aggression: West's Encyclopedia of American Law (Full Article) from Answers.com

Aggression has been defined as a behaviour characterized by the intention of inflicting unpleasant stimulation on another individual — but this underestimates its subtlety and omits some important features. 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.

Distinctions have been made between different types of aggression, largely on the basis of the context in which it occurs or the stimuli that provoke it: (i) inter-male, or inter-female (territorial, social conflict, etc.) ; (ii) maternal; (iii) self defence; and (iv) infanticide. Aggression towards members of the same species has been divided more simply into ‘offence’ and ‘defence’. Predation, the hunting of other species, is sometimes included in discussions of aggressive behaviour, but is more properly classed as feeding behaviour.

Human aggression has been separated into ‘emotional’ aggression, carried out by people with the main intention of harming someone, and ‘instrumental’ aggression, with some other objective, such as to obtain something rewarding, rather than specifically to injure a victim. In general, both the form of the aggressive act and the context in which it occurs have to be taken into account.

Social factors

Most species, including human beings, live in social groups whose structure affects access by individuals to items in short supply — such as food, mates, or shelter. Direct aggressive confrontation may be used to determine which individual has priority, but it is more usual that animals come to know, through a process of social learning, who is likely to win in such an encounter. This determines their strategy, and also gives the group its dominance structure. Animals, or people, low in the hierarchy may not challenge those higher in the scale, presumably because of the cost in terms of potential injury. This mechanism of social control, based on previous aggressive interactions, functions to reduce aggression; but it does have potent effects on individuals. If it is to be effective, social control by hierarchy requires extremely sophisticated neural processing; indeed, there are those who claim that the primary function of the human brain is to facilitate social interaction.

Gender influences

There are marked individual and gender-related differences in aggression. In most mammalian species aggression is more common between males than between females. Exposure of the brain to testosterone in the womb may alter infant behaviour: young males show more aggressive-like play than females. Testosterone may also sensitize the individual to the later effects of the same hormones: for example, increasing the likelihood of adult aggressive behaviour, particularly in the context of competition for desirable, but limited, resources. However, giving excess testosterone to men has had rather inconspicuous effects on their aggressive behaviour or tendencies (including thoughts), though levels of testosterone in the saliva have been shown to correlate positively with violent crimes in male prison inmates. But social status also alters testosterone, so the relation between individual differences in aggressivity and testosterone may be indirect. A variety of studies, in both human and non-human primates, has shown that social ‘stress’ (that is, demands made by the social or working environment) lowers testosterone and that ‘dominant’ males have higher levels. However, injecting ‘subordinate’ monkeys with testosterone does not improve their position in the hierarchy, or make them more aggressive.

Aggression is not a male prerogative. It also occurs in females, particularly when they need to defend their young. For example, lactating rats are highly aggressive to intruding males (rather than females). In this context, different hormones may play a role. This aggressive reaction seems to depend upon suckling, and has the obvious biological function of protecting the young. Testosterone given to lactating females actually reduces their aggressive reaction to males.

Are specific parts of the brain involved in aggression?

Since much of aggression in the biological world is part of another behaviour, it is difficult to separate those areas of the brain responsible for the underlying behaviour (getting food, winning mates, etc.) from those associated with the particular behavioural strategy of aggression to achieve these ends. It has been known for many years that damage to a part of the brain called the amygdala results in ‘tameness’ and reduced aggression. The amygdala is part of the limbic system, a set of brain structures particularly concerned with survival, adaptation, and the defence of the body against the metabolic or social demands that constitute stress. The amygdala is closely involved in the ability of the brain to classify stimuli in a motivationally and emotionally meaningful way. Its role is not, therefore, restricted to aggression, but this along with many other behaviours is dependent on proper functioning of this part of the brain. Human cases are known in which disturbances of the amygdala have led to inappropriate or excessive aggression.

Another area of the brain implicated in aggression is the hypothalamus. Lesions or stimulation in several areas of the hypothalamus have altered aggressive interactions. The hypothalamus is implicated in other behaviours. For example, part of it has well-established roles in sexual and maternal behaviour, and it is prominently involved in the regulation of feeding and drinking. Bearing in mind the relation between aggression and other categories of adaptive behaviour, it is clear that there is still uncertainty about the exact role of the hypothalamus in aggression, and whether this can be truly separated from its other adaptive and homeostatic functions. Nevertheless, there are well-documented cases describing humans with tumours in the particular parts of the hypothalamus who became highly aggressive, responding with aggression to stimuli they would have previously considered only annoying. ‘Sedative’ surgical interventions, involving the hypothalamus, have been used in the treatment of aggressive patients.

We have seen that aggression forms an important part of social regulation and social interaction. This is known to involve the cortex of the frontal lobes of the brain. The frontal cortex is also intimately connected with both the amygdala and the hypothalamus and is therefore in a position to influence these other brain centres that control aggression. This behaviour can occur as a feature of frontal lobe damage in man. Patients with damage in one region of the frontal cortex react impulsively, without planning or taking into account the consequences of their behaviour; they are irritable and have short tempers, responding to minor provocation. But the frontal cortex is a complex area of the brain, and it is still not very clear whether particular parts may have distinct roles in aggression.

Are there specific aggression-related chemicals in the brain?

The brain is a chemical machine, and the recognition that different parts can be defined by the chemical transmitters that they use offers a different perspective. In humans, changes in the level and metabolism of serotonin have been correlated with affective behaviour in general and more specifically with aggressive behaviour. Serotonin has become the major focus of biological studies of suicidal behaviour (defined as ‘self-aggression’) and impulsive aggressive behaviour in humans. An association has been reported between low serotonin concentration in the brain and impulsive, destructive behaviours, particularly when aggression and violence are involved. Studies in animals show that a wide range of aggressive behaviours are sensitive to manipulations of the serotonergic system. Depletion of brain serotonin increases aggression. Conversely, serotonergic-enhancing drugs, such as the specific serotonin-uptake inhibitors (SSRIs), reduce aggression. A class of drugs acting on serotonin are known as ‘serenics’; these reduce aggression.

But serotonin is not the only neurochemical implicated in aggression. Animal studies suggest that increasing brain dopamine activity creates a state in which they are more prepared to respond aggressively to stimuli in the environment. Antagonists of dopamine receptors are the most frequently used therapeutic agents in the management of violent patients. However, dopamine has important roles in generalized behavioural categories such as reward or punishment; this may be the real reason why it contributes to the display of aggression.

Hyperactivity of noradrenaline in the brain has been found to correlate with aggressive behaviour in humans, and noradrenergic receptor blockade is clinically useful in its treatment. This is supported by the effects on aggressive behaviours in isolated mice of drugs that modify noradrenaline activity in the brain.

Many peptides are found in the brain, particularly in the limbic system, that act as neurotransmitters. One of these, corticotrophin releasing factor, is present throughout the limbic system. It has an important role in organizing the co-ordinated response to stress; this includes behaviour, hormones, and the emergency systems regulating the cardiovascular and other autonomic responses. It may also increase aggression. Vasopressin (first known as a pituitary hormone) is another peptide found in the limbic system, and microinjections of this into the hypothalamus and amygdala increased offensive aggression in rodents. Although alterations in several peptides, as well as other substances, are known to change aggression, no single one so far has been specifically associated with this behaviour. Clearly, given the current preoccupation with understanding and controlling aggression in man, the existence of such compounds, should they be proved, would be most important.

The complexity of aggression — the behaviour pattern, the contexts in which it occurs, and the uses to which it is put — means that there can never be a single, definable system underlying it. Nevertheless, attempts should continue to define aggression more precisely, since this offers not only greater understanding of the relation between this behaviour and others but also direct help to those who try to control undesirable aggression in either animals or humans.

— J. Herbert

Bibliography

Albert, D. J., Walsh, M. L., and Jonik, R. H. (1993). Aggression in humans: what is its biological foundation? Neuroscience and Biobehavioural Reviews, 17, 405-25.
Archer, J. (1988). Behavioural biology of aggression. Cambridge University Press.
Valzellli, L. (1981). Psychobiology of aggression and violence. Raven Press Books, New York

See also hormones; peptides; serotonin; violence.

For other uses, see Aggression (disambiguation).

Aggression, in its broadest sense, is behavior, or a disposition towards behavior, that is forceful, hostile or attacking. It may occur either in retaliation or without provocation. In narrower definitions that are commonly used in psychology and other social and behavioral sciences, aggression involves an intention to cause harm, even if only as a means to an end. It has alternatively been defined as acts intended to increase relative social dominance. Predatory or defensive behavior between members of different species may not be considered aggression in the same sense. Aggression can take a variety of forms and can be physical or be communicated verbally or non-verbally. Aggression differs from what is commonly called assertiveness, although the terms are often used interchangeably among laypeople, e.g. an aggressive salesperson.^{[citation needed]}

Overview

Two broad categories of aggression are commonly defined. One is described as affective (emotional), hostile or retaliatory aggression, and the other as instrumental, goal-oriented or predatory aggression.^[1] In the context of violence, data from a range of disciplines lend some support to a distinction between affective and predatory aggression.^[2] However, others question the usefulness of a hostile vs instrumental distinction in humans, despite its ubiquity in research, on the basis that in real life most cases involve mixed motives and many factors.^[3]

Another distinction drawn is between aggression that is enacted physically, and relational aggression. The latter can include covert bullying and social manipulation, including isolating others. It has also been understood and dealt with by distinguishing between reactive versus instrumental types.^[4]

A number of other classifications and dimensions of aggression have been advanced. Some questions that have been considered are whether harm to others is intended or not; whether the aggression is verbal or physical; whether it is carried out actively or expressed passively; and whether it is aimed directly or indirectly. Related emotions (e.g. anger) and mental states (e.g. impulsivity, hostility) have also been addressed, as well as behaviors in and of themselves (aggression in the strictest sense).^[5] Aggression may occur in response to non-social as well as social factors, and can have a close relationship with stress coping style.^[6] Aggression may be displayed in order to intimidate.

Aggression may be defined in particular ways within some moral or political views, for example in the non-aggression principle. There are also specific definitions used in relations between countries, such as in the International Criminal Court's proposed jurisdiction to prosecute crimes of aggression.^[7] Furthermore, there may be particular attitudes towards aggression, both sanctioned and unsanctioned, in competitive sports.^[8] Other specific contexts may also be examined, such as workplace aggression.

Etymology

The term aggression comes from the Latin aggressio, meaning attack. The Latin was itself a joining of ad- and gradi-, which meant to step or to go. The first known use dates back to 1611, in the sense of an unprovoked attack.^[9] A psychological sense of 'hostile or destructive behavior' dates back to 1912, in an English translation of the writing of Sigmund Freud.^[10] Alfred Adler had theorized about an 'aggressive drive' in 1908. Child raising experts began to refer to aggression rather than anger from the 1930s.^[11]

Ethology

Male elephant seals fighting

Aggression is one type of behavior addressed by the scientific study of animal behavior known as ethology. Aggression may be involved in the gaining and securing of territories and other resources including food, water and mating opportunities, as well as to protect the self or offspring.^[12] Direct aggression can involve bodily contact such as biting, hitting or pushing. Threat displays and intimidating thrusts may also be classed as aggression, and the majority of conflicts are settled in this way; including through display of body size, antlers, claws or teeth; stereotyped signals within a species; vocalizations including bird song; chemicals; pigments, and aggressive facial expressions.^[13] The concept of agonistic behaviour may alternatively be used to refer to all of the above.

Aggression against outsiders

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

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.^[14] However, others refer to it as predatory aggression, and point out cases that show more similarity such as mouse-killing by rats.^[15] Aggressive mimicry refers to cases where a predator has the appearance of a harmless organism or object, which then attracts the prey, which the predator then attacks.

An animal defending itself against a predator may become aggressive in order to survive and to ensure the survival of its offspring. It may engage in either "fight or flight" in response to predator attack or threat of attack, depending on how strong they gauge the predator to be relative to themselves. Different animals may also use a range of antipredator adaptations, including alarm signals.

In terms of aggression between groups, defined as a willingness to enter a fight, studies suggest that animals may take into account numerical advantage, distance from home territories, how often the groups encounter each other, competitive abilities, differences in body size, and who is intruding on who. In addition, any given individual is more likely to become aggressive the more other aggressive group members there are nearby.^[16] One particular phenomenon - the formation of intense coordinated coalitions to raid neighbouring territories to kill conspecifics - has only been documented in two species in the animal kingdom: 'common' chimpanzees and humans.^[17]

Aggression within a group

Aggression between conspecifics in a group occurs in a number of contexts having to do with access to resources and breeding. One of the most common is in 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.^[18] In general, the more dominant animals will be more aggressive than their subordinates.^[19]^[20] The majority of conspecific aggression ceases about 24 hours after the introduction of the animals being tested.^[18]^[21] Aggression has been defined from this viewpoint as "behavior which is intended to increase the social dominance of the organism relative to the dominance position of other organisms".^[22] Losing confrontations may be referred to as social defeat, and winning or losing is associated with a range of practical and psychological consequences.^[23]

Conflicts of interest occur in many contexts in animal life, such as between potential mating partners, between parents and offspring, and between competitors for resources. Group-living animals may also dispute over the direction of travel or the allocation of time to joint activities. Various factors limit the escalation of aggression, including communicative displays, conventions and routines. In addition, following incidents, various forms of conflict resolution have many observed in mammal species, particularly in gregarious primates. This can mitigate or repair possible ongoing consequences, especially for the recipient of aggression who may become vulnerable to attacks, including possibly from other individuals joining in. Conciliatory acts vary by species and may simply involve more proximity and interaction between the individuals involved in an incident, or specific gestures. Conflicts over food are rarely followed by postconflict reunions, however, even though they are the most frequent type as foraging takes up a large amount of the time of wild primates.^[24]

Questions that have been considered in the study of primate aggression, including in humans, is how aggression maintains the organization of a group; what costs are incurred by aggression; and why some primates avoid aggressive behavior.^[25] For example, bonobo chimpanzee groups are known for low levels of aggression within a partially matriarchal society. Captive animals including primates may show abnormal levels of social aggression and self-harm related to aspects of the physical, sensory or social environment; this depends on the species and individual factors such as gender, age and background (e.g. raised wild or captive).^[26]

Evolutionary explanations

Like many behaviors, aggression can be examined in terms of its ability to help an animal survive and reproduce, or alternatively to risk survival and reproduction. This cost-benefit analysis can be looked at in terms of evolution. There are profound differences in the extent of acceptance of a biological or evolutionary basis for human aggression, however.^[27]

Violence and conflict

Aggression can involve violence that may be adaptive under certain circumstances in terms of natural selection. This is most obviously the case in terms of attacking prey to obtain food, or in anti-predatory defense. It may also be the case in competition between members of the same species or subgroup, if the average reward (e.g. status, access to resources, protection of self or kin) outweighs average costs (e.g. injury, exclusion from the group, death). There are some hypotheses of specific adaptions for violence in humans under certain circumstances, including for homicide, but it is often unclear what behaviors may have been selected for and what may have been a byproduct, as in the case of collective violence.^[28]^[29]^[30]^[31]

Although aggressive encounters are ubiquitous in the animal kingdom, with often high stakes, most are resolved through posturing, displays and trials of strength. Game theory is used to understand how such behaviors might spread by natural selection within a population, and potentially become 'Evolutionary Stable Strategies'. An initial model of resolution of conflicts is the Hawk-Dove game; others include the Sequential assessment model and the Energetic war of attrition. These try to understand not just one-off encounters but protracted stand-offs, and mainly differ in the criteria by which an individual decides to give up rather than risk loss and harm in physical conflict (such as through estimates of Resource holding potential).^[32]

Gender

There are different theories that seek to explain findings that males and females of the same species can have differing aggressive behaviors. In general, sexual dimorphism can be attributed to greater intraspecific competition in one sex, either between rivals for access to mates and/or to be chosen by mates. This may stem from the other gender being constrained by providing greater parental investment, in terms of factors such as gamete production, gestation, lactation, or upbringing of young. There is much variation in the pattern in species but the more physically aggressive sex is usually the male, particularly in mammals. In species where parental care by both sexes is required there tend to be lesser differences, and when the female can leave the male to care then females may be the larger and more physically aggressive; competitiveness despite parental investment has also been observed in some species.^[33] A related factor is the rate at which males and females are able to mate again after producing offspring, and the basic principles of sexual selection are also influenced by ecological factors affecting the ways or extent to which one sex can compete for the other. The role of such factors in human evolution is controversial. The pattern of male and female aggression is argued to be consistent with evolved sexually-selected behavioral differences, while alternative or complimentary views emphasize conventional social roles stemming from physical evolved differences.^[34] Aggression in women may have evolved to be, on average, less physically dangerous and more covert or indirect.^[35]^[36] However, there are extensive critiques of the use of animal behavior to explain human behavior and the application of evolutionary explanations to contemporary human behavior, including differences between the genders.^[37]

Regarding sexual dimorphism, humans falls into an intermediate group with moderate sex differences in body size but relatively large testes. This is a typical pattern of primates where several males and females live together in a group and the male faces an intermediate amount of challenges from other males compared to exclusive polygyny and monogamy but frequent sperm competition.^[38]

Evolutionary psychology and sociobiology have also discussed and produced theories for some specific forms of male aggression such as sociobiological theories of rape and theories regarding the Cinderella effect.

Physiology

Brain pathways

Many researchers focus on the brain to explain aggression. Numerous circuits within both neocortical and subcortical structures play a central role in controlling aggressive behavior, depending on the species, and the exact role of pathways may vary depending on the type of trigger or intention.

In mammals, the hypothalamus and periaqueductal gray of the midbrain are critical areas, as shown in studies on cats, rats, and monkeys. These brain areas control the expression of both behavioral and autonomic components of aggression in these species, including vocalization. Electrical stimulation of the hypothalamus causes aggressive behavior^[39] and the hypothalamus has receptors that help determine aggression levels based on their interactions with serotonin and vasopressin.^[40] These midbrain areas have direct connections with both the brainstem nuclei controlling these functions, and with structures such as the amygdala and prefrontal cortex.

Stimulation of the amygdala results in augmented aggressive behavior in hamsters,^[41]^[42] while lesions of an evolutionarily homologous area in the lizard greatly reduce competitive drive and aggression (Bauman et al. 2006).^[43] In rhesus monkeys, neonatal lesions in the amygdala or hippocampus results in reduced expression of social dominance, related to the regulation of aggression and fear.^[44] Several experiments in attack-primed Syrian Golden hamsters, for example, support the claim of circuity within the amygdala being involved in control of aggression.^[45] The role of the amygdala is less clear in primates and appears to depend more on situational context, with lesions leading to increases in either social affiliatory or aggressive responses.

The broad area of the cortex known as the prefrontal cortex (PFC) has been implicated in aggression, along with many other functions including inhibition of emotions. Reduced activity of the prefrontal cortex, in particular its medial and orbitofrontal portions, has been associated with violent/antisocial aggression.^[46]

The role of brain chemicals, particularly neurotransmitters, has also been examined. This can vary depending on the pathway, the context and factors such as gender. A deficit in serotonin has been thought to have a primary role in causing impulsivity and/or aggression, and be predictive of response to drugs, but the link is now thought to be less clear cut. Nevertheless, low levels of serotonin transmission may explain in part a vulnerability to impulsivity, and potentially aggression, and may also have an effect through interactions with other neurochemical systems. These include dopamine systems which are generally associated with attention and motivation toward rewards, and operate at various levels. Norepinephrine, also known as noradrenaline, may influence aggression responses both directly and indirectly through the hormonal system, the sympathetic nervous system or the central nervous system (including the brain). It appears to have different effects depending on the type of triggering stimulus, for example social isolation/rank versus shock/chemical agitation, and to not have a linear relationship with aggression. Similarly, GABA, although associated with inhibitory functions at many CNS synapses, sometimes shows a positive correlation with aggression, including when potentiated by alcohol.^[47]^[48]

The hormonal neuropeptides vasopressin and oxytocin play a key role in complex social behaviours in many mammals, regulating attachment, social recognition and aggression. Vasopressin has been mainly implicated in male-typical social behaviours including aggression. Oxytocin may have a particular role in regulating female bonds with offspring and mates, including the use of protective aggression. Initial studies in humans suggest some similar effects.^[49]^[50]

Testosterone

Hormones are chemicals that circulate in the body affecting cells and the nervous system, including the brain. Testosterone is a steroid hormone from the androgen group, which is most linked to the prenatal and postnatal development of the male gender and physique, which in turn has been linked on average to more physical aggression in many species. Testosterone is present to a lesser extent in females, who may be more sensitive to its effects. Animal studies have also indicated a link between incidents of aggression and the individual level of circulating testosterone. However, results in relation to primates, particularly humans, are less clear cut and are at best only suggestive of a positive association in some contexts.^[51]

Challenge Hypothesis

Washington State Song Sparrow

The challenge hypothesis outlines the dynamic relationship between plasma testosterone levels and aggression in mating contexts in many species. It proposes that testosterone is linked to aggression when it is beneficial for reproduction, such as in mate guarding and preventing the encroachment of intrasexual rivals. The challenge hypothesis predicts that seasonal patterns in testosterone levels in a species are a function of mating system (monogamy versus polygyny), paternal care, and male-male aggression in seasonal breeders. This pattern between testosterone and aggression was first observed in seasonally breeding birds, such as the Song Sparrow, where testosterone levels rise modestly with the onset of the breeding season to support basic reproductive functions.^[52] It has subsequently expanded and been modified to predict relationships between testosterone and aggression in other species. For example, chimpanzees, which are continuous breeders, show significantly raised testosterone levels and aggressive male-male interactions when receptive and fertile females are present.^[53] Currently, no research has specified a relationship between the modified challenge hypothesis and human behavior, or the human nature of concealed ovulation, although some suggest it may apply.^[54]

Effects on the nervous system

Testosterone to Estradiol conversion

Another line of research has focused on the proximate effects of circulating testosterone on the nervous system, as mediated by local metabolism within the brain. Testosterone can be metabolized to 17b-estradiol by the enzyme aromatase, or to 5-alpha-dihydrotestosterone (DHT) 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 with estradiol partially restored aggressive behavior, suggesting that the neural conversion of circulating testosterone to estradiol and its effect on estrogen receptors influences inter-male aggression. In addition, two different estrogen receptors, ERa and ERb, have been identified as having the ability to exert different effects on aggression in mice. However, the effect of estradiol appears to vary depending on the strain of mouse, and in some strains it reduces aggression during long days (16 h of light), while during short days (8 h of light) estradiol rapidly increases aggression.^[55]

Another hypothesis is that testosterone influences brain areas that control behavioral reactions. Studies in animal models indicate that aggression is affected by several interconnected cortical and subcortical structures within the so-called social behavior network. A study involving lesions and electrical-chemical stimulation in rodents and cats revealed that such a neural network consists of the medial amygdala, medial hypothalamus and periaqueductal grey (PAG), and it positively modulates reactive aggression.^[56] Moreover, a study done in human subjects showed that prefrontal-amygdala connectivity is modulated by endogenous testosterone during social emotional behavior.^[57]

In human studies, testosterone-aggression research has also focused on the role of the orbitofrontal cortex (OFC). This brain area is strongly associated with impulse control and self-regulation systems that integrate emotion, motivation, and cognition to guide context-appropriate behavior.^[58] Patients with localized lesions to the OFC engage in heightened reactive aggression.^[59] Aggressive behavior may be regulated by testosterone via reduced medial OFC engagement following social provocation.^[58] When measuring participants’ salivary testosterone, higher levels can predict subsequent aggressive behavioral reactions to unfairness faced during a task. Moreover, brain scanning with fMRI shows reduced activity in the medial OFC during such reactions. Such findings may suggest that a specific brain region, the OFC, is a key factor in understanding reactive aggression.

General associations with behavior

Research on the relationship between testosterone and aggression in people is difficult since the only reliable measurement of brain testosterone is by a lumbar puncture, which is not done for research purposes. Studies therefore have often instead used more unreliable measurements from blood or saliva.^[60]

In humans, males engage in crime and especially violent crime more than females. The involvement in crime usually rises in the early teens to mid teens which happen at the same time as testosterone levels rise. Most studies support a link between adult criminality and testosterone although the relationship is modest if examined separately for each sex. However, nearly all studies of juvenile delinquency and testosterone are not significant. Most studies have also found testosterone to be associated with behaviors or personality traits linked with criminality such as antisocial behavior and alcoholism.^[60]

Many studies have been done on the relationship between more general aggressive behavior/feelings, including antisocial behavior and crime, and testosterone. About half the studies have found a relationship and about half no relationship.^[60] Concentration of testosterone may most clearly correlate with aggressive responses involving provocation. However, a correlation between testosterone levels and aggression does not prove a causal role for testosterone.

Studies of testosterone levels of male athletes before and after a competition revealed that testosterone levels rise shortly before their matches, as if in anticipation of the competition, and are dependent on the outcome of the event: testosterone levels of winners are high relative to those of losers. No specific response of testosterone levels to competition was observed in female athletes, although a mood difference was noted.^[61] In addition, some experiments have failed to find a relationship between testosterone levels and aggression in humans.^[62]^[63]^[64]

The possible correlation between testosterone and aggression could explain the "roid rage" that can result from anabolic steroid use,^[65]^[66] although an effect of abnormally high levels of steroids does not prove an effect at physiological levels.

Dehydroepiandrosterone

Dehydroepiandrosterone (DHEA) is the most abundant circulating androgen hormone 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.^[55]

Glucocorticoids

Glucocorticoid hormones have 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.^[55]

Glucocorticoids are released by the hypothalamic pituitary adrenal (HPA) axis in response to stress, of which cortisol is the most prominent in humans. Results in adults suggest that reduced levels of cortisol, linked to lower fear or a reduced stress response, can be associated with more aggression. However, it may be that proactive aggression is associated with low cortisol levels while reactive aggression may be accompanied by elevated levels. Differences in assessments of cortisol may also explain a diversity of results, particularly in children.^[51]

The HPA axis is related to the general fight-or-flight response or acute stress reaction, and the role of catecholamines such as epinephrine, popularly known as adrenaline.

Pheromones

In many animals, aggression can be linked to pheromones released between conspecifics. In mice, major urinary proteins (Mups) have been demonstrated to promote innate aggressive behavior in males.^[67]^[68] Mups activate olfactory sensory neurons in the vomeronasal organ (VNO), a subsystem of the nose known to detect pheromones via specific sensory receptors, of mice^[68] and rats.^[69] Pheremones have also been identified in fruit flies, detected by neurons in the antenna, that send a message to the brain eliciting aggression; it has been noted that aggression pheremones have not been identified in humans.^[70]

Genetics

Main article: Genetics of aggression

In general, differences in a continuous phenotype such as aggression are likely to result from the action of a large number of genes each of small effect, which interact with each other and the environment through development and life.

In a non-mammalian example of genes related to aggression, the fruitless gene in fruit flies is a critical determinant of certain sexually dimorphic behaviors, and its artificial alteration can result in a reversal of stereotypically male and female patterns of aggression in fighting. However, in what was thought to be a relatively clear case, inherent complexities have been reported in deciphering the connections between interacting genes in an environmental context and a social phenotype involving multiple behavioral and sensory interactions with another organism.^[71]

In mice, candidate genes for differentiating aggression between the sexes are the Sry (sex determining region Y) gene, located on the Y chromosome and the Sts (steroid sulfatase) gene. The Sts gene encodes the steroid sulfatase enzyme, which is pivotal in the regulation of neurosteroid biosynthesis. It is expressed in both sexes, is correlated with levels of aggression among male mice, and increases dramatically in females after parturition and during lactation, corresponding to the onset of maternal aggression.^[45]

In humans, there is good evidence that the basic human neural architecture underpinning the potential for flexible aggressive responses is influenced by genes as well as environment. In terms of variation between individual people, more than 100 twin and adoption studies studies have been conducted in recent decades examining the genetic basis of aggressive behavior and related constructs such as conduct disorders. According to a meta-analysis published in 2002, approximately 40% of variation between individuals is explained by differences in genes, and 60% by differences in environment (mainly non-shared environmental influences rather than those that would be shared by being raised together). However, such studies have depended on self-report or observation by others including parents, which complicates interpretation of the results. The few laborotory-based analyses have not found significant amounts of individual variation in aggression explicable by genetic variation in the human population. Furthermore, linkage and association studies that seek to identify specific genes, for example that influence neurotransmitter or hormone levels, have generally resulted in contradictory findings characterized by failed attempts at replication. One possible factor is an allele (variant) of the MAO-A gene which, in interaction with certain life events such as childhood maltreatment (which may show a main effect on its own), can influence development of brain regions such as the amygdala and as a result some types of behavioral response may be more likely. The generally unclear picture has been compared to equally difficult findings obtained in regard to other complex behavioral phenotypes.^[72]^[73]

In humans

Humans share aspects of aggression with non-human animals, and have specific aspects and complexity related to factors such as genetics, early development, social learning and flexibility, culture and morals.

Culture

Culture is a factor that plays a role in aggression.

Tribal or band societies existing before or outside of modern states have sometimes been depicted as peaceful 'noble savages' or alternatively as brutish 'beasts'. The Kung Bushmen were described as 'The Harmless People' in a popular work by Elizabeth Marshall Thomas in 1958,^[74] while Lawrence Keeley's 1996 War Before Civilization suggested that regular warfare without modern technology was conducted by most groups throughout human history, including most Native American tribes.^[75] Studies of hunter-gatherers show a range of different societies. In general, aggression, conflict and violence sometimes occur, but direct confrontation is generally avoided and conflict is socially managed by a variety of verbal and non-verbal methods. Different rates of aggression or violence, currently or in the past, within or between groups, have been linked to the structuring of societies and environmental conditions influencing factors such as resource or property acquisition, land and subsistence techniques, and population change.^[76]

Analyzing aggression culturally or politically is complicated by the fact that the label 'aggressive' can itself be used as a way of asserting a judgement from a particular point of view. Whether a coercive or violent method of social control is perceived as aggression - or as legitimate versus illegitimate aggression - depends on the position of the relevant parties in relation to the social order of their culture. This in turn can relate to factors such as: norms for coordinating actions and dividing resources; what is considered self-defense or provocation; attitudes towards 'outsiders', attitudes towards specific groups such as women, the disabled or the lower status; the availability of alternative conflict resolution strategies; trade interdependence and collective security pacts; fears and impulses; and ultimate goals regarding material and social outcomes.^[77]

Insignia of the 18th Aggressor Squadron

Cross-cultural research has found differences in attitudes towards aggression in different cultures. In one questionnaire study of university students, in addition to men overall justifying some types of aggression more than women, USA respondents justified defensive physical aggression more readily than Japanese or Spanish respondents, whereas Japanese students preferred direct verbal aggression (but not indirect) more than their American and Spanish counterparts.^[78] Within American culture, southern men were shown in a study on university students to be more affected and to respond more aggressively than northerners when randomly insulted after being bumped into, which was theoretically related to a traditional culture of honor in the Southern United States.^[79] A similar sociological concept that may be applied in different cultures is 'face'. Other cultural themes sometimes applied to the study of aggression include individualistic versus collectivist styles, which may relate, for example, to whether disputes are responded to with open competition or by accommodating and avoiding conflicts. Other comparisons made in relation to aggression or war include democratic versus authoritarian political systems and egalitarian versus stratified societies.^[77] The economic system known as capitalism has been viewed by some as reliant on the leveraging of human competitiveness and aggression in pursuit of resources and trade, which has been considered in both positive and negative terms.^[80] Attitudes about the social acceptability of particular acts or targets of aggression are also important factors. This can be highly controversial, as for example in disputes between religions or nation states, for example in regard to the Arab–Israeli conflict.^[81]^[82]

Media

Main article: Media violence research

Some scholars believe that behaviors like aggression may be partially learned by watching and imitating the behavior of others. Some scholars have concluded that media may have some small effects on aggression.^[83] There is also research questioning this view.^[84] For instance, a recent long-term outcome study of youth found no long-term relationship between playing violent video game and youth violence or bullying.^[85] One study suggested there is a smaller effect of violent video games on aggression than has been found with television violence on aggression. This effect is positively associated with type of game violence and negatively associated to time spent playing the games.^[86] The author concluded that insufficient evidence exists to link video game violence with aggression. However, another study suggested links to aggressive behavior.^[87] One study suggested that adults (i.e. parents) suffering from dissociative symptoms related to post-traumatic stress disorder may be more likely to expose their children to violent programs and video games; links between these issues were also related to poverty.^[88]

Children

The frequency of physical aggression in humans peaks at around 2–3 years of age. It then declines gradually on average.^[89]^[90] These observations suggest that physical aggression is not only a learned behavior but that development provides opportunities for the learning and biological development of self-regulation. However, a small subset of children fail to acquire all the necessary self-regulatory abilities and tend to show atypical levels of physical aggression across development. These may be at risk for later violent behavior or, conversely, lack of aggression that may be considered necessary within society. Some findings suggest that early aggression does not necessarily lead to aggression later on, however, although the course through early childhood is an important predictor of outcomes in middle childhood. In addition, physical aggression that continues is likely occurring in the context of family adversity, including socioeconomic factors. Moreover, 'opposition' and 'status violations' in childhood appear to be more strongly linked to social problems in adulthood than simply aggressive antisocial behavior.^[91]^[92] Social learning through interactions in early childhood has been seen as a building block for levels of aggression which play a crucial role in the development of peer relationships in middle childhood.^[93] Overall, an interplay of biological, social and environmental factors can be considered.^[94]

What is typically expected of children?

Young children preparing to enter kindergarten need to develop the socially important skill of being assertive. Examples of assertiveness include asking others for information, initiating conversation, or being able to respond to peer pressure.
In contrast, some young children use aggressive behavior, such as hitting or biting, as a form of communication.
Aggressive behavior can impede learning as a skill deficit, while assertive behavior can facilitate learning. However, with young children, aggressive behavior is developmentally appropriate and can lead to opportunities of building conflict resolution and communication skills.
By school age, children should learn more socially appropriate forms of communicating such as expressing themselves through verbal or written language; if they have not, this behavior may signify a disability or developmental delay

What triggers aggressive behavior in children?

Corporal punishment such as spanking increases subsequent aggression in children.^[95]

The Bobo doll experiment was conducted by Albert Bandura in 1961. In this work, Bandura found that children exposed to an 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.^[96]

Summary points from recommendations by national associations

American Academy of Pediatrics (2011): "The best way to prevent aggressive behavior is to give your child a stable, secure home life with firm, loving discipline and full-time supervision during the toddler and preschool years. Everyone who cares for your child should be a good role model and agree on the rules he’s expected to observe as well as the response to use if he disobeys."^[97]
National Association of School Psychologists (2008): "Proactive aggression is typically reasoned, unemotional, and focused on acquiring some goal. For example, a bully wants peer approval and victim submission, and gang members want status and control. In contrast, reactive aggression is frequently highly emotional and is often the result of biased or deficient cognitive processing on the part of the student."^[98]

Gender

Gender is a factor that plays a role in both human and animal aggression. Males are historically believed to be generally more physically aggressive than females from an early age,^[99]^[100] and men commit the vast majority of murders (Buss 2005). This is one of the most robust and reliable behavioral sex differences, and it has been found across many different age groups and cultures. There is evidence that males are quicker to aggression (Frey et al. 2003) and more likely than females to express their aggression physically.^[101] When considering indirect forms of non-violent aggression, such as relational aggression and social rejection, some scientists argue that females can be quite aggressive although female aggression is rarely expressed physically.^[102]^[103]^[104]

Although females are less likely to initiate physical violence, they can express aggression by using a variety of non-physical means. Exactly which method women use to express aggression is something that varies from culture to culture. On Bellona Island, a culture based on male dominance and physical violence, women tend to get into conflicts with other women more frequently than with men. When in conflict with males, instead of using physical means, they make up songs mocking the man, which spread across the island and humiliate him. If a woman wanted to kill a man, she would either convince her male relatives to kill him or hire an assassin. Although these two methods involve physical violence, both are forms of indirect aggression, since the aggressor herself avoids getting directly involved or putting herself in immediate physical danger.^[105]

See also the sections on testosterone and evolutionary explanations for gender differences above.

Situational factors

Aggression as a positive adaptation theory

Some recent scholarship has questioned traditional psychological conceptualizations of aggression as universally negative.^[22] Most traditional psychological definitions of aggression focus on the harm to the recipient of the aggression, implying this is the intent of the aggressor, however this may not always be the case.^[109] From this alternate view, although the recipient may or may not be harmed, the intent is to increase the status of the aggressor, not necessarily to harm the recipient.^[110] Such scholars contend that traditional definitions of aggression have no validity.^{[citation needed]}

From this view, rather than concepts such as assertiveness, aggression, violence and criminal violence existing as distinct constructs, they exist instead along a continuum with moderate levels of aggression being most adaptive.^[22] Such scholars do not consider this a trivial difference, noting that many aggression measures may measure outcomes lower down in the continuum, at levels which are adaptive, yet generalize their findings to non-adaptive levels of aggression, thus losing precision.^[111]

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^ Bongers, I. L.; Koot, H. M., van der Ende, J., Verhulst, F. C. (30 November 2007). "Predicting young adult social functioning from developmental trajectories of externalizing behaviour". Psychological Medicine 38 (7). doi:10.1017/S0033291707002309. http://web.up.ac.za/sitefiles/File/2561/OUB804/Aggressionoutcomesdevelopmental.pdf.
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^ National Association of School Psychologists (2008) Angry and Aggressive Students
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^ Bjorkqvist, Kaj; Lagerspetz, Kirsti M.; Osterman, Karin (1994). "Sex Differences in Covert Aggression". Aggressive Behavior 202: 27–33. http://www.vasa.abo.fi/svf/up/articles/sexdiff_in_covert.pdf.
^ Archer, J. (2004). "Sex differences in aggression in real-world settings: A meta-analytic review". Review of General Psychology 8 (4): 291–322. doi:10.1037/1089-2680.8.4.291.
^ Card, N.A.; Stucky, B.D.; Sawalani, G.M.; Little, T.D. (2008). "Direct and indirect aggression during childhood and adolescence: A meta-analytic review of gender differences, intercorrelations, and relations to maladjustment". Child Development 79 (5): 1185–1229. doi:10.1111/j.1467-8624.2008.01184.x. PMID 18826521.
^ Hines, Denise A.; Saudino, Kimberly J. (2003). "Gender Differences in Psychological, Physical, and Sexual Aggression Among College Students Using the Revised Conflict Tactics Scales". Violence and Victims 18 (2): 197–217. doi:10.1891/vivi.2003.18.2.197. PMID 12816404.
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External links

Look up aggression or aggressive in Wiktionary, the free dictionary.

Wikiquote has a collection of quotations related to: Aggression

Wikimedia Commons has media related to: Aggression

When Family Life Hurts: Family experience of aggression in children – Parentline plus, 31 October 2010
Aggression and Violent Behavior, a Review Journal
International Society for Research on Aggression (ISRA)
Problems in the Concepts and Definitions of Aggression, Violence and some Related Terms by Johan van der Dennen, originally published in 1980

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aggression

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Aggression

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Aggression

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Aggression

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