From Wikipedia, the free encyclopedia
Aggression is overt or covert, often harmful, social
interaction with the intention of inflicting damage or other harm upon
another individual. It may occur either reactively or without
provocation. In humans, aggression can be caused by various triggers,
from frustration due to blocked goals to feeling disrespected. Human aggression can be classified into direct and indirect aggression;
whilst the former is characterized by physical or verbal behavior
intended to cause harm to someone, the latter is characterized by
behavior intended to harm the social relations of an individual or
group.
In definitions commonly used in the social sciences and behavioral sciences, aggression is an action or response by an individual that delivers something unpleasant to another person. Some definitions include that the individual must intend to harm another person.
In an interdisciplinary perspective, aggression is regarded as
“an ensemble of mechanism formed during the course of evolution in order
to assert oneself, relatives or friends against others, to gain or to
defend resources (ultimate causes) by harmful damaging means [...] These
mechanisms are often motivated by emotions like fear, frustration,
anger, feelings of stress, dominance or pleasure (proximate causes)
[...] Sometimes aggressive behavior serves as a stress relief or a
subjective feeling of power." 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, which may be expressed physically, or communicated verbally
or non-verbally: including anti-predator aggression, defensive
aggression (fear-induced), predatory aggression, dominance aggression,
inter-male aggression, resident-intruder aggression, maternal
aggression, species-specific aggression, sex-related aggression,
territorial aggression, isolation-induced aggression, irritable
aggression, and brain-stimulation-induced aggression (hypothalamus).
There are two subtypes of human aggression: (1) controlled-instrumental
subtype (purposeful or goal-oriented); and (2) reactive-impulsive
subtype (often elicits uncontrollable actions that are inappropriate or
undesirable). Aggression differs from what is commonly called assertiveness, although the terms are often used interchangeably among laypeople (as in phrases such as "an aggressive salesperson").
Overview
Dollard et al. (1939) proposed that aggression was due to frustration, which was described as an unpleasant emotion resulting from any interference with achieving a rewarding goal. Berkowitz extended this frustration–aggression hypothesis
and proposed that it is not so much the frustration as the unpleasant
emotion that evokes aggressive tendencies, and that all aversive events
produce negative affect and thereby aggressive tendencies, as well as fear tendencies. Besides conditioned stimuli, Archer categorized aggression-evoking (as well as fear-evoking) stimuli into three groups; namely, pain, novelty, and frustration, although he also described "looming," which refers to an object rapidly moving towards the visual sensors of a subject, and can be categorized as "intensity."
Aggression can have adaptive benefits or negative effects.
Aggressive behavior is an individual or collective social interaction
that is a hostile behavior with the intention of inflicting damage or harm. Two broad categories of aggression are commonly distinguished. One includes affective (emotional) and hostile, reactive, or retaliatory aggression that is a response to provocation, and the other includes instrumental, goal-oriented or predatory, in which aggression is used as a means to achieve a goal.
An example of hostile aggression would be a person who punches someone
who insulted him or her. An instrumental form of aggression would be armed robbery. Research on violence from a range of disciplines lend some support to a distinction between affective and predatory aggression.
However, some researchers question the usefulness of a hostile versus
instrumental distinction in humans, despite its ubiquity in research,
because most real-life cases involve mixed motives and interacting
causes.
A number of classifications and dimensions of aggression have
been suggested. These depend on such things as whether the aggression is
verbal or physical; whether or not it involves relational aggression such as covert bullying and social manipulation;
whether harm to others is intended or not; whether it is carried out
actively or expressed passively; and whether the aggression is aimed
directly or indirectly. Classification may also encompass
aggression-related emotions (e.g. anger) and mental states (e.g. impulsivity, hostility).
Aggression may occur in response to non-social as well as social
factors, and can have a close relationship with stress coping style. Aggression may be displayed in order to intimidate.
The operative definition of aggression may be affected by moral or political views. Examples are the axiomatic moral view called the non-aggression principle and the political rules governing the behavior of one country toward another. Likewise in competitive sports, or in the workplace, some forms of aggression may be sanctioned and others not (see Workplace aggression). Aggressive behaviors are associated with adjustment problems and several psychopathological symptoms such as Antisocial Personality Disorder, Borderline Personality Disorder, and Intermittent Explosive Disorder.
Biological approaches conceptualize aggression as an internal
energy released by external stimuli, a product of evolution through
natural selection, part of genetics, a product of hormonal fluctuations.
Psychological approaches conceptualize aggression as a destructive
instinct, a response to frustration, an affect excited by a negative
stimulus, a result of observed learning of society and diversified
reinforcement, a resultant of variables that affect personal and
situational environments.
Etymology
The term aggression comes from the Latin word aggressio, meaning attack. The Latin was itself a joining of ad- and gradi-, which meant step at. The first known use dates back to 1611, in the sense of an unprovoked attack. A psychological sense of "hostile or destructive behavior" dates back to a 1912 English translation of Sigmund Freud's writing. Alfred Adler theorized about an "aggressive drive" in 1908. Child raising experts began to refer to aggression, rather than anger, from the 1930s.
Ethology
Ethologists study aggression as it relates to the interaction and evolution
of animals in natural settings. In such settings aggression can involve
bodily contact such as biting, hitting or pushing, but most conflicts
are settled by threat displays and intimidating thrusts that cause no
physical harm. This form of aggression may include the display of body
size, antlers, claws or teeth; stereotyped signals including facial
expressions; vocalizations such as bird song; the release of chemicals;
and changes in coloration. The term agonistic behaviour is sometimes used to refer to these forms of behavior.
Most ethologists believe that aggression confers biological advantages. Aggression may help an animal secure territory,
including resources such as food and water. Aggression between males
often occurs to secure mating opportunities, and results in selection of
the healthier/more vigorous animal. Aggression may also occur for
self-protection or to protect offspring.
Aggression between groups of animals may also confer advantage; for
example, hostile behavior may force a population of animals into a new
territory, where the need to adapt to a new environment may lead to an
increase in genetic flexibility.
Between species and groups
The most apparent type of interspecific aggression is that observed in the interaction between a predator and its prey. However, according to many researchers, predation is not aggression. A cat does not hiss or arch its back when pursuing a rat, and the active areas in its hypothalamus resemble those that reflect hunger rather than those that reflect aggression.
However, others refer to this behavior as predatory aggression, and
point out cases that resemble hostile behavior, such as mouse-killing by
rats. In aggressive mimicry
a predator has the appearance of a harmless organism or object
attractive to the prey; when the prey approaches, the predator attacks.
An animal defending against a predator may engage in either "fight or flight" or "tend and befriend"
in response to predator attack or threat of attack, depending on its
estimate of the predator's strength relative to its own. Alternative
defenses include a range of antipredator adaptations, including alarm signals. An example of an alarm signal is nerol, a chemical which is found in the mandibular glands of Trigona fulviventris individuals.
Release of nerol by T. fulviventris individuals in the nest has been
shown to decrease the number of individuals leaving the nest by fifty
percent, as well as increasing aggressive behaviors like biting.
Alarm signals like nerol can also act as attraction signals; in T.
fulviventris, individuals that have been captured by a predator may
release nerol to attract nestmates, who will proceed to attack or bite
the predator.
Aggression between groups is determined partly by willingness to
fight, which depends on a number of factors including numerical
advantage, distance from home territories, how often the groups
encounter each other, competitive abilities, differences in body size,
and whose territory is being invaded. Also, an individual is more likely to become aggressive if other aggressive group members are nearby. One particular phenomenon – the formation of coordinated coalitions that raid neighbouring territories to kill conspecifics – has only been documented in two species in the animal kingdom: 'common' chimpanzees and humans.
Within a group
Aggression
between conspecifics in a group typically involves access to resources
and breeding opportunities. One of its most common functions is to
establish a dominance hierarchy.
This occurs in many species by aggressive encounters between contending
males when they are first together in a common environment. Usually the more aggressive animals become the more dominant. In test situations, most of the conspecific aggression ceases about 24 hours after the group of animals is brought together.
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". Losing confrontations may be called social defeat, and winning or losing is associated with a range of practical and psychological consequences.
Conflicts between animals occur in many contexts, such as between
potential mating partners, between parents and offspring, between
siblings and between competitors for resources. Group-living animals may
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 aggressive incidents, various forms of conflict resolution
have been observed in mammalian species, particularly in gregarious
primates. These can mitigate or repair possible adverse consequences,
especially for the recipient of aggression who may become vulnerable to
attacks by other members of a group. Conciliatory acts vary by species
and may involve specific gestures or simply more proximity and
interaction between the individuals involved. However, conflicts over
food are rarely followed by post conflict reunions, even though they are
the most frequent type in foraging primates.
Other questions that have been considered in the study of primate
aggression, including in humans, is how aggression affects the
organization of a group, what costs are incurred by aggression, and why
some primates avoid aggressive behavior. 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 that are related to aspects of the physical or
social environment; this depends on the species and individual factors
such as gender, age and background (e.g. raised wild or captive).
Aggression, fear and curiosity
Within ethology, it has long been recognized that there is a relation between aggression, fear, and curiosity. A cognitive approach to this relationship puts aggression in the broader context of inconsistency reduction,
and proposes that aggressive behavior is caused by an inconsistency
between a desired, or expected, situation and the actually perceived
situation (e.g., "frustration"), and functions to forcefully manipulate the perception into matching the expected situation.
In this approach, when the inconsistency between perception and
expectancy is small, learning as a result of curiosity reduces
inconsistency by updating expectancy to match perception. If the
inconsistency is larger, fear or aggressive behavior may be employed to
alter the perception in order to make it match expectancy, depending on
the size of the inconsistency as well as the specific context.
Uninhibited fear results in fleeing, thereby removing the inconsistent
stimulus from the perceptual field and resolving the inconsistency. In
some cases thwarted escape may trigger aggressive behavior in an attempt
to remove the thwarting stimulus.
Evolutionary explanations
Like
many behaviors, aggression can be examined in terms of its ability to
help an animal itself survive and reproduce, or alternatively to risk
survival and reproduction. This cost-benefit analysis can be looked at in terms of evolution.
However, there are profound differences in the extent of acceptance of a
biological or evolutionary basis for human aggression.
According to the male warrior hypothesis,
intergroup aggression represents an opportunity for men to gain access
to mates, territory, resources and increased status. As such, conflicts
may have created selection evolutionary pressures for psychological
mechanisms in men to initiate intergroup aggression.
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.
Although aggressive encounters are ubiquitous in the animal
kingdom, with often high stakes, most encounters that involve aggression
may be resolved through posturing, or displaying and trial 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).
Gender
General
Gender plays an important role in human aggression.
There are multiple theories that seek to explain findings that males
and females of the same species can have differing aggressive behaviors.
One review concluded that male aggression tended to produce pain or
physical injury whereas female aggression tended towards psychological
or social harm.
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. Although there is much variation in species,
generally the more physically aggressive sex is the male, particularly
in mammals.
In species where parental care by both sexes is required, there tends
to be less of a difference. When the female can leave the male to care
for the offspring, then females may be the larger and more physically
aggressive. Competitiveness despite parental investment has also been
observed in some species.
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 complementary views emphasize conventional social roles stemming from physical evolved differences. Aggression in women may have evolved to be, on average, less physically dangerous and more covert or indirect.
However, there are critiques for using animal behavior to explain human
behavior. Especially in the application of evolutionary explanations to
contemporary human behavior, including differences between the genders.
According to the 2015 International Encyclopedia of the Social & Behavioral Sciences, sex differences in aggression is one of the most robust and oldest findings in psychology.
Past meta-analyses in the encyclopedia found males regardless of age
engaged in more physical and verbal aggression while small effect for
females engaging in more indirect aggression such as rumor spreading or
gossiping. It also found males tend to engage in more unprovoked aggression at higher frequency than females. This analysis also conforms with the Oxford Handbook of Evolutionary Psychology
which reviewed past analysis which found men to use more verbal and
physical aggression with the difference being greater in the physical
type.
There are more recent findings that show that differences in male and
female aggression appear at about two years of age, though the
differences in aggression are more consistent in middle-aged children
and adolescence. Tremblay, Japel and Pérusse (1999) asserted that
physically aggressive behaviors such as kicking, biting and hitting are
age-typical expressions of innate and spontaneous reactions to
biological drives such as anger, hunger, and affiliation. Girls' relational aggression,
meaning non-physical or indirect, tends to increase after age two while
physical aggression decreases. There was no significant difference in
aggression between males and females before two years of age.
A possible explanation for this could be that girls develop language
skills more quickly than boys, and therefore have better ways of
verbalizing their wants and needs. They are more likely to use
communication when trying to retrieve a toy with the words "Ask nicely"
or "Say please."
According to the journal of Aggressive Behaviour, an analysis across 9 countries found boys reported more in the use of physical aggression. At the same time no consistent sex differences emerged within relational aggression.
It has been found that girls are more likely than boys to use reactive
aggression and then retract, but boys are more likely to increase rather
than to retract their aggression after their first reaction. Studies
show girls' aggressive tactics included gossip, ostracism,
breaking confidences, and criticism of a victim's clothing, appearance,
or personality, whereas boys engage in aggression that involves a
direct physical and/or verbal assault. This could be due to the fact that girls' frontal lobes develop earlier than boys, allowing them to self-restrain.
One factor that shows insignificant differences between male and
female aggression is in sports. In sports, the rate of aggression in
both contact and non-contact sports is relatively equal. Since the
establishment of Title IX, female sports have increased in
competitiveness and importance, which could contribute to the evening of
aggression and the "need to win" attitude between both genders. Among
sex differences found in adult sports were that females have a higher
scale of indirect hostility while men have a higher scale of assault. Another difference found is that men have up to 20 times higher levels of testosterone than women.
In intimate relationships
Some
studies suggest that romantic involvement in adolescence decreases
aggression in males and females, but decreases at a higher rate in
females. Females will seem more desirable to their mate if they fit in
with society and females that are aggressive do not usually fit well in
society, they can often be viewed as antisocial. Female aggression is
not considered the norm in society and going against the norm can
sometimes prevent one from getting a mate.
However, studies have shown that an increasing number of women are
getting arrested for domestic violence charges. In many states, women
now account for a quarter to a third of all domestic violence arrests,
up from less than 10 percent a decade ago. The new statistics reflect a
reality documented in research: women are perpetrators as well as
victims of family violence.
However, another equally possible explanation is a case of improved
diagnostics: it has become more acceptable for men to report female
domestic violence to the authorities while at the same time actual
female domestic violence has not increased at all. This can be the case
when men have become less ashamed of reporting female violence against
them, therefore an increasing number of women are arrested, although the
actual number of violent women remains the same.
In addition, males in competitive sports are often advised by
their coaches not to be in intimate relationships based on the premises
that they become more docile and less aggressive during an athletic
event. The circumstances in which males and females experience
aggression are also different. A study showed that social anxiety and
stress was positively correlated with aggression in males, meaning as
stress and social anxiety increases so does aggression. Furthermore, a
male with higher social skills has a lower rate of aggressive behavior
than a male with lower social skills. In females, higher rates of
aggression were only correlated with higher rates of stress. Other than
biological factors that contribute to aggression there are physical
factors are well.
Physiological factors
Regarding
sexual dimorphism, humans fall 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 number of
challenges from other males compared to exclusive polygyny and monogamy but frequent sperm competition.
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. Another evolutionary theory explaining gender differences in aggression is the Male Warrior hypothesis,
which explains that males have psychologically evolved for intergroup
aggression in order to gain access to mates, resources, territory and
status.
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
and the hypothalamus has receptors that help determine aggression
levels based on their interactions with serotonin and vasopressin. In rodents, activation of estrogen receptor-expressing neurons in the ventrolateral portion of the ventromedial hypothalamus (VMHvl) was found to be sufficient to initiate aggression in both males and females. Midbrain areas involved in aggression 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, while lesions of an evolutionarily homologous area in the lizard greatly reduce competitive drive and aggression (Bauman et al. 2006). 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.
Several experiments in attack-primed Syrian golden hamsters, for
example, support the claim of circuity within the amygdala being
involved in control of aggression.
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. Amygdalotomy, which involves removing or destroying parts of the amygdala, has been performed on people to reduce their violent behaviour.
The broad area of the cortex known as the prefrontal cortex (PFC) is crucial for self-control
and inhibition of impulses, including inhibition of aggression and
emotions. Reduced activity of the prefrontal cortex, in particular its
medial and orbitofrontal portions, has been associated with violent/antisocial aggression. In addition, reduced response inhibition has been found in violent offenders, compared to non-violent offenders.
The role of the chemicals in the brain, particularly neurotransmitters,
in aggression has also been examined. This varies depending on the
pathway, the context and other factors such as gender. A deficit in serotonin
has been theorized to have a primary role in causing impulsivity and
aggression. At least one epigenetic study supports this supposition.
Nevertheless, low levels of serotonin transmission may explain a
vulnerability to impulsiveness, potential aggression, and may 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 which appears not to 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.
The hormonal neuropeptides vasopressin and oxytocin
play a key role in complex social behaviours in many mammals such as
regulating attachment, social recognition, and aggression. Vasopressin
has been implicated in male-typical social behaviors which includes
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.
In human, aggressive behavior has been associated with abnormalities in three principal regulatory systems in the body serotonin systems, catecholamine systems, and the hypothalamic–pituitary–adrenal axis. Abnormalities in these systems also are known to be induced by stress, either severe, acute stress or chronic low-grade stress
Testosterone
Early androgenization has an organizational effect on the developing
brains of both males and females, making more neural circuits that
control sexual behavior as well as intermale and interfemale aggression
become more sensitive to testosterone.
There are noticeable sex differences in aggression. 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.
In humans, there is a seasonal variation in aggression associated with changes in testosterone.
For example, in some primate species, such as rhesus monkeys and
baboons, females are more likely to engage in fights around the time of
ovulation as well as right before menstruation.
If the results were the same in humans as they are in rhesus monkeys
and baboons, then the increase in aggressive behaviors during ovulation
is explained by the decline in estrogen levels. This makes normal
testosterone levels more effective.
Castrated mice and rats exhibit lower levels of aggression. Males
castrated as neonates exhibit low levels of aggression even when given
testosterone throughout their development.
Challenge hypothesis
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.
The hypothesis has been subsequently expanded and 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.
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.
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 estradiol by the enzyme aromatase, or to dihydrotestosterone (DHT) by 5α-reductase.
Aromatase is highly expressed in regions involved in the
regulation of aggressive behavior, such as the amygdala and
hypothalamus. In studies using genetic knockout 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, ERα and ERβ,
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.
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.
Moreover, a study done in human subjects showed that
prefrontal-amygdala connectivity is modulated by endogenous testosterone
during social emotional behavior.
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. Patients with localized lesions to the OFC engage in heightened reactive aggression. Aggressive behavior may be regulated by testosterone via reduced medial OFC engagement following social provocation.
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
Scientists
have for a long time been interested in the relationship between
testosterone and aggressive behavior. In most species, males are more
aggressive than females. Castration
of males usually has a pacifying effect on aggressive behavior in
males. 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. Research on the relationship between testosterone and aggression
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.
The Handbook of Crime Correlates, a review of crime
studies, states 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.
Many studies have also been done on the relationship between more
general aggressive behavior/feelings and testosterone. About half the
studies have found a relationship and about half no relationship.
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. In addition, some experiments have failed to find a relationship between testosterone levels and aggression in humans.
The possible correlation between testosterone and aggression could explain the "roid rage" that can result from anabolic steroid use, 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.
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.
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.
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, and can be mediated by neuromodulatory systems. 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 and rats. 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.
Genetics
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.
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.
At least one study has found a possible epigenetic signature (i.e.
decreased methylation at a specific CpG site on the promoter region) of
the serotonin receptor 5-HT3a that is associated with maternal
aggression among human subjects.
Mice with experimentally elevated sensitivity to oxidative stress (through inhibition of copper-zinc superoxide dismutase, SOD1 activity) were tested for aggressive behavior. Males completely deficient in SOD1 were found to be more aggressive than both wild-type males and males that express 50% of this antioxidant
enzyme. They were also faster to attack another male. The causal
connection between SOD1 deficiency and increased aggression is not yet
understood.
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
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
laboratory-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. For example, both 7R and 5R, ADHD-linked VNTR alleles of dopamine receptor D4 gene are directly associated with the incidence of proactive aggression in the men with no history of ADHD.
Society and culture
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.
Konrad Lorenz stated in his 1963 classic, On Aggression, that
human behavior is shaped by four main, survival-seeking animal drives.
Taken together, these drives—hunger, fear, reproduction, and
aggression—achieve natural selection. E. O. Wilson elaborated in On Human Nature
that aggression is, typically, a means of gaining control over
resources. Aggression is, thus, aggravated during times when high
population densities generate resource shortages.
According to Richard Leakey and his colleagues, aggression in humans
has also increased by becoming more interested in ownership and by
defending his or her property. However, UNESCO adopted the Seville Statement of Violence in 1989 that refuted claims, by evolutionary scientists, that genetics by itself was the sole cause of aggression.
Social and cultural aspects may significantly interfere with the
distinct expression of aggressiveness. For example, a high population
density, when associated with a decrease of available resources, might
be a significant intervening variable for the occurrence of violent
acts.
Culture
Culture is one 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'. The ǃKung people were described as 'The Harmless People' in a popular work by Elizabeth Marshall Thomas in 1958, 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. 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.
American psychologist Peter Gray hypothesizes that band hunter-gatherer societies are able to reduce aggression while maintaining relatively peaceful, egalitarian relations between members through various methods, such as fostering a playful
spirit in all areas of life, the use of humor to counter the tendency
of any one person to dominate the group, and non-coercive or "indulgent"
child-rearing practices. Gray likens hunter-gatherer bands to social
play groups, while stressing that such play is not frivolous or even
easy at all times.
According to Gray, "Social play—that is, play involving more than one
player—is necessarily egalitarian. It always requires a suspension of
aggression and dominance along with a heightened sensitivity to the
needs and desires of the other players".
Joan Durrant at the University of Manitoba writes that a number of studies have found physical punishment
to be associated with "higher levels of aggression against parents,
siblings, peers and spouses", even when controlling for other factors. According to Elizabeth Gershoff at the University of Texas at Austin,
the more that children are physically punished, the more likely they
are as adults to act violently towards family members, including
intimate partners.
In countries where physical punishment of children is perceived as
being more culturally accepted, it is less strongly associated with
increased aggression; however, physical punishment has been found to
predict some increase in child aggression regardless of culture. While these associations do not prove causality, a number of longitudinal studies suggest that the experience of physical punishment has a direct causal effect on later aggressive behaviors. In examining several longitudinal studies that investigated the path from disciplinary spanking
to aggression in children from preschool age through adolescence,
Gershoff concluded: "Spanking consistently predicted increases in
children's aggression over time, regardless of how aggressive children
were when the spanking occurred". similar results were found by Catherine Taylor at Tulane University in 2010. Family violence researcher Murray A. Straus
argues, "There are many reasons this evidence has been ignored. One of
the most important is the belief that spanking is more effective than
nonviolent discipline and is, therefore, sometimes necessary, despite
the risk of harmful side effects".
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.
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, United States 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. 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, or "saving 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.
In a study including 62 countries school principals reported aggressive
student behavior more often the more individualist, and hence less
collectivist, their country's culture. Other comparisons made in relation to aggression or war include democratic versus authoritarian political systems and egalitarian versus stratified societies. 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.
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.
Media
Some scholars believe that behaviors like aggression may be partially
learned by watching and imitating people's behavior, while other
researchers have concluded that the media may have some small effects on
aggression. There is also research questioning this view.
For instance, a long-term outcome study of youth found no long-term
relationship between playing violent video games and youth violence or
bullying. 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.
The author concluded that insufficient evidence exists to link video
game violence with aggression. However, another study suggested links to
aggressive behavior.
Fear-induced aggression
According to philosopher and neuroscientist Nayef Al-Rodhan, "fear(survival)-induced pre-emptive aggression" is a human reaction to injustices that are perceived to threaten survival. It is often the root of the unthinkable brutality
and injustice perpetuated by human beings. It may occur at any time,
even in situations that appear to be calm and under control. Where there
is injustice that is perceived as posing a threat to survival,
"fear(survival)-induced pre-emptive aggression" will result in
individuals taking whatever action necessary to be free from that
threat.
Nayef Al-Rodhan argues that humans' strong tendency towards
"fear(survival)-induced pre-emptive aggression" means that situations of
anarchy or near anarchy should be prevented at all costs. This is because anarchy provokes fear, which in turn results in aggression, brutality, and injustice. Even in non-anarchic situations, survival instincts
and fear can be very powerful forces, and they may be incited
instantaneously. "Fear(survival)-induced pre-emptive aggression" is one
of the key factors that may push naturally amoral humans to behave in
immoral ways.
Knowing this, Al-Rodhan maintains that we must prepare for the
circumstances that may arise from humans' aggressive behavior. According
to Al-Rodhan, the risk of this aggression and its ensuing brutality
should be minimized through confidence-building measures and policies that promote inclusiveness and prevent anarchy.
Children
The frequency of physical aggression in humans peaks at around 2–3 years of age. It then declines gradually on average.
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.
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. Overall, an interplay of biological, social and environmental factors can be considered. Some research indicates that changes in the weather can increase the likelihood of children exhibiting deviant behavior.
Typical expectations
- 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.
Aggression triggers
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 affect the way they react and handle
situations.
- 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."
- 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."
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,
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. However, some empirical studies
have found the discrepancy in male and female aggression to be more
pronounced in childhood and the gender difference in adults to be modest
when studied in an experimental context.
Still, there is evidence that males are quicker to aggression (Frey et
al. 2003) and more likely than females to express their aggression
physically. 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. An exception is intimate partner violence that occurs among couples who are engaged, married, or in some other form of intimate relationship.
Although females are less likely than males 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.
See also the sections on testosterone and evolutionary explanations for gender differences above.
Situational factors
There has been some links between those prone to violence and their
alcohol use. Those who are prone to violence and use alcohol are more
likely to carry out violent acts.
Alcohol impairs judgment, making people much less cautious than they
usually are (MacDonald et al. 1996). It also disrupts the way
information is processed (Bushman 1993, 1997; Bushman & Cooper
1990).
Pain and discomfort also increase aggression. Even the simple act
of placing one's hands in hot water can cause an aggressive response.
Hot temperatures have been implicated as a factor in a number of
studies. One study completed in the midst of the civil rights movement
found that riots were more likely on hotter days than cooler ones
(Carlsmith & Anderson 1979). Students were found to be more
aggressive and irritable after taking a test in a hot classroom
(Anderson et al. 1996, Rule, et al. 1987). Drivers in cars without air
conditioning were also found to be more likely to honk their horns
(Kenrick & MacFarlane 1986), which is used as a measure of
aggression and has shown links to other factors such as generic symbols
of aggression or the visibility of other drivers.
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
to demonstrate how unexpected frustration leads to increased
aggression, Kulik & Brown (1979) selected a group of students as
volunteers to make calls for charity donations. One group was 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
that expected success was more upset when no one was pledging than the
group who did not expect success (everyone actually had horrible
success). This research suggests that when an expectation does not
materialize (successful collections), unexpected frustration arises
which increases aggression.
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.
A new proposal links military experience to anger and aggression,
developing aggressive reactions and investigating these effects on
those possessing the traits of a serial killer. Castle and Hensley
state, "The military provides the social context where servicemen learn
aggression, violence, and murder." Post-traumatic stress disorder
(PTSD) is also a serious issue in the military, also believed to
sometimes lead to aggression in soldiers who are suffering from what
they witnessed in battle. They come back to the civilian world and may
still be haunted by flashbacks and nightmares, causing severe stress. In
addition, it has been claimed that in the rare minority who are claimed
to be inclined toward serial killing, violent impulses may be
reinforced and refined in war, possibly creating more effective
murderers.
As a positive adaptation theory
Some recent scholarship has questioned traditional psychological conceptualizations of aggression as universally negative.
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.
From this alternate view, although the recipient may or may not be
harmed, the perceived intent is to increase the status of the aggressor,
not necessarily to harm the recipient.
Such scholars contend that traditional definitions of aggression have
no validity because of how challenging it is to study directly.
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.
Such scholars do not consider this a trivial difference, noting that
many traditional researchers' aggression measurements may measure
outcomes lower down in the continuum, at levels which are adaptive, yet
they generalize their findings to non-adaptive levels of aggression,
thus losing precision.
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