonnegligent manslaughter (5.9 per 100,000 inhabitants) and 791,988 cases of aggravated assault (301.6 per 100,000 inhabitants). These statistics help to clarify the scope of violent interpersonal aggression, but they do not provide any information regarding noncriminal or unreported aggression.
Studies show that aggressive behavior is exhibited across the life span. Studies of children age 12–18 months show that approximately half of children’s interpersonal exchanges can be defined as conflict-based or disruptive. For most children, levels of physical aggression decrease over time, although a minority demonstrate consistently high levels of physical aggression throughout childhood. This decrease in physical aggression appears to be greater for girls than for boys (Côté, Vaillancourt, & Barker, 2007). Indirect aggression follows a different pattern, appearing in low rates consistently throughout childhood for the majority of children and increasing over the course of childhood for a minority of children.
Data tend to show a gender disparity in physical aggression, although there may be specific situations in which women are as aggressive as or more aggressive than men. Meta-analysis of physical aggression research reveals a gender difference that is moderate in size (d = .5). A smaller effect size is found for gender differences in verbal aggression (d = .09) (Archer, 2004). In the United States between 1961 and 1999, 85% to 91% of homicides were committed by males. These data are consistent with homicide data across several countries (e.g., Canada, England, and Australia). However, research on domestic violence has shown that there may not be a significant gender difference in the number of physically aggressive acts committed within a relationship (Straus & Ramirez, 2007). This research also showed that when only one partner in the relationship engaged in aggression, it was twice as likely to be the female partner.
Regional differences in rates of aggression also exist. Homicide rates vary widely across the globe. Even within a single country, differences in aggression may emerge between regions. In the southern United States, rates of homicide, self-reported violence, and executions are all higher than in the northern United States.
Factors that influence group and individual variation in aggression can be identified at the biological, sociological, and psychological levels. There appears to be a genetic vulnerability to aggression. Studies of childhood and juvenile antisocial behavior have shown higher concordance rates for self-reported aggressiveness, delinquency, and conduct disorder symptoms for identical (monozygotic) than for nonidentical (dizygotic) twins, suggesting a significant contribution of genetics (Carey, 1996). In studies of adult antisocial behavior, the findings have been similar. Adoption studies also suggest that the adopted-out children of mothers and fathers with a criminal conviction are more likely to have an arrest or conviction than the adopted-out children of noncriminal mothers and fathers.
Biological approaches to aggression focus on the role of the hormone testosterone. Testosterone is present in higher levels in males than in females, it is tied to masculine physical development, and it is associated with masculine-typed social behaviors, including aggressiveness. Consistent with this association, Dabbs (2000) and colleagues have found that prison inmates with higher testosterone levels commit more violent crimes, on average, than inmates with lower testosterone levels. Further, in nonincarcerated samples, higher testosterone levels are associated with higher rates of delinquency, a greater history of violent behaviors, and more trouble with the law. Finally, individuals given increasing doses of testosterone, either as part of an experiment or as the result of medical sex-change procedures, show associated increases in aggressive behavior.
According to evolutionary psychology theories, human aggression results from the competition for mates. Specifically, males compete—often aggressively—for status and resources as a means of improving their reproductive odds. Consistent with this proposal, males tend to show higher levels of aggression at those ages when competition for mates is most important (i.e., late teens and early 20s).
Sociological theories also emphasize motivations for status and dominance as explanations for aggressive behavior. According to this approach, aggression is a tool that individuals use to gain resources and status when poverty or deprivation has limited other options (Hovland & Sears, 1940). Consistent with this proposal, Dabbs (2000) and colleagues have shown that although high testosterone levels are strongly associated with aggressive behavior among low-status men, among men of higher status, testosterone levels do not reliably relate to adult antisocial behavior.
Regional differences in aggression in the United States are also explained on the basis of status and dominance theories. According to Cohen and Nisbett (1997), the differences in aggression between the northern and southern States reflect the presence of a southern “culture of honor,” wherein perceived disrespect must be answered with aggression. Such aggression is meant to preserve social status. In support of this model, it has been shown that homicide rates are higher in the South only for cases involving argument or insult (i.e., homicides committed in the course of a robbery are not elevated). In an experimental situation, southern participants who were insulted showed higher levels of anger and higher levels of testosterone following the insult than northern participants. The proposal that perceived disrespect will increase the likelihood of aggressive behavior is also consistent with work by Dodge (1986) and colleagues showing that children with severe conduct problems are more likely than other children to view an ambiguous provocation as an act of hostility and to retaliate aggressively.
This latter finding emphasizes the role of social information processing in aggression. The social information processing approach emphasizes the role of aggressive schemas and scripts in aggression and proposes that aggression is more common when nonaggressive responses are less well learned, less accessible, and viewed as less likely to be effective. The cognitive-neoassociation theory (Berkowitz, 1993) focuses on the importance of the accessibility of aggressive cognitions. According to this model, anything in the environment that increases the accessibility of aggressive thoughts will increase the likelihood of aggressive behavior. Consistent with this theory, the presence of a weapon has been shown to significantly increase the likelihood that research participants will engage in aggressive retaliation against an individual who has angered them.
Social psychological research has also provided evidence for a link between exposure to media violence and aggressive behavior. Correlational studies reveal an association between exposure to violent media in childhood and criminal (including violent) behavior in adulthood. This relationship has also been demonstrated in the laboratory, where exposure to violent media (film clips and video game play) results in short-term increases in aggression. These effects are strongest when the viewer identifies with the aggressor portrayed in the clip and when the aggression is portrayed as justified.
Other situational factors can increase aggression. Heat is correlated with aggression. There are higher rates of violent (but not nonviolent) crime in cities with hotter climates, and rates of violence tend to peak in summer months. This effect of heat may exemplify a broader category of causal factors. Any situation that increases discomfort may increase the likelihood of aggression. For example, individuals are also more likely to aggress when exposed to physical pain, hunger, overcrowding, noxious smells, and insults.
Personality theorists account for differences in aggression at the trait level. For example, the psychopathic personality, which is characterized by callousness, lack of empathy, fearlessness, and impulsivity, is also associated with elevated rates of both instrumental and hostile aggression (Hare, 2003). Given the heterogeneity of aggressive behavior, it is unlikely that any single perspective will address all forms of aggression. Thus, prevention and treatment efforts must be developed at the biological, sociological, and psychological levels.
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