Aggression Definition

In sports and in business, the term aggressive is frequently used when the term assertive, enthusiastic, or confident would be more accurate. For example, an aggressive salesperson is one who tries really hard to sell you something. Within psychology, the term aggression means something different. Most social psychologists define human aggression as any behavior that is intended to harm another person who wants to avoid the harm. This definition includes three important features. First, aggression is a behavior. You can see it. For example, you can see a person shoot, stab, hit, slap, or curse someone. Aggression is not an emotion that occurs inside a person, such as feeling angry. Aggression is not a thought that occurs inside someone’s brain, such as mentally rehearsing a murder one is about to commit. Aggression is a behavior you can see. Second, aggression is intentional. Aggression is not accidental, such as when a drunk driver accidentally runs over a child on a tricycle. In addition, not all intentional behaviors that hurt others are aggressive behaviors. For example, a dentist might intentionally give a patient a shot of novocaine (and the shot hurts!), but the goal is to help rather than hurt the patient. Third, the victim wants to avoid the harm. Thus, again, the dental patient is excluded, because the patient is not seeking to avoid the harm (in fact, the patient probably booked the appointment weeks in advance and paid to have it done!). Suicide would also be excluded, because the person who commits suicide does not want to avoid the harm. Sadomasochism would likewise be excluded, because the masochist enjoys being harmed by the sadist.

AggressionThe motives for aggression might differ. Consider two examples. In the first example, a husband finds his wife and her lover together in bed. He takes his hunting rifle from a closet and shoots and kills both individuals. In the second example, a “hitman” uses a rifle to kill another person for money. The motives appear quite different in these two examples. In the first example, the man appears to be motivated by anger. He is enraged when he finds his wife making love to another man, so he shoots both of them. In the second example, the hit-man appears to be motivated by money. The hitman probably does not hate his victim. He might not even know his victim, but he kills that person anyway for the money. To capture different types of aggression based on different motives, psychologists have made a distinction between hostile aggression (also called affective, angry, impulsive, reactive, or retaliatory aggression) and instrumental aggression (also called proactive aggression). Hostile aggression is “hot,” impulsive, angry behavior that is motivated by a desire to harm someone. Instrumental aggression is “cold,” premeditated, calculated behavior that is motivated by some other goal (e.g., obtain money, restore one’s image, restore justice).

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One difficulty with the distinction between hostile and instrumental aggression is that the motives for aggression are often mixed. Consider the following example. On April 20, 1999, the 110th anniversary of Adolf Hitler’s birthday, Eric Harris and Dylan Klebold entered their high school in Littleton, Colorado (United States), with weapons and ammunition. They murdered 13 students and wounded 23 others before turning the guns on themselves. Harris and Klebold were repeatedly angered and provoked by the athletes in their school. However, they planned the massacre more than a year in advance, did research on weapons and explosives, made drawings of their plans, and conducted rehearsals. Was this an act of hostile or instrumental aggression? It is hard to say. That is why some social psychologists have argued that it is time to get rid of the distinction between hostile and instrumental aggression.

Another distinction is between displaced and direct aggression. Displaced aggression (also called the “kicking the dog” effect) involves substituting the target of aggression: The person has an impulse to attack one person but attacks someone else instead. Direct aggression involves attacking the person who provoked you. People displace aggression for several reasons. Directly aggressing against the source of provocation may be unfeasible because the source is unavailable (e.g., the provoker has left the situation) or because the source is an intangible entity (e.g., hot temperature, loud noise, foul odor). Fear of retaliation or punishment from the provoker may also inhibit direct aggression. For example, an employee who is reprimanded by his boss may be reluctant to retaliate because he does not want to lose his job.

Violence is aggression that has extreme physical harm as its goal, such as injury or death. For example, one child intentionally pushing another off a tricycle is an act of aggression but is not an act of violence. One person intentionally hitting, kicking, shooting, or stabbing another person is an act of violence. Thus, all violent acts are aggressive acts, but not all aggressive acts are violent; only the extreme ones are.

Is Aggression Innate or Learned?

For decades, psychologists have debated whether aggression is innate or learned. Instinct theories propose that the causes of aggression are internal, whereas learning theories propose that the causes of aggression are external. Sigmund Freud argued that human motivational forces such as sex and aggression are based on instincts. In his early writings, Freud proposed the drive for sensory and sexual gratification as the primary human instinct, which he called eros. After witnessing the horrors of World War I, however, Freud proposed that humans also have a destructive, death instinct, which he called thanatos.

According to Konrad Lorenz, a Nobel Prize-winning scientist, aggressive behavior in both humans and nonhumans comes from an aggressive instinct. This aggressive instinct presumably developed during the course of evolution because it promoted survival of the human species. Because fighting is closely linked to mating, the aggressive instinct helped ensure that only the strongest individuals would pass on their genes to future generations.

Other psychologists have proposed that aggression is not an innate drive, like hunger, in search of gratification. According to Albert Bandura’s social learning theory, people learn aggressive behaviors the same ways they learn other social behaviors—by direct experience and by observing others. When people observe and copy the behavior of others, this is called modeling. Modeling can weaken or strengthen aggressive responding. If the model is rewarded for behaving aggressively, aggressive responding is strengthened in observers. If the model is punished for behaving aggressively, aggressive responding is weakened in observers.

This nature versus nurture debate has frequently generated more heat than light. Many experts on aggression favor a middle ground in this debate. There is clearly a role for learning, and people can learn how to behave aggressively. Given the universality of aggression and some of its features (e.g., young men are always the most violent individuals), and recent findings from heritability studies, there may be an innate basis for aggression as well.

Some Factors Related to Aggression

Frustration and Other unpleasant Events

In 1939, a group of psychologists from Yale University published a book titled Frustration and Aggression. In this book, they proposed the frustration-aggression hypothesis, which they summarized on the first page of their book with these two statements: (1) “The occurrence of aggressive behavior always presupposes the existence of frustration” and (2) “the existence of frustration always leads to some form of aggression.” They defined frustration as blocking goal-directed behavior, such as when someone crowds in front of you while you are waiting in a long line. Although they were wrong in their use of the word always, there is no denying the basic truth that aggression is increased by frustration.

Fifty years later, Leonard Berkowitz modified the frustration-aggression hypothesis by proposing that all unpleasant events—instead of only frustration—deserve to be recognized as causes of aggression. Other examples of unpleasant events include hot temperatures, crowded conditions, foul odors, secondhand smoke, air pollution, loud noises, provocations, and even pain (e.g., hitting your thumb with a hammer).

All of these unpleasant environmental factors probably increase aggression because they make people feel bad and grumpy. But why should being in a bad mood increase aggression? One possible explanation is that angry people aggress because they think it will make them feel better. Because many people believe that venting is a healthy way to reduce anger and aggression, they might vent by lashing out at others to improve their mood. However, research has consistently shown that venting anger actually increases anger and aggression.

It is important to point out that like frustration, being in a bad mood is neither a necessary nor a sufficient condition for aggression. All people in a bad mood do not behave aggressively, and all aggressive people are not in a bad mood.

Aggressive Cues

Weapons. Obviously using a weapon can increase aggression and violence, but can just seeing a weapon increase aggression? The answer is yes. Research has shown that the mere presence of a weapon increases aggression, an effect called the weapons effect.

Violent Media. Content analyses show that violence is a common theme in many types of media, including television programs, films, and video games. Children are exposed to approximately 10,000 violent crimes in the media per year. The results from hundreds of studies have shown that violent media increase aggression. The magnitude of the effect of violent media on aggression is not trivial either. The correlation between TV violence and aggression is only slightly smaller than that correlation between smoking and lung cancer. Smoking provides a useful analogy for thinking about media violence effects. Not everyone who smokes gets lung cancer, and not everyone who gets lung cancer is a smoker. Smoking is not the only factor that causes lung cancer, but it is an important factor. Similarly, not everyone who consumes violent media becomes aggressive, and not everyone who is aggressive consumes violent media. Media violence is not the only factor that causes aggression, but it is an important factor. Like the first cigarette, the first violent movie seen can make a person nauseous. After repeated exposure, however, the person craves more and more. The effects of smoking and viewing violence are cumulative. Smoking one cigarette probably will not cause lung cancer. Likewise, seeing one violent movie probably will not make a person more aggressive. But repeated exposure to both cigarettes and media violence can have harmful long-term consequences.

Chemical Influences

Numerous chemicals have been shown to influence aggression, including testosterone, cortisol, serotonin, and alcohol.

Testosterone. Testosterone is the male sex hormone. Both males and females have testosterone, but males have a lot more of it. Testosterone has been linked to aggression. Robert Sapolsky, author of The Trouble With Testosterone, wrote, “Remove the source of testosterone in species after species and levels of aggression typically plummet. Reinstate normal testosterone levels afterward with injections of synthetic testosterone, and aggression returns.”

Cortisol. A second hormone that is important to aggression is cortisol. Cortisol is the human stress hormone. Aggressive people have low cortisol levels, which suggests that they experience low levels of stress. How can this explain aggression? People who have low cortisol levels do not fear the negative consequences of their behavior, so they might be more likely to engage in aggressive behavior. Also, people who have low cortisol become easily bored, which might lead to sensation-seeking behavior such as aggression.

Serotonin. Another chemical influence is serotonin. In the brain, information is communicated between neurons (nerve cells) by the movement of chemicals across a small gap called the synapse. The chemical messengers are called neurotransmitters. Serotonin is one of these neurotransmitters. It has been called the “feel good” neurotransmitter. Low levels of serotonin have been linked to aggression in both animals and humans. For example, violent criminals have a serotonin deficit.

Alcohol. Alcohol has long been associated with violent and aggressive behavior. Well over half of violent crimes are committed by individuals who are intoxicated. Does all of this mean that aggression is somehow contained in alcohol? No. Alcohol increases rather than causes violent or aggressive tendencies. Factors that normally increase aggression, such as provocation, frustration, aggressive cues, and violent media, have a much stronger effect on intoxicated people than on sober people.

Self and Culture

Norms and Values. Amok is one of the few Indonesian words used in the English language. The term dates back to 1665, and describes a violent, uncontrollable frenzy. Running amok roughly translated means “going berserk.” A young Malay man who had suffered some loss of face or other setback would run amok, recklessly performing violent acts. The Malays believed it was impossible for young men to restrain their wild, aggressive actions under those circumstances. However, when the British colonial administration disapproved of the practice and began to hold the young men responsible for their actions, including punishing them for the harm they did, most Malays stopped running amok.

The history of running amok thus reveals three important points about aggression. First, it shows the influence of culture: The violence was accepted in one culture and prohibited in others, and when the local culture changed, the practice died out. Second, it shows that cultures can promote violence without placing a positive value on it. There is no sign that the Malays approved of running amok or thought it was a good, socially desirable form of action, but positive value wasn’t necessary. All that was needed was for the culture to believe that it was normal for people to lose control under some circumstances and act violently as a result. Third, it shows that when people believe their aggression is beyond control, they are often mistaken—the supposedly uncontrollable pattern of running amok died out when the British cracked down on it. The influence of culture was thus mediated through self-control.

Self-Control. In 1990, two criminologists published a book called A General Theory of Crime. Such a flamboyant title was bound to stir controversy. After all, there are many crimes and many causes, and so even the idea of putting forward a single theory as the main explanation was pretty bold. What would their theory feature: Poverty? Frustration? Genetics? Media violence? Bad parenting? As it turned out, their main theory boiled down to poor self-control. The authors provided plenty of data to back up their theory. For one thing, criminals seem to be impulsive individuals who simply don’t show much respect for norms, rules, and standards of behavior. If self-control is a general capacity for bringing one’s behavior into line with rules and standards, criminals lack it. Another sign is that the lives of criminals show low self-control even in behaviors that are not against the law (e.g., smoking cigarettes).

Social psychology has found many causes of violence, including frustration, anger or insult, alcohol intoxication, violence in the media, and hot temperatures. This raises the question of why there isn’t more violence than there is. After all, who hasn’t experienced frustration, anger, insult, alcohol, media violence, or hot weather in the past year? Yet most people do not hurt or kill anyone. These factors may give rise to violent impulses, but most people restrain themselves. Violence starts when self-control stops.

Culture of Honor. The southern United States has long been associated with greater levels of violent attitudes and behaviors than the northern United States. In comparison to northern states, southern states have more homicides per capita, have fewer restrictions on gun ownership, allow people to shoot assailants and burglars without retreating first, are more accepting of corporal punishment of children at home and in schools, and are more supportive of any wars involving U.S. troops.

Social psychologist Richard Nisbett hypothesized that these regional differences are caused by a southern culture of honor, which calls for violent response to threats to one’s honor. This culture apparently dates back to the Europeans who first came to the United States. The northern United States was settled by English and Dutch farmers, whereas the southern United States was settled by Scottish and Irish herders. A thief could become rich quick by stealing another person’s herd. The same was not true of agricultural crops in the North. It is difficult to quickly steal 50 acres of corn. Men had to be ready to protect their herds with a violent response. A similar culture of violence exists in the western United States, or the so-called Wild West, where a cowboy could also lose his wealth quickly by not protecting his herd. (Cowboys herded cows, hence the name.) This violent culture isn’t confined to the southern and western United States; cultural anthropologists have observed that herding cultures throughout the world tend to be more violent than agricultural cultures.

Humiliation appears to be the primary cause of violence and aggression in cultures of honor. Humiliation is a state of disgrace or loss of self-respect (or of respect from others). It is closely related to the concept of shame. Research shows that feelings of shame frequently lead to violent and aggressive behavior. In cultures of honor there is nothing worse than being humiliated, and the appropriate response to humiliation is swift and intense retaliation.

Age and Aggression

Research has shown that the most aggressive human beings are toddlers, children 1 to 3 years old. Researchers observing toddlers in daycare settings have found that about 25% of the interactions involve some kind of physical aggression (e.g., one child pushes another child out of the way and takes that child’s toy). High aggression rates in toddlers are most likely due to the fact that they still lack the means to communicate in more constructive ways. No adult group, not even violent youth gangs or hardened criminals, resorts to physical aggression 25% of the time.

Young children do not commit many violent crimes, especially as compared to young men. This is most likely due to the fact that young children can’t do much physical damage, because they are smaller and weaker.

Longitudinal studies show that serious aggressive and violent behavior peaks just past the age of puberty. After the age of 19, aggressive behaviors begin to decline. However, a relatively small subgroup of people continue their aggressive behavior after adolescence. These “career criminals” typically started violent offending in early life. The earlier the onset of aggressive or violent behavior is, the greater is the likelihood that it will continue later in life.

Gender and Aggression

In all known societies, young men just past the age of puberty commit most of the violent crimes. Rarely women. Rarely older men. Rarely young children. Research shows that males are more physically aggressive than females, but this difference shrinks when people are provoked. Males are also more verbally aggressive than females, although the difference is much smaller. Females are often taught to be less direct in expressing aggression, so they often resort to more indirect forms of aggression. When it comes to relational aggression, for example, females are more aggressive than males. Relational aggression is defined as intentionally harming someone’s relationships with others. Some examples of relational aggression include saying bad things about people behind their backs, withdrawing affection to get what you want, and excluding others from your circle of friends. Thus, rather than simply stating that males are more aggressive than females, it is more accurate to state that both sexes can behave aggressively, but they tend to engage in different types of aggression.

Aggression and Biased Social Information Processing

People do not passively respond to the things happening around them, but they actively try to perceive, understand, and attach meaning to these events. For example, when someone bumps a shopping cart into your knee in the local supermarket, you will likely do more than just feel the pain and take another carton of milk from the shelf. Instead, you will try to make sense of what happened to you (often this occurs automatically and so fast that you’re not even aware of it): Why did this person bump me? Was it an accident or was it intentional?

According to the social information processing model, the way people process information in a situation can have a strong influence on how they behave. In aggressive people, the processing of social information takes a different course than in nonaggressive people. For example, aggressive people have a hostile perception bias. They perceive social interactions as more aggressive than nonaggressive people do. Aggressive people pay too much attention to potentially hostile information and tend to overlook other types of information. They see the world as a hostile place. Aggressive people have a hostile expectation bias. They expect others to react to potential conflicts with aggression. Furthermore, aggressive people have a hostile attribution bias. They assume that others have hostile intentions. When people perceive ambiguous behaviors as stemming from hostile intentions, they are much more likely to behave aggressively than when they perceive the same behaviors as stemming from other intentions. Finally, aggressive people are more likely than others to believe that “aggression pays.” In estimating the consequences of their behavior, they are overly focused on how to get what they want, and they do not focus much on maintaining good relationships with others. This is why aggressive people often choose aggressive solutions for interpersonal problems and ignore other solutions.

Intervening With Aggression and Violence

Most people are greatly concerned about the amount of aggression in society. Most likely, this is because aggression directly interferes with people’s basic needs of safety and security. Accordingly, it is urgent to find ways to reduce aggression. Aggression has multiple causes. Unpleasant events, biased social information processing, violent media, and reduced self-control are just some of the factors that can increase aggression. The fact that there is no single cause for aggression makes it difficult to design effective interventions. A treatment that works for one individual may not work for another individual. One subgroup of extremely aggressive and violent people, psychopaths, is even believed to be untreatable. Indeed, many people have started to accept the fact that aggression and violence have become an inevitable, intrinsic part of society.

This being said, there certainly are things that can be done to reduce aggression and violence. Although aggression intervention strategies will not be discussed in detail here, there are two important general points to be made. First, successful interventions target as many causes of aggression as possible and attempt to tackle them collectively. Most often, these interventions are aimed at reducing factors that promote aggression in the direct social environment (family, friends), general living conditions (housing and neighborhood, health, financial resources), and occupation (school, work, spare time). Common interventions include social competence training, family therapy, parent management training (in children and juveniles), or a combination of these. Interventions that are narrowly focused at removing a single cause of aggression, however well conducted, are bound to fail.

Aggression is often stable over time, almost as stable as intelligence. If young children display excessive levels of aggression (often in the form of hitting, biting, or kicking), it places them at high risk for becoming violent adolescents and even violent adults. It is much more difficult to alter aggressive behaviors when they are part of an adult personality than when they are still in development. Thus, as a second general rule, it is emphasized that aggressive behavior problems are best treated in early development, when they are still malleable. The more able professionals are to identify and treat early signs of aggression, the safer our communities will be.


  1. Anderson, C. A., & Bushman, B. J. (2002). Human aggression. Annual Review of Psychology, 53, 27-51.
  2. Bushman, B. J., & Anderson, C. A. (2001). Is it time to pull the plug on the hostile versus instrumental aggression dichotomy? Psychological Review, 108, 273-279.