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Humans and other animals exhibit aggression, defined as action or threat of action that causes harm against another. Violence is differentiated as escalated aggression that includes the use of physical force against another. As in most species, males tend to show more physical aggression than females, although females typically exhibit indirect aggression. Among humans it is likely that both biological and cultural aspects come into play in the gender differences of violence. A few species, such as bonobos and spotted hyenas which have female-dominated social systems show the opposite trend.
All primates exhibit aggression, in the context of direct or indirect competition over limited resources, i.e. food and access to mates. Direct competition involves an immediate, perceived threat. Examples include forced copulation, territory defense, and competition over food. Indirect competition involves immediate aggression with potential resource re-distribution displaced in time. For instance, infanticide by males can result in a long-term increase in reproductive success. Male-male aggression can lead to changes in rank, thereby leading to an increase in an alpha male’s reproductive success. Aggression may increase in times of resource instability.
Aggression is modulated (at least in part) by the androgen system. Aggressive behavior can lead to upregulation of dopamine in the nucleus accumbens in mice, indicative of involvement of the reward system during aggressive acts.
Unlike any other animal, humans use their symbolic capacities and language to both inhibit and promote aggressive behavior within and between social groups. Aspects of human violence that are unique are some of those that are mediated through language – blackmail, verbal abuse – although it could be argued that these are similar to the gestural threats present in other animals. Language is a key tool to establish and maintain such social networks and is instrumental in preparing and organizing war. Our abilities and tendencies for (in-group) cooperation and (out-group) aggression seem to go hand in hand. Finally, language itself has become a tool for aggression as it allows to deliver stressful psychological blows by demeaning insult and shaming. In addition to language, the use of tools, especially projectile weapons and traps (both also very effectively used for hunting) add novel dimensions to human aggression.
Human aggression differs in scale. Modern humans engage in world wars and genocides, resulting in the deaths of thousands and even hundreds of thousands of individuals. Human aggressive acts may be driven by perceived (though not necessarily real) threats to a group or status, desire to stop all future attacks, or even ritualistic behavior. It has been speculated that modern humans caused the downfall of Neanderthal through genocide. Humans often rely on cooperation that transcend the immediate social group and draw from potentially large social networks, especially male kin and affine alliances. Such behavior is more reminiscent of that seen in social insects, than in any non-human primates. Human groups can get involved in multi-generational feuding over past aggression, whereby each aggressive act fuels novel retaliation. Humans appear to be the only primates that retaliate as a group.
Sedentarization and the accumulation of property, followed by social stratification, may have led to increased rates of violence and in particular of organized violence (i.e. war). Long-lived reputation (through language) is an important factor in mediating human inter-group aggression.
Human aggression is also tied to cognitive abilities, linking aggressive acts with potential benefits dislocated in time or space. Acts of aggression can lead to resource gain in territories not present or in the future scenarios yet to happen. It can be related to remembered aggression, such as revenge, or to prevent unwanted future aggression.
Only humans have made long-range weapons. These weapons possibly lower the cost benefit ratio of aggression, by reducing the risk of self harm harm while inflicting more damage to the victim. Lastly, only humans attempt suicide, which could be viewed as aggression towards oneself.
Aggression has been recorded in all human societies.
Among humans, the evolution of intra-group cooperation was likely instrumental in a cascade of traits, including the co-evolution of competitive groups. Language also allows for much more abstract in-group/out-group distinctions, both facilitating cooperation within a group and acting as a discriminating marker of membership. Language and theory of mind also underlie premeditated, long-term future acts of aggression.
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