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Aggression is a common behavior in humans. Aggressiveness can be an advantageous quality, but can also have adverse consequences in certain situations. Human aggression encompasses any behavior that is directed towards another individual or group of individuals with the intent to cause harm. An additional qualifier can be added that the recipient of the aggressive behavior is motivated to avoid that behavior. Aggression can be subdivided into hostile and instrumental aggression based on whether intent to harm is the proximate or ultimate goal. Hostile (or unplanned) aggression is an impulsive reaction to some sort of provocation and has the immediate goal of harming the target. Instrumental aggression is a premeditated means of achieving a goal besides harming the target. Aggressiveness is influenced by a host of biological and environmental factors. Another useful classification is direct vs indirect aggression, where direct aggression is physical or verbal action that is intended to cause harm to another, and indirect aggression is the intent to harm social relations of an individual or group.
There are likely cultural differences in humans that affect aggressive behavior. Given its nearly ubiquitous presence in humans as well as many of our primate relatives, and the adaptive benefits that the behavior confers, it is likely that our early human ancestors also exhibited certain forms of aggression. It is unknown whether all the classifications of types of aggression in humans can be applied to non-human animals. In addition to cultural differences, other factors can also influence the expression of aggressive behavior. A number of drugs (both synthetic and naturally occurring, legal and illegal) increase aggression in humans.
Gender plays a clear role in the expression of aggression, and it is further determined by a range of physiological factors and neurobiological mechanisms including serotonergic systems, sex neurosteroids, and brain physiology. Men and women exhibit different aggressive tendencies in both frequency and type, although lab studies indicate that provocation reduces these differences. Additionally, men tend to favor direct aggression while women prefer indirect. Highly aggressive behavior correlates with high levels of testosterone in both humans and animals, however the precise role of testosterone in human aggression is not fully understood. Other hormones such as cortisol, oxytocin, and vasopressin are also likely involved. The influence of these hormones on aggressiveness in women are even less well understood.
Several specific sections of the human brain seem to be directly connected to aggressiveness. The Prefrontal Cortex is the most frequently and extensively associated brain region in human impulsive aggression. Lesion studies on both the PFC and Orbitofrontal Cortex showed increased aggression scores among participants. Finally, the amygdala is also considered to be one of the most important brain regions involved in human aggression due to its role in processing stimuli and emotions.
The difference in aggressive displays between humans and non-human primates seems to be a difference in terms of degree, and not presence/absence. Some may argue that instrumental aggression is a uniquely human feature, however, chimpanzees also seem to be capable of this form of aggression in certain circumstances.
Aggression is common in many species of non-human primates as well as other vertebrates, and in spite of the risk is clearly an adaptive strategy. Frequently aggressive behavior is linked to competition. Competition for resources such as food and mates (sexual coercion) elicits aggression, as does territorial disputes, and social standing. In many species of non-human primates (such as chimpanzees, baboons, and gorillas) aggression is used to establish and maintain social order and enforce dominance hierarchies within communities. Aggressiveness on the community level also serves a protective function against outside individuals and groups.
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