Aggression is overt, often harmful, social interaction with the intention of inflicting damage or other unpleasantness upon another individual.
It may occur either in retaliation or without provocation.
Most ethologists believe that aggression confers biological advantages.
Human aggression can be classified into direct and indirect aggression, whilst the first is characterized by physical or verbal behavior intended to cause harm to someone, the second one is characterized by a behavior intended to harm social relations of an individual or a group.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. Aggression may be displayed in order to intimidate.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).
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.