Clearly factors within a person, as well as the situation, influence who is more and less likely to be aggressive. In this section we briefly consider eight such individual difference variables: gender, age, trait aggressiveness, intelligence, narcissism, low and unstable self-esteem, and the ability to control one’s thoughts and behavior.

Over the course of history and across the globe today, men are more likely than women to be physically aggressive (Archer, 2004; Card et al., 2008). Men commit the vast majority of violent and homicidal aggressive acts, such as murder, armed robbery, and aggravated assault (e.g., Daly & Wilson, 1988). However, women are more likely than men to engage in acts of verbal aggression by spreading malicious rumors and gossip, excluding others from desirable events and groups, and threatening to end friendships (Archer, 2004; Card et al., 2008; Crick & Grotpeter, 1995). Men and women do not differ in their overall level of aggression as much as they do in their preferred mode of aggression—physical for men, verbal for women.

Although verbal aggression does not physically harm the victims, it can be extremely harmful for emotional and psychological health, as we saw in the case of Chanelle Rae. Victims of verbal aggression are at a high risk for depression and anxiety (Crick & Grotpeter, 1996). The distress and humiliation they experience can even lead them to take their own lives.

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That said, because the consequences of physical aggression are usually more immediately apparent and severe, considerable research has sought to understand why men are more physically aggressive than women. One theory traces this gender difference back to physiological differences between men and women (Maccoby & Jacklin, 1974). Recall that testosterone level is correlated with aggressive behavior, particularly in response to provocation, and that on average men secrete more testosterone than women. Another factor is that men may be more likely than women to engage in physical aggression and do more physical harm because they are generally larger and physically stronger than women. A third factor is that men are more likely than women to interpret other people’s actions as intended to provoke them, for example, by insulting their reputations (Crick & Dodge, 1994; Dodge & Coie, 1987). As we’ve seen, this interpretation of others’ actions as provoking is the most common trigger of an aggressive response.

Another way to understand the gender difference in physical aggressiveness is to ask, Why aren’t women more aggressive? According to Eagly and Steffen (1986), boys and girls are socialized with different normative expectations about what men and women should and should not do. Social norms dictate that a certain amount of physical aggression is acceptable among young boys, but not among young girls (e.g., Björkqvist et al., 1992). Compared with men, women are more likely to consider physical aggression inappropriate. As a result, they inhibit their aggressive impulses to avoid the stronger experience of guilt or anxiety that would result if they did act on their aggression (Brock & Buss, 1964; Eagly & Steffen, 1986; Wyer et al., 1965).

Such evidence suggests that when women are in situations where they feel less constrained by traditional gender-role norms, they should be less likely to inhibit themselves from physically aggressing. Some research supports this idea (e.g., Bettencourt & Miller, 1996). Thus, when asking why men are more physically aggressive than women, one area we need to look at is gender-role norms.

Even if some people are more aggressive than others, how do these propensities change over time? Here we note some general trends in aggression levels at different phases of the life span. We’ll also discuss the time line of the gender differences mentioned in the previous section.

Pint-sized Aggressors

Most of us think that children are bundles of joy during the preschool years (ages 2 to 3), but toddlers physically aggress at a level that even the most hardened criminal would find impressive: Fully 25% of preschoolers’ social interactions involve some form of physical aggression, whether it is pushing other children or taking their toys (Tremblay, 2000). The level of aggression is so high partly because, at this age, children are just beginning to experiment with how much control they have over their environment. Also, their abilities to control their impulses, let alone communicate their desires, are not yet fully developed.

Learning About Self and Others

In the first few years after toddlerhood, most children’s brains develop in ways that give them more control over their actions, and they are socialized to learn that kicking and punching are often not the most effective means of getting what they want. As a result, they begin to express their desires and frustrations in nonaggressive ways. Children also develop the ability to think about other people’s intentions.

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Boys Get Physical, Girls Get Verbal

As children move through elementary school, gender differences in preferred mode of aggression become more noticeable. Around age 11, girls begin to engage in significantly more verbal aggression than boys, while boys start to surpass girls in physical aggression (Lagerspetz et al., 1988). Longitudinal studies provide some evidence that socialization places more pressure on girls to inhibit their aggression than it does on boys: Among those eight-year-old children nominated by their peers as high in aggressiveness, 47% of the boys continued to be high in physical aggressiveness into middle adulthood, whereas only 18% of the girls continued to be highly aggressive into middle adulthood (Huesmann et al., 2009). These findings suggest that socialization reduces physical aggressiveness in girls more than it does in boys.

The Hazards of Puberty

Gender differences in preferred mode of aggression peak in adolescence and early adulthood. Between the ages of 15 and 30, males show a dramatic spike in the rate of violent criminal offenses. For instance, collapsing over about a dozen-year period, the average homicide perpetrator in the United States is a 27-year-old male (Federal Bureau of Investigation, 2009). One reason for this spike in physical aggression in adolescence is that this is also when testosterone levels peak. In fact, differences in verbal aggression also are at their height around the same time range: 18-year-old women show higher levels of verbal aggression than men of the same age (Lagerspetz & Björkqvist, 1994).

Well-behaved Adults

After early adulthood, rates of interpersonal aggression and rates of violent criminal offenses decline, even among men. One reason is that men secrete less testosterone after they reach age 25. In addition, by adulthood both men and women are employing verbal means of aggression more often (Björkqvist et al., 1994). Of course, these developmental trends are also affected by the social norms of the society in question, but at least among middle-class, European, and North American males, physical aggression in middle and late adulthood is commonly considered acceptable only in self-defense or in defense of others (Holm, 1983).

Some people are more likely to aggress than others over time and across situations. People who are high in such trait aggressiveness are susceptible to hostile thoughts, are likely to express anger, and tend to engage in physical and verbal aggression. Researchers often measure trait aggressiveness in adults by giving them questionnaires that ask how much they agree with statements such as “Once in a while I can’t control the urge to strike another person” and “I sometimes feel like a powder keg ready to explode” (Buss & Perry, 1992).

Individual differences in trait aggressiveness emerge as early as age three (Olweus, 1979). These differences are stable across the life span, with trait aggressiveness in childhood correlating highly with aggressiveness as much as 40 years later (Huesmann et al., 2009). Indeed, the continuity of aggression across the life span is as stable as the continuity of IQ. Laboratory studies show that individuals who are high in trait aggressiveness engage in higher levels of aggressive behavior under both neutral and provoking conditions (Bettencourt et al., 2006; Bushman, 1995).

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Why Are Some People High in Trait Aggressiveness?

A complete answer to this question would likely involve virtually all the factors we have already discussed in this chapter. But let’s take a brief look at two basic influences, one nurture, the other nature.

Bad Parenting

Coercive parenting styles, inconsistent discipline, physical abuse, and exposure to family conflicts all contribute to the child’s tendency to behave in an aggressive and antisocial manner later in life (Rhee & Waldman, 2002). In fact, bad parenting can set into motion a chain of aggressiveness that spans three generations (Capaldi et al., 2003; Conger et al., 2003; Hops et al., 2003; Thornberry et al., 2003).

From the social cognitive perspective, part of what children learn from more aggressive parents is not just aggressive behavior but also aggressive interpretation of social information. When a person is provoked, the degree to which he or she infers that another’s actions were committed with hostile intent (a hostile attribution) strongly predicts whether the person will react aggressively (a probability of about 76%). If the same person infers that the actions were committed benignly, the probability of an aggressive behavioral reaction is much lower (about 25%) (Dodge, 1980). The link between hostile attributions and aggressive behavior persists across ages, demographic and cultural groups, and social contexts (de Castro et al., 2003). It is not surprising that children classified as highly aggressive (e.g., by their peers and teachers) are more likely than their less aggressive peers to attribute hostile intent to others’ actions, even when others’ intentions are benign (Dodge, 1980; Nasby et al., 1980). This has been called the hostile attribution bias (Crick & Dodge, 1994). In hostile family environments, children quickly learn to attribute hostile motives to others. The experience of physical and/or psychological abuse by one’s parents during the first five years of life predicts a tendency toward the hostile attribution bias, which in turn predicts aggressive behavior even years later (Dodge et al., 1990; Weiss et al., 1992).

Genetic Factors

The clearest evidence for the role of genetic influences on aggressiveness comes from studies of twins (Miles & Carey, 1997; Rhee & Waldman, 2002). In one study (Rushton et al., 1986), monozygotic twin pairs (twins who are genetically identical) were more similar in aggressiveness than were dizygotic twin pairs (those who on average share only half of their genes). In another study (Eley et al., 2003), researchers looked at over 1,000 identical and fraternal pairs of Swedish twins and had parents rate their children’s aggressiveness first as children (ages 8 to 9) and then again as adolescents (ages 13 to 14). Twins’ aggressiveness over time was highly correlated, particularly when they were identical, suggesting that aggressive behavior might be influenced by genetic factors.

Although there have been few successes so far in identifying any single gene that makes people aggressive, studies have found interesting links between aggressiveness and genes involved in the production of serotonin. As noted earlier, serotonin is a neurotransmitter that helps regulate stress. Low levels of serotonin are associated with high levels of aggression. For the body to metabolize and secrete serotonin, it needs an enzyme called monoamine oxidase A, or “MAO-A” for short. A rare genetic variant that causes low levels of MAO-A has been linked to violent and antisocial behavior (Brunner et al., 1993, Munafo et al., 2003), perhaps because individuals with this genetic variant have greater difficulty metabolizing serotonin to help them deal with stress (Meyer-Lindenberg et al., 2006).

It’s important to keep in mind that this genetic factor—referred to by some researchers as the warrior gene—does not universally predict aggressive behavior. It does so largely in response to enduring and situational stressors. For example, the genetic variant causing low MAO-A levels (and thus disrupted use of stress-damping serotonin) predicts aggressive behavior only among people who had been exposed to high levels of maltreatment and stress during childhood (Caspi et al., 2002; Kim-Cohen et al., 2006). If an individual had been mistreated early in life, this genetic variant increases the risk of aggressiveness later in life. But if the individual was reared in an environment that was nurturing and supportive, this genetic variant does not predict aggressiveness.

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The genetic variant indicating low MAO-A also predicts higher levels of aggression when people are currently provoked. In one experiment, participants were told that another person in the study had taken money from them (McDermott et al., 2009). Other participants were not given this provocation. All participants later had the opportunity to determine how much hot sauce that person had to consume. The researchers found that MAO-A did not predict aggression in the absence of provocation. However, when participants were told the other person had taken money from them, low levels of MAO-A predicted greater allocations of hot sauce (i.e., more aggression). Thus, we should consider this genetic factor a biological predisposition that interacts with the person’s environment, rather than as a strict determinant of the person’s behavior.

Poor intellectual functioning is linked to high aggressiveness, especially in children (e.g., Pitkanen-Pulkinen, 1979). This link is the product of a number of processes. For one, if people are less able to process the subtleties of a social situation and the intentions behind other people’s actions, they may be more likely to infer automatically that other people are deliberately trying to offend them (Guerra et al., 1994).

Poor intellectual functioning also makes it more likely that people will feel frustrated in their lives. This may be especially evident in school, where students with deficits in reading comprehension and mathematical reasoning may be continually frustrated by the tasks assigned to them. Intellectual deficits may also make it difficult to understand the inappropriateness of aggression, consider future consequences, or to think of nonaggressive means of responding to frustrating situations (Geen, 2001; Slaby & Guerra, 1988).

Finally, not only does poor intellectual functioning lead to aggression, but aggression can in turn impair intellectual functioning (Huesmann, 1988). Children who tend to lash out aggressively often end up disrupting good relationships with their teachers and peers, foreclosing opportunities to learn problem-solving skills and advance intellectually. In fact, a 22-year longitudinal study showed that aggressiveness in children at age 8 predicted poor intellectual functioning at age 30 better than intellectual functioning at age 8 predicts adult aggressiveness (Huesmann & Eron, 1984).

We’ve seen that perceived provocation is a major trigger of aggressive behavior. A number of personality traits predict how strongly people react to provocation, and thus how likely they are to retaliate with anger and aggression.

Narcissism and Deficits in Self-esteem

A long-standing belief of many researchers and lay individuals alike is that low self-esteem contributes to aggression. Research backs this up. Physically abusive parents and spouses and aggressive children tend to have lower self-esteem than their nonaggressive counterparts (Anderson & Lauderdale, 1982; Burdett & Jensen, 1983; Goldstein & Rosenbaum, 1985; Tangney et al., 2011). Given that provocations are often threats to self-esteem, those who have lower self-esteem also react more emotionally than others to failure, negative social feedback, and social rejection.

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More recently, researchers have focused on those who have unstable self-esteem. Narcissists, people who have a grandiose but fragile view of themselves (Baumeister et al., 1996; Thomaes & Bushman, 2011), tend to agree with statements such as “If I ruled the world, it would be a much better place.” Nevertheless, narcissists exhibit low self-esteem when it is measured implicitly. Individuals with unstable self-esteem have views of themselves that fluctuate radically in response to social situations (Kernis et al., 1989). People who are high in narcissism or unstable self-esteem respond to provocations with higher levels of aggressive behavior than those who are comparatively low in these traits (Bushman & Baumeister, 1998). Unlike trait aggressiveness, which predicts aggressive behavior under both neutral and provocation conditions, narcissism and unstable self-esteem seem to make people particularly reactive to threats to their self-views (Bettencourt et al., 2006).

Individual Differences in Impulsivity

As we saw in the earlier section on biology and human aggression, human evolution has endowed us with sophisticated cognitive abilities. These abilities enable us to reflect on the likely consequences of our actions, allowing us to inhibit or override an aggressive impulse. However, people differ in their ability to control their thoughts and behaviors. Individuals who are high in impulsivity tend to react to situations without thinking through the consequences of their actions (Barratt, 1994). They respond affirmatively to questions such as “Do you do things on the spur of the moment?” They also tend to respond to insults, attacks, and frustrations with angry outbursts, whereas low-impulsivity individuals tend to stay calm, refrain from overt signs of anger, and inhibit their urge to behave aggressively after being provoked (Caprara et al., 2002). For example, high- and low-impulsivity individuals can experience equivalent levels of negative affect and anger in response to a provocation, but the high-impulsivity individuals are less able to resist the urge to lash out aggressively (e.g., Hynan & Grush, 1986). Consequently, high impulsivity is correlated with aggressive behaviors, and has in fact been found to be “one of the strongest known correlates of crime” (Pratt & Cullen, 2000, p. 952).

It is interesting to note that a certain subgroup of violent criminals does not fit this high-impulsivity pattern. On the one hand, studies of violent criminals reveal one group of individuals who often have a history of impulsive actions and assaults (D’Silva & Duggan, 2010; Du Toit & Duckitt, 1990; Megargee, 1966). These individuals seem to have undercontrolled aggressive impulses. But a second group of violent criminals often have no prior assaultive history; they seem to be low in impulsivity and have rigid inhibitions against expressing anger. Researchers suggest that these overcontrolled offenders allow their frustrations and hostilities to build up until they boil over into an extreme act of aggression.

The social neuroscience perspective has provided evidence regarding brain regions that may contribute to the behavior of the undercontrolled violent person (Davidson et al., 2000; Raine, 2008). As we mentioned earlier in this chapter and discussed further in chapter 5, the prefrontal cortex is the part of the brain that governs our ability to monitor and control our behavior. Individuals with antisocial personality disorder have an 11% to 14% deficit in prefrontal gray matter relative to matched controls and individuals with other psychiatric disorders (Raine et al., 2000). Other research showed that convicted murderers displayed less glucose metabolism in their prefrontal regions than a control group of persons of the same sex and age (Raine et al., 1997). Differences in the functioning of prefrontal regions also predict aggressive responses to provocation. Participants with weak frontal–lobe functioning tend to be more aggressive (administering painful electric shocks) toward a confederate who has previously provoked them (who had administered painful shocks to the participants themselves) than when they were not provoked. In contrast, the effect of provocation is substantially weaker among individuals with strong frontal-lobe functioning (Lau et al., 1995).

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