Aggression
[7] 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.
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.
[35] One particular phenomenon – the formation of coordinated coalitions that raid neighboring territories to kill conspecifics – has only been documented in two species in the animal kingdom: 'common' chimpanzees and humans.
[49] According to the male warrior hypothesis, intergroup aggression represents an opportunity for men to gain access to mates, territory, resources and increased status.
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).
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.
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.
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.
[73] Regarding sexual dimorphism, humans fall into an intermediate group with moderate sex differences in body size but relatively large testes.
[77] 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.
[82] 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.
[83] Several experiments in attack-primed Syrian golden hamsters, for example, support the claim of circuitry within the amygdala being involved in control of aggression.
[81] 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.
For example, chimpanzees, which are continuous breeders, show significantly raised testosterone levels and aggressive male-male interactions when receptive and fertile females are present.
This brain area is strongly associated with impulse control and self-regulation systems that integrate emotion, motivation, and cognition to guide context-appropriate behavior.
[112] 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.
[81] 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.
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.
[115][116] 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.
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.
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.
[128] 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.
[129] 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.
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, disabled people or those with 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.
"[136] 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.
[123] 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.
Males are historically believed to be generally more physically aggressive than females from an early age,[156][157] and men commit the vast majority of murders (Buss 2005).
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.
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.
