In psychology and other social and behavioral sciences, aggression refers to behavior that is intended to cause harm. Aggression can be physical, mental, or verbal. Aggression is not the same thing as assertiveness.
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.
The hypothalamus and periaqueductal gray of the midbrain are the most critical areas controlling aggression in mammals, as shown in studies on cats, rats, and monkeys. These brain areas control the expression of all the behavioral and autonomic components of aggression in these species, including vocalization. They have direct connections with both the brainstem nuclei controlling these functions and areas such as the amygdala and prefrontal cortex.
Electrical stimulation of the hypothalamus causes aggressive behavior the hypothalamus expresses receptors that help determine aggression levels based on their interactions with the neurotransmitters serotonin and vasopressin.
The amygdala is also critically involved in aggression. Stimulation of the amygdala results in augmented aggressive behavior in hamsters, while lesions of an evolutionarily homologous area in the lizard greatly reduce competitive drive and aggression(Bauman et al 2006). Several experiments in attack-primed Syrian Golden Hamsters support the claim of the amygdala being involved in control of aggression. Using expression of c-fos as a neuroanatomically localized marker of activity, the neural circuitry involved in the state of “attack readiness” in attack primed hamsters was studied. The results showed that certain structures of the amygdala were involved in aggressiveness: the medial nucleus and the cortical nuclei showed distinct differences in involvement as compared to other structures such as the lateral and basolateral nuclei and central nucleus of the amygdala, which were not associated with any substantial changes in aggressiveness. In addition, c-fos expression was found most clearly in the most dorsal and caudal aspects of the corticomedial amygdala (CMA). In the same study, it was also shown that lesions of the CMA significantly reduced the number of aggressive behaviors. Eight of eleven subjects failed to attack. Also a correlation between lesion site and attack latency was determined: the more anterior the lesion, the longer mean elapsed time to the aggressive behavior .
The prefrontal cortex (PFC) has been implicated in aggressive psychopathology. Reduced activity of the prefrontal cortex, in particular its medial and orbitofrontal portions, has been associated with violent/antisocial aggression. Specifically, regulation of the levels of the neurotransmitter serotonin in the PFC has been connected with a particular type of pathological aggression, induced by subjecting genetically predisposed, aggressive, wild-type mice to repeated winning experience; the male mice selected from aggressive lines had lower serotonin tissue levels in the PFC than the low-aggressive lines in this study .
Another line of research has focused more on the effects of circulating testosterone on the nervous system mediated by local metabolism within the brain. Testosterone can be metabolized to 17b-estradiol by the enzyme aromatase or to 5a-dihydrotestosterone by 5a-reductase. Aromatase is highly expressed in regions involved in the regulation of aggressive behavior, such as the amygdala and hypothalamus. In studies using genetic knock out techniques in inbred mice, male mice that lacked a functional aromatase enzyme displayed a marked reduction in aggression. Long-term treatment of these mice with estradiol partially restored aggressive behavior, suggesting that the neural conversion of circulating testosterone to estradiol and its effect on estrogen receptors affects inter-male aggression. Also, two different estrogen receptors, ERa and ERb, have been identified as having the ability to exert different effects on aggression. In studies using estrogen receptor knockout mice, individuals lacking a functional ERa displayed markedly reduced inter-male aggression while male mice that lacked a functional ERb exhibited normal or slightly elevated levels of aggressive behavior. These results imply that ERa facilitates male-male aggression, where as ERb may inhibit aggression. However, different strains of mice show the opposite pattern in that aromatase activity is negatively correlated with aggressive behavior. Also, in a different strain of mice the behavioral effect of estradiol is dependent on daylength: under long-days (16h of light) estradiol reduces aggression, and under short-days (8h of light) estradiol rapidly increases aggression .
Glucocorticoids also play an important role in regulating aggressive behavior. In adult rats, acute injections of corticosterone promote aggressive behavior and acute reduction of corticosterone decreases aggression; however, a chronic reduction of corticosterone levels can produce abnormally aggressive behavior. In addition, glucocorticoids affect development of aggression and establishment of social hierarchies. Adult mice with low baseline levels of corticosterone are more likely to become dominant than are mice with high baseline corticosterone levels .
Dehydroepiandrosterone (DHEA) is the most abundant circulating androgen and can be rapidly metabolized within target tissues into potent androgens and estrogens. Gonadal steroids generally regulate aggression during the breeding season, but non-gonadal steroids may regulate aggression during the non-breeding season. Castration of various species in the non-breeding season has no effect on territorial aggression. In several avian studies, circulating DHEA has been found to be elevated in birds during the non-breeding season. These data support the idea that non-breeding birds combine adrenal and/or gonadal DHEA synthesis with neural DHEA metabolism to maintain territorial behavior when gonadal testosterone secretion is low. Similar results have been found in studies involving different strains of rats, mice, and hamsters. DHEA levels also have been studied in humans and may play a role in human aggression. Circulating DHEAS (its sulfated ester) levels rise during adrenarche (~7 years of age) while plasma testosterone levels are relatively low. This implies that aggression in pre-pubertal children with aggressive conduct disorder might be correlated with plasma DHEAS rather than plasma testosterone, suggesting an important link between DHEAS and human aggressive behavior .
Another chemical messenger with implications for aggression is the neurotransmitter serotonin. In various experiments, serotonin action was shown to be negatively correlated with aggression(Delville et al. 1997). This correlation with aggression helps to explain the aggression-reducing effects of selective serotonin reuptake inhibitors such as fluoxetine (Delville et al. 1997), aka prozac.
While serotonin and testosterone have been the two most researched chemical messengers with regards to aggression, other neurotransmitters and hormones have been shown to relate to aggressive behavior as well. The neurotransmitter vasopressin causes an increase in aggressive behavior when present in large amounts in the anterior hypothalamus (Delville et al. 1997). The effects of norepinephrine, cortisol, and other neurotransmitters are still being studied.
Like most behaviors, aggression can be examined in terms of its ability to help an animal reproduce and survive. Animals may use aggression to gain and secure territories, as well as other resources including food, water, and mating opportunities. Researchers have theorized that aggression and the capacity for murder are products of our evolutionary past.
An animal defending itself against a predator becomes aggressive in order to survive and to ensure the survival of its offspring. 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. 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 it's relatives and progeny would be more able to avoid predators.
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.
There are many different theories that try to explain how males and females developed these different aggressive tendencies. One theory states that in species where one sex makes a higher parental investment than the other, the higher investing sex is a resource for which the other sex competes: in the majority of species, females are the higher investing sex. It also holds that the main objective of survival is to have the chance to pass on an individual’s genetic material. For males, it is of crucial importance to establish dominance and resource holding to obtain reproductive opportunities in order to pass on their genetics. Unlike females, whose reproductive success is constrained by long gestation and lactation periods, male reproductive success is constrained by the number of partners they can mate with. As a result, males employ physical aggression more often than females; they take more risks in order to compete with other males and gain an elevation of status. Males even go as far as killing one another and offspring belonging to other males. Males demonstrate less concern for their physical welfare in such competitions. In contrast, females compete with one another for resources, which can be converted to offspring. The establishment of dominance is more costly for females than for males and females have less to gain from achieving status. The female presence is more critical to the offspring’s survival and hence her reproductive success than is the father’s. It is only logical then that the health and well being of females would cause them to use less aggressive, low risk, and indirect strategies to acquire resources. As a result, in the majority of female-female conflicts, females rarely inflict serious damage to one another over resources. When translated to human, these facts suggest that women should be expected to show less evidence of dominance hierarchies than men do. In society, aggression in boys becomes increasingly motivated by issues of social status and self-esteem, which are usually decided by varying degrees of aggressive reactivity to personal challenge. Aggression in girls, focusing mainly on resource acquisition and not status, is more likely to take less physically dangerous and more covert forms of indirect aggression . There are, however, extensive critiques of the use of animal behavior to explain human behavior and the application of evolutionary explanations of contemporary human behavior.
Although humans are similar to non-human animals in some aspects of aggression, they differ from most of these animals in the complexity of their aggression because of factors such as culture, morals, and social situations. A wide variety of studies have been done on these situations.
Empirical cross-cultural research has found differences in the level of aggression between cultures. In one study, American men resorted to physical aggression more readily than Japanese or Spanish men, whereas Japanese men preferred direct verbal conflict more than their American and Spanish counterparts (Andreu et al. 1998). Within American culture, southerners were shown to become more aroused and to respond more aggressively than northerners when affronted (Bowdle et al. 1996). There is also a higher homicide rate among young white southern men than among white northern men in the United States (Nisbett 1993). Changes in dominant behavior or in social status causes changes in testosterone levels. Reports of changes in testeosterone of young men during athletic events, which involve face-to-face competition with a winner and a loser, reveal that testosterone rises shortly before their matches, as if in anticipation of the competition. Also, one to two hours after the competitive match, the testosterone levels of the winners are high relative to those levels of the losers . It is also important to take into account the type of conflict that is occurring when assessing aggression. Is the conflict between groups, within a group, within a family? The sex of those involved in the conflict is also critical. Male-male, male-female and female-female encounters should all be clearly distinguished from one another. Same sex encounters are more frequent than inter-sex encounters and this could affect the level of aggression present .
Behaviors like aggression can be learned by watching and imitating the behavior of others. A considerable amount of evidence suggests that watching violence on television increases the likelihood of short-term aggression in children (Aronson, Wilson, & Akert, 2005), though for a dissenting viewpoint, see Freedman (2002). Individuals may differ in how they respond to violence. The greatest impact is on those who are already prone to violent behavior. Adults may be influenced by violence in 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 (Aronson, Wilson, & Akert, 2005).
Although exposure to violence in media is associated with likelihood of short-term increases in aggression, none of these studies provide evidence for a definitive causal mechanism. Instead, violence in media may be one of many factors, or it may play a maintenance role since violent media tend to be selected by people who are prone to violence.
Frustration is another major cause of aggression. The frustration-aggression theory states that aggression increases if a person feels that he or she is being blocked from achieving a goal (Aronson et al. 2005). 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 line (Harris 1974). 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 (Kulik & Brown 1979).
There is some evidence to suggest that the presence of violent objects such as a gun can trigger aggression. In a study done by Leonard Berkowitz and Anthony Le Page (1967), college students were made angry and then left in the presence of a gun or badminton racket. They were then led to believe they were delivering electric shocks to another student, as in the Milgram experiment. Those who had been in the presence of the gun administered more shocks. It is possible that a violence-related stimulus increases the likelihood of aggressive cognitions by activating the semantic network.
The Bobo doll experiment was conducted by Albert Bandura in 1961. In this work, Bandura found that children exposed to aggressive adult model acted more aggressively than those who were exposed to a nonaggressive adult model. This experiment suggests that anyone who comes in contact with and interacts with children can have an impact on the way they react and handle situations.. Summaries of best practice recommendations
(1) American Academy of Pediatrics : 'Set firm, consistent limits to help children self monitor emotions and behavior; make sure all care takers agree to the same limits. Provide examples of effective and socially acceptable ways of managing anger; be careful not to reinforce aggression with aggressive forms of punishment. Also, model acceptable behavior as a caretaker by managing your own temper. Remember that occasional outbursts are normal. If aggressive behavior continues for more than a few weeks, consult a pediatrician or mental health professional.'
(2) National Association of School Psychologists: 'Overly aggressive behavior can signify a social skills deficit; direct instruction, modeling, and coaching can help children acquire the skill of assertion, which as a replacement behavior may help prevent aggressive behavior.' (NASP Best Practices in School Psychology 2002):