Are we natural-born killers? When we reflect on how common aggression is in virtually every society, both past and present, it’s tempting to conclude that aggressive tendencies are an inherent part of human nature. For example, before World War I, Sigmund Freud (1920/1955) maintained that people are motivated by an inborn instinct to seek pleasure and to create, which he called eros. But after observing how willing and even eager people were to torture, maim, and murder one another during the war, he proposed that humans are also born with an aggressive instinct, which he called thanatos, that seeks to destroy life. Is Freud right? Is aggressiveness biologically programmed into human nature? There is no simple answer to this question. Some evidence on the “yes” side comes from comparisons of humans with other animal species.

Ethology, or behavioral biology, is the study of animal behavior in its natural context. Konrad Lorenz, a Nobel Prize–winning Austrian ethologist, posited that if we observe humans and other species displaying similar aggressive behaviors under roughly similar situations, we can infer that those behaviors helped humans and other species alike to survive and reproduce (Lorenz, 1966). Such evidence would suggest a shared, innate psychological mechanism for aggression.

Ethologists draw attention to a number of interesting parallels in the aggressive behavior of humans and other animal species. For example, in species as diverse as chimpanzees, crayfish, and bald eagles, an organism will usually aggress against another organism that attempts to acquire or gain control over material resources that are necessary for survival, such as food, nesting sites, and feeding sites (Enquist & Leimar, 1983). Similarly, humans display anger and aggression when others attempt to take control of their property. Imagine that you’ve claimed a table at the library for yourself by setting up your laptop and books, but on returning from a quick trip to the vending machine you find that someone has taken your spot. This violation of your personal space would probably evoke anger, if not aggression (Worchel & Teddie, 1976).

Animal species also routinely exhibit threat displays and aggression when they or their offspring are attacked, tendencies that are also seen in humans. Indeed, laws in most cultures formally recognize that aggression in defense of self or others is justified. Aggression in the animal world is also commonly seen in competition over social status. In many social species such as monkeys, apes, and hyenas, members of a group are organized in a dominance hierarchy. Lower-status group members (typically males) sometimes try to achieve a higher status to gain access to more material resources, sexual partners, and control over others’ behavior. Of course, high-status group members are reluctant to relinquish their dominant position and respond to a competitor with overt displays of anger. If the competitor doesn’t back down, physical combat may ensue.

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Analogously, in humans, threats to reputation are a primary trigger for aggression. From the tribal communities of Highland New Guinea (Sargent, 1974) to the Jivaro people of the western Amazon (Karsten, 1935) to the street gangs of America’s inner cities (Toch, 1969), threats to one’s face or status are often the trigger for violent or even homicidal aggressive acts among young men. Also consistent with the idea that anger and aggression function to maintain one’s reputation, people are especially likely to retaliate against someone who insults them when they know that an audience has witnessed the insult (Brown, 1968; Felson, 1982; Kim et al., 1998).

Across species, aggression is often triggered by the perception that others are making one’s life difficult, either by imposing costs (e.g., direct threats to survival, stealing resources) or denying benefits (e.g., preventing a rise in status). In addition, anger and aggression seem to have similar functions across species. Anger displays (e.g., baring one’s teeth) deter others who might challenge one’s status (Sell, 2011). If displays of anger are not effective at changing others’ behavior, aggression is a way to reinforce the message, reducing the probability that others will repeat their harmful actions in the future (Clutton-Brock & Parker, 1995). On the basis of these parallels, it seems reasonable to conclude that the human mind has inherited from natural selection a propensity to respond to certain situations with anger and, when need be, aggression.

If the human mind evolved over millions of years to respond to certain situations with anger and aggression, these adaptations should be reflected in our physiology. Research has indeed discovered physiological mechanisms involved in the detection of social threat, the experience of anger, and engaging in aggressive behavior.

Brain Regions

One region of the brain involved in the detection of social threat is the dorsal anterior cingulate cortex, or dACC (see FIGURE 12.1).

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One function of the dACC is to alert us when there is a conflict between our expectations and the situation we are in (Bush et al., 2000). Imagine, for example, that you are driving in your car and the idiot driver to your left suddenly cuts in front of you, forcing you to swerve to avoid a collision. Blood rushes to your dACC to alert you that something is wrong. Suddenly your drive is not proceeding as you expected, and you need to straighten things out.

Insults and other provocations have been shown to activate the dACC because those situations create a conflict between how we feel we should be treated and how we actually are treated. The relation between the dACC and reaction to provocation is seen in a study by Denson and colleagues (2009). Participants were asked to complete difficult puzzles and to state their answers to the puzzles out loud to the experimenter. The experimenter pretended not to hear the participants and politely prompted them to speak louder. But on the third such “mishearing,” the experimenter insulted the participants by saying in an irate and condescending tone, “Look, this is the third time I’ve had to say this! Can’t you follow directions?” Relative to the baseline measure taken before the provocation, participants showed increased activation in the dACC after the insult. This activation was positively correlated with how much anger they felt toward the experimenter. Related research shows that the dACC is activated when we feel rejected by others (Eisenberger & Lieberman, 2004). Rejection is an important cause of aggressive behavior, as we will see later in this chapter. What’s more, the more people show dACC activation in response to a provocation by another person, the more willing they are to retaliate against that person by subjecting him or her to blasts of painful noise (Krämer et al., 2007).

Although the dACC sounds the neural alarm when we are faced with unjustified wrongdoing, regions in the limbic system, especially the hypothalamus and the amygdala (see FIGURE 12.2), are involved in emotional experiences of fear and anger, which often elicit aggressive behaviors.

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When we are faced with threat, the hypothalamus kicks into high-arousal mode, preparing our body for “fight or flight,” that is, fleeing from danger or preparing to aggress against the threatening stimulus. The amygdala responds to threatening stimuli with processes generating fear or anger. When the emotion is fear, the behavior is avoidance, or “flight.” When the emotion is anger, the behavior is aggression, or “fight.” In experiments with cats and monkeys, a lesion to the amygdala leads them to be excessively tame, whereas stimulation to the amygdala leads them to display signs of anger such as shrieks and hisses. Neuroimaging studies with humans show that the amygdala is activated by the presentation of threatening faces (Pezawas et al., 2005), the perception of anger in others, and the experience of anger in oneself (Murphy et al., 2003). Studies also show that regions of the limbic system are activated in response to interpersonal provocations, such as unfair allocation of resources by a peer (Meyer-Lindenberg et al., 2006).

Body Chemistry

A number of hormones play a role in the experience of anger and aggressive behavior. The most widely studied is the sex hormone testosterone. The more people secrete testosterone, the more predisposed they are to aggress. For example, convicted male criminals who had committed aggressive crimes (e.g., assault) showed higher concentrations of testosterone in their blood than did males who had committed nonaggressive crimes (Dabbs et al., 1987, 1995). Of course, such correlational findings don’t prove causation. What happens when testosterone levels change? One way researchers have addressed this question is by looking at individuals undergoing sex-change operations. Although testosterone level predicts aggression in both sexes (Dabbs & Hargrove, 1997; Sapolsky, 1998), males secrete more testosterone. Females transforming into males receive androgen therapy, which entails taking drugs to increase testosterone levels, whereas males transforming into females receive testosterone-reducing drugs. Female-to-male transsexuals became considerably more aggressive in the first three months of androgen therapy, whereas male-to-female transsexuals became less aggressive (Van Goozen et al., 1995).

Testosterone levels most clearly correlate with aggressiveness in situations involving provocation and interpersonal conflict. One study of Swedish boys (Olweus et al., 1980) showed that boys with higher testosterone levels were more physically and verbally aggressive, especially in response to provocations. A similar study showed that in response to mounting intensity of provocation by another participant, men with high levels of testosterone were more physically aggressive than those with lower levels of testosterone (Berman et al., 1993). These pieces of evidence have led some researchers to suggest that testosterone leads people to seek out and maintain a sense of status relative to others (Josephs et al., 2006).

Before we conclude from this biological evidence that humans are born to aggress, we need to examine the other side of the biological coin. There are numerous reasons to think that both humans and other animals may have evolved not to be particularly aggressive. For one, even if an animal ends up dominating another animal in a competition, it probably will suffer injuries that will take energy to heal and that may ultimately be fatal. Also, those who are likely to aggress risk getting a bad reputation. We see this in monkeys, for example, when group members known to be overly aggressive are usually rejected by other members (Higley et al., 1994). In humans, people who are overly aggressive—either by physically attacking others or by trying to damage others’ peer relationships (e.g., by spreading hateful gossip)—increase their chances of being rejected by their peers in the future, if not imprisoned (Crick & Grotpeter, 1995; Dishion et al., 1994; Dodge, 1983).

Also, although aggressing against someone who harmed us may deter them in the future, it usually damages our relationship with that person. As a result, we miss out on any benefits that might have come from that relationship. Therefore, some theorists argue that natural selection may have shaped the human mind to forgive valued relationship partners despite the harm they have caused us (McCullough, 2008). If ancestral humans forgave an offending relationship partner and reconciled after interpersonal conflict, they could restore relationships that, on average, would have increased their chances of surviving and reproducing. Finally, humans have evolved to be especially capable of learning, perspective-taking, empathy, imagining future events, internalizing morals, considering fairness, and controlling impulses. As we will see throughout this chapter, these capabilities often counteract inclinations to aggress.

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Two differences between humans and animals stand out when comparing aggressive behavior in humans and other species. One makes us more aggressive than other animals, the other makes us less.

Technology outstrips natural controls on aggression. When two members of the same species fight, they almost never kill each other. Over the course of evolution, these species have developed strategies to avoid unnecessary injury. For example, animals are very good at gauging each other’s physical strength and fighting ability. If one animal recognizes that it will probably lose, it avoids further conflict. Also, if the animals do end up fighting, once one animal has clearly dominated the other, the loser assumes a subordinate position, such as lying on its back or baring its throat. It is as if the loser is saying to the dominant animal, “All right, you win, champ; let’s both walk away.” At that point the dominant animal does not go in for the kill; rather, it breaks off the combat, and in some cases even makes a friendly gesture to the subordinate animal. In this way, both animals avoid unnecessary injury (Lorenz, 1966).

If you’ve noticed that human aggression doesn’t seem to work by these rules, you are absolutely right. Whereas other species have natural controls that restrain violence before it gets out of hand, humans seem to have far fewer qualms about killing each other. Why is this? An important part of the answer proposed by Lorenz (1966) is that our technological capacity for violence has outstripped any inhibitions against killing that we may have. The ability to pull triggers, plant bombs, and launch missiles means we can cause lethal harm so quickly and from such a distance that evolved controls over violence that might lead to less lethal actions, such as empathy and signs of submission and suffering, have no opportunity to intercede and limit the damage. Because our technological proficiency in killing is so great, we humans kill more members of our own species because of how we are different from other animals, rather than because of how we are similar to them.

The human mind specializes in self-control. Parallels between humans and other species are also limited because of the unique self-regulatory capacities afforded by our cerebral cortex. We have the ability to think abstractly and self-reflectively on our past experiences and future possibilities, reappraise emotional events, and reason about right and wrong.

Indeed, our brains are equipped with regions that allow us to inhibit aggressive responses. Earlier we saw that brain regions such as the dACC and the limbic system act as a neural alarm system that alerts us when something is wrong. In some circumstances, these regions produce an impulsive desire to retaliate aggressively to perceived hostile treatment. But obviously we don’t act on every aggressive impulse that we experience. Rather, these impulses are regulated by regions of our prefrontal cortex, in particular the medial prefrontal cortex, or MPFC, and the dorsolateral prefrontal cortex, or DLPFC, which we discussed in chapter 11 (FIGURE 12.3) (Davidson et al., 2000; Raine, 2008; Siever, 2008).

These regions of the prefrontal cortex are active when we introspect, consider our morals and the consequences of our actions, reflect on our emotional responses to distressing stimuli, and control our behavior. Also, they share many neural connections with the dACC and limbic structures such as the amygdala. Through these connections, they act as an emergency brake on the aggressive impulses generated by those structures (Inzlicht & Gutsell, 2007). For example, individuals who suffer injury to the prefrontal cortex become dramatically more irritable, hostile, and aggressive (Grafman et al., 1996).

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What’s more, our prefrontal cortex has many receptors for a neurotransmitter called serotonin, which damps our angry and aggressive impulses. Neurotransmitters are chemicals that carry messages between brain cells. Serotonin, often called the “feel-good” neurotransmitter, helps the prefrontal cortex put the brakes on impulsive responses to distressing events (Soubrié, 1986). High levels of serotonin in the nervous system of humans and other animals are correlated with low levels of aggression (Ferris et al., 1997; Suarez & Krishnan, 2006). Experimentally boosting serotonin’s activity reduces aggression (Berman et al., 2009; Carrillo et al., 2009; Cleare & Bond, 1995). Reducing serotonin levels leads to more aggressive responses to distressing events such as unfair treatment by a peer (Crockett et al., 2008).

This means that if another driver cuts us off on the freeway, we may experience an aggressive impulse. Whether or not we act on that aggressive impulse will depend on a number of factors, one of which is the activation in the prefrontal cortex self-regulatory control regions of the brain. It is precisely our ability to engage in the sophisticated regulation of our emotions and control of our behavior that distinguishes human aggression from the more reactive aggression observed in other animals. As we extend our analysis, we will see how factors that impede higher-order cognitive functioning block this capacity for control, making aggressive responses more likely.

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