44-3 How does psychology’s definition of aggression differ from everyday usage? What biological factors make us more prone to hurt one another?

Prejudice hurts, but aggression often hurts more. In psychology, aggression is any unwanted behavior intended to harm someone, whether done out of hostility or as a calculated means to an end. The assertive, persistent salesperson is not aggressive. Nor is the dentist who makes you wince with pain. But the person who passes along a vicious rumor about you, the person who bullies you in person or online, and the attacker who mugs you for your money are aggressive.

Aggressive behavior emerges from the interaction of biology and experience. For a gun to fire, the trigger must be pulled; with some people, as with hair-trigger guns, it doesn’t take much to trip an explosion. Let’s look first at some biological factors that influence our thresholds for aggressive behavior, then at the psychological factors that pull the trigger.

Aggression varies too widely from culture to culture, era to era, and person to person to be considered an unlearned instinct. But biology does influence aggression. We can look for biological influences at three levels—genetic, neural, and biochemical.

Genetic InfluencesGenes influence aggression. We know this because animals have been bred for aggressiveness—sometimes for sport, sometimes for research. The effect of genes also appears in human twin studies (Miles & Carey, 1997; Rowe et al., 1999). If one identical twin admits to “having a violent temper,” the other twin will often independently admit the same. Fraternal twins are much less likely to respond similarly. Researchers continue to search for genetic markers in those who commit violent acts. One is already well known and is carried by half the human race: the Y chromosome. Another such marker is the monoamine oxidase A (MAOA) gene, which helps break down neurotransmitters such as dopamine and serotonin. Sometimes called the “warrior gene,” people who have low MAOA gene expression tend to behave aggressively when provoked. In one experiment, low (compared with high) MAOA gene carriers gave more unpleasant hot sauce to someone who provoked them (McDermott et al., 2009).

Neural InfluencesThere is no one spot in the brain that controls aggression. Aggression is a complex behavior, and it occurs in particular contexts. But animal and human brains have neural systems that, given provocation, will either inhibit or facilitate aggression (Denson, 2011; Moyer, 1983; Wilkowski et al., 2011). Consider:

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Biochemical InfluencesOur genes engineer our individual nervous systems, which operate electrochemically. The hormone testosterone, for example, circulates in the bloodstream and influences the neural systems that control aggression. A raging bull becomes a gentle Ferdinand when castration reduces its testosterone level. Conversely, when injected with testosterone, gentle, castrated mice once again become aggressive.

Humans are less sensitive to hormonal changes. But as men age, their testosterone levels—and their aggressiveness—diminish. Hormonally charged, aggressive 17-year-olds mature into hormonally quieter and gentler 70-year-olds. Men more than women tend to have wide faces, a testosterone-linked trait, rather than roundish or long faces. And men’s facial width is a predictor of their aggressiveness and prejudicial attitudes (Carré et al., 2009; Hehman et al., 2013; Stirrat & Perrett, 2010). Women apparently pick up on this by perceiving men with higher facial width-to-height ratios as more dominant (Valentine et al., 2014).

High testosterone correlates with irritability, assertiveness, impulsiveness, and low tolerance for frustration—qualities that predispose somewhat more aggressive responses to provocation or competition for status (Dabbs et al., 2001b; McAndrew, 2009; Montoya et al., 2012). Among both teenage boys and adult men, high testosterone levels correlate with delinquency, hard drug use, and aggressive-bullying responses to frustration (Berman et al., 1993; Dabbs & Morris, 1990; Olweus et al., 1988). Drugs that sharply reduce testosterone levels subdue men’s aggressive tendencies.

Another drug that sometimes circulates in the bloodstream—alcohol—unleashes aggressive responses to frustration. Across police data, prison surveys, and experiments, aggression-prone people are more likely to drink, and to become violent when intoxicated (White et al., 1993). National crime data indicate that 73 percent of Russian homicides and 57 percent of U.S. homicides are alcohol-influenced (Landberg & Norström, 2011). Alcohol effects aggression both biologically and psychologically (Bushman, 1993; Ito et al., 1996; Taylor & Chermack, 1993). Just thinking you’ve imbibed alcohol can increase aggression (Bègue et al., 2009). But so, too, does unknowingly ingesting alcohol slipped into a drink. Unless people are distracted, alcohol tends to focus their attention on a provocation rather than on inhibitory cues (Giancola & Corman, 2007). Alcohol also inclines people to interpret ambiguous acts (such as a bump in a crowd) as provocations (Bègue et al., 2010).

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44-4 What psychological and social-cultural factors may trigger aggressive behavior?

Biological factors influence how easily aggression is triggered. But what psychological and social-cultural factors pull the trigger?

Aversive EventsSuffering sometimes builds character. In laboratory experiments, however, those made miserable have often made others miserable (Berkowitz, 1983, 1989). This phenomenon is called the frustration-aggression principle: Frustration creates anger, which can spark aggression. One analysis of 27,667 hit-by-pitch Major League Baseball incidents between 1960 and 2004 revealed this link (Timmerman, 2007). Pitchers were most likely to hit batters when

Other aversive stimuli—hot temperatures, physical pain, personal insults, foul odors, cigarette smoke, crowding, and a host of others—can also evoke hostility. In laboratory experiments, when people get overheated, they think, feel, and act more aggressively. Simply thinking about words related to hot temperatures is enough to increase hostile thoughts (DeWall & Bushman, 2009). In baseball games, the number of hit batters rises with the temperature (Reifman et al., 1991; see FIGURE 44.6). In the wider world, violent crime and spousal abuse rates have been higher during hotter years, seasons, months, and days (Anderson et al., 1997). Other studies from archaeology, economics, geography, political science, and psychology converge in finding that throughout human history, higher temperatures have predicted increased individual violence, wars, and revolutions (Hsiang et al., 2013). From the available data, Craig Anderson and his colleagues (2000, 2011) have projected that, other things being equal, global warming of 4 degrees Fahrenheit (about 2 degrees Celsius) could induce tens of thousands of additional assaults and murders—and that’s before the added violence inducements from climate change-related drought, poverty, food insecurity, and migration.

Reinforcement, Modeling, and Self-ControlAggression may naturally follow aversive events, but learning can alter natural reactions. We learn when our behavior is reinforced, and we learn by watching others.

In situations where experience has taught us that aggression pays, we are likely to act aggressively again. Children whose aggression has successfully intimidated other children may become bullies. Animals that have successfully fought to get food or mates become increasingly ferocious. To foster a kinder, gentler world we had best model and reward sensitivity and cooperation from an early age, perhaps by training parents to discipline without modeling violence.

Parent-training programs often advise parents to avoid modeling violence by screaming and hitting. Instead, parents should reinforce desirable behaviors and frame statements positively. (“When you finish loading the dishwasher you can go play,” rather than “If you don’t load the dishwasher, there’ll be no playing.”)

One aggression-replacement program worked with juvenile offenders and gang members and their parents. It taught both generations new ways to control anger, and more thoughtful approaches to moral reasoning (Goldstein et al., 1998). The result? The youths’ re-arrest rates dropped more than is typical.

Different cultures model, reinforce, and evoke different tendencies toward violence. For example, crime rates are higher and average happiness is lower in times and places marked by a great disparity between rich and poor (Messias et al., 2011; Oishi et al., 2011; Wilkinson & Pickett, 2009). In the United States, cultures and families in which fathers are minimally involved also have high violence rates (Triandis, 1994). Even after controlling for parental education, race, income, and teen motherhood, American male youths from father-absent homes are incarcerated at twice the rate of their peers (Harper & McLanahan, 2004).

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Violence can vary by culture within a country. Richard Nisbett and Dov Cohen (1996) analyzed violence among White Americans in southern towns settled by Scots-Irish herders whose tradition emphasized “manly honor,” the use of arms to protect one’s flock, and a history of coercive slavery. Compared with their White counterparts in New England towns settled by the more traditionally peaceful Puritan, Quaker, and Dutch farmer-artisans, the cultural descendants of those herders have had triple the homicide rates and were more supportive of physically punishing children, of warfare initiatives, and of uncontrolled gun ownership. “Culture-of-honor” states also have higher rates of students bringing weapons to school and of school shootings (Brown et al., 2009).

Media Models for Sexual ViolenceParents are hardly the only aggression models. In the United States and elsewhere, TV, films, video games, and the Internet offer supersized portions of violence. Repeatedly viewing on-screen violence tends to make us less sensitive to cruelty (Montag et al., 2012). It also primes us to respond aggressively when provoked. And it teaches us social scripts—culturally provided mental files for how to act. When we find ourselves in new situations, uncertain how to behave, we rely on social scripts. After watching so many action films, adolescent boys may acquire a script that plays in their head when they face real-life conflicts. Challenged, they may “act like a man” by intimidating or eliminating the threat. More than 100 studies together confirm that people sometimes imitate what they’ve viewed. Watching risk-glorifying behaviors (dangerous driving, extreme sports, unprotected sex) increases viewers’ real-life risk-taking (Fischer et al., 2011).

Music lyrics also write social scripts. In experiments, German university men who listened to woman-hating song lyrics administered the most hot chili sauce to a woman. They also recalled more negative feelings and beliefs about women. Man-hating song lyrics had a similar effect on the aggressive behavior of women listeners (Fischer & Greitemeyer, 2006).

Sexual scripts depicted in pornographic films are often toxic. Researchers have found that repeatedly watching pornographic films, even nonviolent films, makes sexual aggression seem less serious (Harris, 1994). In one experiment, undergraduates viewed six brief, sexually explicit films each week for six weeks (Zillmann & Bryant, 1984). A control group viewed films with no sexual content during the same six-week period. Three weeks later, both groups read a newspaper report about a man convicted of raping a hitchhiker and were asked to suggest an appropriate prison term. Sentences recommended by those viewing the sexually explicit films were only half as long as the sentences recommended by the control group.

While nonviolent sexual content affects aggression-related sexual attitudes, violent sexual content can also increase men’s readiness to actually behave aggressively toward women. A statement by 21 social scientists noted, “Pornography that portrays sexual aggression as pleasurable for the victim increases the acceptance of the use of coercion in sexual relations” (Surgeon General, 1986). Contrary to much popular opinion, viewing such scenes does not provide an outlet for bottled-up impulses. Rather, “in laboratory studies measuring short-term effects, exposure to violent pornography increases punitive behavior toward women.”

To a lesser extent, nonviolent pornography can also influence aggression. One set of studies exploring pornography’s effects on aggression toward relationship partners found that pornography consumption predicted both self-reported aggression and participants’ willingness to administer laboratory noise blasts to their partner (Lambert et al., 2011). Abstaining from one’s customary pornography consumption decreased aggression. Abstaining from a favorite food did not.

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Do Violent Video Games Teach Social Scripts for Violence?Experiments in North America, Western Europe, Singapore, and Japan indicate that playing positive games produces positive effects (Greitemeyer & Osswald, 2010, 2011; Prot et al., 2014). For example, playing Lemmings, where a goal is to help others, increases real-life helping. So, might a parallel effect occur after playing games that enact violence? Violent video games became an issue for public debate after teenagers in more than a dozen places seemed to mimic the carnage in the shooter games they had so often played (Anderson, 2004a, 2013).

In 2002, three young men in Michigan spent part of a night drinking beer and playing Grand Theft Auto III. Using simulated cars, they ran down pedestrians, then beat them with fists, leaving a bloody body behind (Kolker, 2002). These same young men then went out for a real drive. Spotting a 38-year-old man on a bicycle, they ran him down with their car, got out, stomped and punched him, and returned home to play the game some more. (The victim, a father of three, died six days later.)

Such violent mimicry causes some to wonder: What are the effects of actively role-playing aggression? Does it cause young people to become less sensitive to violence and more open to violent acts? Nearly 400 studies of 130,000 people offer an answer, report some researchers (Anderson et al., 2010). Video games can prime aggressive thoughts, decrease empathy, and increase aggression. University men who spend the most hours playing violent video games have also tended to be the most physically aggressive (Anderson & Dill, 2000). (For example, they more often acknowledged having hit or attacked someone else.) And people randomly assigned to play a game involving bloody murders with groaning victims (rather than to play nonviolent Myst) became more hostile. On a follow-up task, they were more likely to blast intense noise at a fellow student. Studies of young adolescents reveal that those who play a lot of violent video games become more aggressive and see the world as more hostile (Gentile, 2009; Hassin et al., 2013). Compared with nongaming kids, they get into more arguments and fights and earn poorer grades.

Ah, but is this merely because naturally hostile kids are drawn to such games? Apparently not. Comparisons of gamers and nongamers who scored low on hostility measures revealed a difference in the number of fights they reported. Almost 4 in 10 violent-game players had been in fights, compared with only 4 in 100 of the non-gaming kids (Anderson, 2004a). Some researchers believe that, due partly to the more active participation and rewarded violence of game play, violent video games have even greater effects on aggressive behavior and cognition than do violent TV shows and movies (Anderson & Warburton, 2012).

Other researchers are unimpressed by such findings (Ferguson, 2013, 2014). They note that from 1996 to 2006, youth violence declined while video game sales increased, and argue that other factors—depression, family violence, peer influence—better predict aggression. Also, game-playing keeps people off the streets and out of trouble (Engelstätter et al., 2011). Moreover, some point out that avid game players are quick and sharp: They develop speedy reaction times and enhanced visual skills (Dye et al., 2009; Green et al., 2010). The focused fun of game playing can satisfy basic needs for a sense of competence, control, and social connection (Granic et al., 2014).

In 2011, a U.S. Supreme Court decision overturned a California state law that banned violent video game sales to children (modeled after the bans on sales of sexually explicit materials to children). The First Amendment’s free speech guarantee protects even offensive games, said the court’s majority, which was unpersuaded by the evidence of harm. So, the debate continues.

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To sum up, research reveals biological, psychological, and social-cultural influences on aggressive behavior. Complex behaviors, including violence, have many causes, making any single explanation an oversimplification. Asking what causes violence is therefore like asking what causes cancer. Those who study the effects of asbestos exposure on cancer rates may remind us that asbestos is indeed a cancer cause, but it is only one among many. Like so much else, aggression is a biopsychosocial phenomenon (FIGURE 44.7).

A happy concluding note: Historical trends suggest that the world is becoming less violent over time (Pinker, 2011). That people vary across time and place reminds us that environments differ. Yesterday’s plundering Vikings have become today’s peace-promoting Scandinavians. Like all behavior, aggression arises from the interaction of persons and situations.