Aggression is defined as
The act of initiating hostilities or invasion
The practice or habit of launching attacks
Hostile or destructive behavior or actions
In psychology, aggression encompasses many different types of behaviour, some of which are not clearly related to each other. Consequently, aggression has been a difficult term to provide one concise definition for.
Moyer (1968) presented an early, and highly influential, classification of seven different forms of aggression.
Predatory aggression: attack on prey by a predator
Inter-male aggression: competition between males of the same species over access to females, dominance status etc.
Fear-induced aggression: aggression associated with attempts to flee from a threat
Irritable aggression: aggression directed towards an available target induced by some sort of frustration (eg schedule-induced aggression)
Territorial aggression: defence of a fixed space against intruders, typically conspecifics.
Maternal aggression: a female's aggression to protect her offspring from a threat
Instrumental aggression: aggression directed towards obtaining some goal, maybe a learned response to a situation
Aggression is a basic drive of life and part of the process of survival and evolution.
Aggression in humans is partly genetic, with origins going as far as to our reptilian ancestors, and partly a result of the upbringing. Enhanced levels of aggression in male mice and monkeys have been associated with the hormone monoamine oxidase A, MAO-A. However, studies in macaque and humans showed that its negative effects can usually be compensated by parenting.
The easiest aggression to explain is that of a group defending itself against a predator to prevent being eaten.
Either the strong will form a circle around the weak and defend them, thus using aggression and also endangering themselves, or the group will flee and the weak gets eaten, which may eventually lead to extinction.
One function of aggression is that the leader of a group is determined or the pecking order.
Another purpose of aggression is the gaining of an own territory. Initially that means scaring away the competition of the own kind. An effect of this is that the own kind will spread itself over the available space, each having its own territory. Once all territory has been used up, the aggression will, instead of scaring away, lead to death.
Since aggression is part of life and partly genetic, not being able to express aggression can be unhealthy. If people are locked up together for a long time they may fight with each other. Aggression can be experienced as fun when it fulfills this drive, in particular if one does not run any risk himself, which may explain the origin of bullying.
Aggression is one of the most important and most controversial kinds of motivation. Its use as a category in the psychology of motivation has often been criticised, because it is clear that it encompasses a vast range of phenomena, from modern war to squabbles between individuals, and it is far from clear that these have anything in common other than the risk that someone gets hurt. There is a constant danger that concepts and explanations that are useful in the study of one kind of aggression will be misapplied in a different field. However, it remains one of the most important topics in many areas of psychology and other social sciences, including:
ethology and comparative psychology
psychoanalysis and other kinds of depth psychology
Not all aggression is direct or readily identifiable. Some aggression may occur in the context of what appear to be a friendship. Such Relational aggression may involve domination, even sadism as the more powerful friend torments the weaker through threats of exclusion. Indirect aggression involves such actions as spreading rumors about others, even lies; as may social aggression which attacks self esteem or social status. Together these are characterized by Rachel Simmons in Odd Girl Out as alternative aggression