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In psychology, aggression refers to behaviour that is intended to cause harm or pain. Aggression can be either physical or verbal, and behaviour is classified as aggression even if it does not succeed in causing harm or pain. Behaviour that accidentally causes harm or pain is not aggression. Aggression is not the same as assertiveness.

Aggression is a perplexing phenomenon. It is well known that aggression is not a simple, single category of behaviour. Several classifications of aggression have been proposed, some more elaborate than others. Aggression, unlike some other behaviours, has no biological function or purpose in isolation. Aggressive interactions occur mostly as part of some other pattern of behaviour; for example, as a strategy to achieve sexual goals or access to preferred foods. More generally they form part of the process whereby individuals define their position in the social groups to which they belong, and hence ensure access to restricted resources without the need for constant conflict — a form of social control. However, this does not rule out the possibility that the performance of aggressive acts can itself be rewarding.

Classification of aggression

Aggression should be viewed as a complex behaviour that is composed of a number of more specific varieties. For example, Moyer (1968) presented an early and influential typology of seven different forms of aggression, from a biological and evolutionary point of view.

  1. Predatory aggression: attack on prey by a predator.
  2. Inter-male aggression: competition between males of the same species over access to females, dominance, status etc.
  3. Fear-induced aggression: aggression associated with attempts to flee from a threat.
  4. Irritable aggression: aggression directed towards an available target induced by some sort of frustration (e.g. schedule-induced aggression).
  5. Territorial aggression: defence of a fixed space against intruders, typically conspecifics.
  6. Maternal aggression: a female's aggression to protect her offspring from a threat. Paternal aggression also exists.
  7. Instrumental aggression: aggression directed towards obtaining some goal, maybe a learned response to a situation.

Currently, there is a consensus for at least two broad categories of aggression, variously known as hostile, affective, or retaliatory aggression, versus instrumental, predatory, or goal-oriented aggression.[1] Empirical research indicates that this is an important difference. Some research indicates that people with tendencies toward affective aggression have lower IQs than those with tendencies toward predatory aggression.[2]

Biology of aggression

Aggression is directed to and often originates from outside stimuli, but has a very distinct internal character. Using various techniques and experiments, scientists have been able to explore the relationships between various parts of the body and aggression.

Aggression in the brain

The area from which all emotion originates is the brain. While scientists continue to test various areas of the brain for their effects on aggression, two areas that directly regulate or affect aggression have been found. The amygdala has been shown to be an area that causes aggression. Stimulation of the amygdala results in augmented aggressive behaviour,[3] while lesions of this area greatly reduce one's competitive drive and aggression.[4] Another area, the hypothalamus, is believed to serve a regulatory role in aggression. The hypothalamus has been shown to cause aggressive behaviour when electrically stimulated[5] but more importantly has receptors that help determine aggression levels based on their interactions with the neurotransmitters serotonin and vasopressin.[6]

Neurotransmitters and hormones

Various neurotransmitters and hormones have been shown to correlate with aggressive behaviour. The most often mentioned of these is the hormone testosterone. Testosterone has been shown to correlate with aggressive behaviour in mice and in some humans,[7] but in contrast to some long-standing theories, various experiments have not shown a relationship between testosterone levels and aggression in humans.[8] The possible correlation between testosterone and aggression could explain the 'roids rage' that results from anabolic steroid use.[9]

Another chemical messenger with implications for aggression is the neurotransmitter serotonin. In various experiments, an serotonin was shown to have a negative correlation with aggression.[10] This correlation with aggression helps to explain the aggression-reducing effects of selective serotonin reuptake inhibitors such as fluoxetine,[11] or as it is commonly known, prozac.

While serotonin and testosterone have been the two most researched chemical messengers with regards to aggression, other neurotransmitters and hormones have been show to relate to aggressive behaviour. The neurotransmitter vasopressin causes an increase in aggressive behaviour when present in large amounts in the anterior hypothalamus.[12] The effects of norepinephrine, cortisol, and other neurotransmitters are still being studied.

Genetics and aggression

Another biological question concerning aggression is that of its heritability. While the exact amount of influence that genetics have is still questioned, studies on animals have shown that a tendency towards aggression is at least partly inherited.[13]

Evolution of aggression

Like most behaviours, aggression can be looked at for its ability to help an animal reproduce and survive, but unlike most behaviours has been shown to decrease an animal's survivorship when increased. There are two general types of aggression, aggression against outsiders and aggression against the own kind. Both types of aggression help and hinder animals evolutionarily.

Aggression against outsiders

The most apparent type of aggression is that seen in the interaction between a predator and its prey.

An animal defending itself against a predator becomes aggressive in order to survive and to ensure the survival of its progeny. Because aggression against a much larger enemy or group of enemies would lead to the death of an animal, animals have developed a good sense of when they are outnumbered or outgunned.[14] This ability to gauge the strength of other animals gives animals a 'fight or flight' response to predators; depending on how strong they gauge the predator to be, animals will either become aggressive or flee.

The need to ensure the continuation of one's genes leads to a phenomenon known as altruism. An example of an altruistic act is the alarm call that is given when a predator is approaching. While this call will inform the community of a predator's presence, it will also inform the predator of the whereabouts of the animal that gave the alarm call. While this would appear to give the alarm caller an evolutionary disadvantage, it would facilitate the continuation of this animal's genes because its relatives and progeny would be more able to avoid predators.[15]

According to many researchers, predation is not aggression. Cats do not hiss or arch their backs when in pursuit of a rat, and the active areas in their hypothalamuses are more similar to those that reflect hunger than those that reflect aggression.[16]

Aggression within a species

Aggression against conspecifics serves a number of purposes having to do with breeding. One of the most common of these purposes is the establishment of a dominance hierarchy. When certain types of animals are first placed in a common environment, the first thing they do is fight to assert their role in the dominance hierarchy.[17] In general, the more dominant animals will be more aggressive than their subordinates.[18] Fighting amongst themselves would not be beneficial for a given species because of the injuries it would cause, so it makes sense that the majority of conspecific aggression ceases about 24 hours after the introduction of the species tested.[19]

In establishing itself in a dominance hierarchy, an animal also sets its attractiveness. In a variety of species, a high social ranking means more and healthier mates. Thus, while aggression carries the risk of wounding the animal it benefits the animal by giving its progeny healthier genes if it is successful in the hierarchy.[20]

Once a female has given birth to an offspring, this female develops maternal aggression. This maternal aggression is directed mainly at conspecifics and is believed intended to prevent the mother's offspring from harassment by other individuals.[21] Maternal aggression is especially strong and often leads to the mother driving away even male conspecific intruders.[22]

Aggression in humans

As can be expected, humans express their aggression differently than animals. While a human is similar to animals in some aspects of aggression, they differ from most animals in the complexity of their aggression because of factors such as culture, morals, and social situations. A wide variety of studies has been done on these situations. Alcohol, pain and discomfort, frustration, and violence on television are just a few of the factors that influence aggression in humans.

Alcohol impairs judgment, making people much less cautious than they usually are.[23] It also disrupts the way information is processed.[24] A drunk person is much more likely to view an accidental event as a purposeful one, and therefore act more aggressively. Pain and discomfort also increase aggression. Even the simple act of placing ones hands in cold water can cause an aggressive response. Heat is very influential as well. A study completed in the midst of the civil rights movement found that riots were more likely on hotter days than colder ones.[25] Students were found to be more aggressive and irritable after taking a test in a hot classroom.[26] Drivers in cars without air conditioning were also found to be more likely to honk their horns.[27]

Frustration may be another major cause of aggression. The frustration-aggression theory states that aggression increases if a person feels that they are being blocked from achieving a goal.[28] Although this is not always the case, it is often true as the following study shows. One study found that the closeness to the goal makes a difference. The study examined people waiting in line and concluded that the 2nd person was more aggressive than the 12th one when someone cut in front of them.[29] Unexpected frustration may be another factor. In a separate study, a group of students were collecting donations over the phone. Some of them were told that the people they would call would be generous and the collection would be very successful. The other group was given no expectations. The group with high expectations was much more upset and became more aggressive when no one was pledging.[30]

Surprisingly, even the presence of violent objects such as a gun can trigger aggression. In a study done by Leonard Berkowitz and Anthony Le Page, college students were made angry and then left in the presence of a gun or badminton racket. Then, they were made to believe they were able to administer shocks to another student. Those who had been in the presence of the gun administered more shocks (1967).

Aggression in the media

There is evidence that aggression can be increased through watching and imitating the behaviour of others. The effect of violence on television has long been a question of interest for psychologists. The majority of the evidence suggests that watching violence on television does increase the likelihood of violent behaviour in children.[31] However, not all humans are equally sensitive to that effect; The greatest impact is on those who are prone to violent behaviour to begin with, but even nonaggressive children become more aggressive as they are exposed to a heavy amount of violent programs. Playing video games has a very similar effect. In one study, a high correlation was found between violent video games and aggressive behaviour, including delinquency. In a follow-up study, researchers used a random sample of children with all levels of aggression and found a 'direct and immediate impact' after the games were played.[32] Of course, adults are influenced by violence in the media as well. A long-term study of over 700 families found 'a significant association' between the amount of time spent watching violent television as a teenager and the likelihood of committing acts of aggression later in life. The results remained the same in spite of factors such as family income, parental education and neighborhood violence.[33] However, history of early childhood aggression has not been controlled. Once again, everyone reacts to aggression differently, but not as differently as some may think.

Although exposure to violence in the media is associated with the risk for violent behaviour, none of these studies provide evidence for a definitive causal mechanism. Instead, violence in the media may have more of a maintenance effect, as violent media tend to be selected by violent prone people. As the developmental work shows (see below) physical aggression tends to decrease over time while cumulative exposure to violence in the media increases. Therefore, there seems to be some important degree of socialization to offset the potential causal effect of violence in the media on human behaviour.

Aggression and gender

Gender is a factor that plays a role in both human and animal aggression, but unlike in animals gender is not indicative of the amount of aggression that a human subject will express. While there is evidence that males are quicker to aggression[34] and more likely than females to express their aggression physically,[35] they are not necessarily more aggressive; women show just as much aggression in a non-physical way.[36]

Aggression and culture

Culture is an entirely humans factor that plays a role in aggression. While other differences in aggression such as gender-related differences remain consistent, cultural differences have a definite relationship with both direct and indirect expressions of aggression. In one study, American men were shown to accept physical aggression more readily than Japanese or Spanish men, while Japanese men preferred direct verbal conflict more than their American and Spanish counterparts.[37] Even within American culture, southerners were shown to generally both become more aroused and to respond more aggressively than northerners when affronted.[38]

Aggression in children

The frequency of physical aggression in humans peaks at around 2-3 years of age. It then declines gradually on average.[39] These observations suggest that physical aggression is mostly not a learned behaviour and that development provides opportunities for the learning of self-regulation. However, a small subset of children fails to acquire the necessary self-regulatory abilities and tends to show atypical levels of physical aggression across development.[40] These may be at risk for later violent behaviour.


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  3. DeCoster et al 1996, Ferris et al 1996
  4. Bauman et al 2006, Crews at al 1984
  5. Hermans et al 1983
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  29. Harris, 1974
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