The most apparent type of aggression in the between species and groups is that observed in the interaction between a predator and its prey. However, according to many researchers, predation is not aggression. A cat does not hiss or arch its back when pursuing a rat, and the active areas in its hypothalamus resemble those that reflect hunger rather than those that reflect aggression. However, others refer to this behavior as predatory aggression, and point out cases that resemble hostile behavior, such as mouse-killing by rats. In aggressive mimicry a predator has the appearance of a harmless organism or object attractive to the prey; when the prey approaches, the predator attacks. An animal defending against a predator may engage in either “fight or flight” in response to predator attack or threat of attack, depending on its estimate of the predator’s strength relative to its own. Alternative defenses include a range of anti-predator adaptations, including alarm signals.

Theories of Aggression

The most influential theories of aggression can be broadly divided into the following:

• Theories that see aggression as an instinct.
• Theories that suggest frustration is an important factor in aggression.
• Theories that suggest aggression is learned behavior.

Aggression as an Instinct

There are a number of theories (e.g. the Psychoanalytic Approach, the Ethological Approach and the Biological Approach) which attempt to explain aggression by suggesting that aggressive behavior occurs because each individual is born with an aggressive instinct. 

Such theories suggest that aggression is innate, i.e. inborn, and, therefore, unavoidable.  Outlets such as competitive sport and creative pursuits serve to reduce socially unacceptable aggressive behavior. 

Frustration as a Factor in Aggression

The Frustration-Aggression theory proposes that aggression occurs as a result of frustration.  The inference is that if individuals are frustrated in achieving their aims, the most likely response is aggression.  Such individuals are motivated to be aggressive towards whoever or whatever is standing in their way.  While frustration often leads to feelings of aggression, critics of this theory have argued that it does not explain all the causes of aggression. 

Aggression as Learned Behavior

The theory of aggression as a learned behavior is called the Social Learning Approach.  This theory argues that aggressive behavior is not inborn, rather it is something that is learned, either through direct experience or through observation or imitation of others.  The more that an individual’s aggression is rewarded, perhaps by getting what is wanted or by parental approval, the more likely that person is to be aggressive in the future.