1. Territorial defense - Aggression occurs when animals attack intruders who enter their 'territory'.

2. Predatory aggression - This is also common when an animal attacks prey. This form of aggression is not believed to be hunger-induced, but rather involves the lateral hypothalamus and the specific 'trigger' stimuli (the animals it typically feeds on - the prey).

3. Inter-male aggression - It occurs when another (stranger) male is present. Androgens (hormones) are believed to be important in this form of aggression.

4. Fear-induced aggression - always preceded by attempts to escape. This form of aggressive behavior is most evident when the animal is 'cornered' and is afraid. They will almost always react with aggression before they attempt to escape. The snake’s or dog’s bite is in the form of fear-induced aggression.

5. Irritable aggression - this will be evoked by any attackable object or other animal. The ventromedial hypothalamus and amygdala are believed to be the crucial brain structures here.

6. Maternal aggression - when a female reacts with aggression in order to protect her young from harm.

7. Instrumental aggression - when aggression was used successfully in a given situation in the past the animal will use this behavior again (in the same or similar situation - therefore the behavior has been reinforced via learning).

In summary, Moyer believed that there were many subcategories of aggressive behavior, and therein lies the problem - defining aggression in an 'all encompassing fashion' is simply not correct.

Moyer believed the environment, the specific aggression, the inducing stimuli and which neural (brain) circuits are involved all must be taken into account when attempting to define aggressive behavior. Discovering which brain circuits - the amygdala, the hypothalamus, the septum and/or the hippocampus - is an obvious difficulty when studying (and defining) aggression in humans. New sophisticated brain-imaging techniques (e.g. MRI) will undoubtedly shed light in this field of study in the near future.

Lets now study about another psychologist’s Paul Brain theory. He in 1979 criticized Moyer's theory however, primarily because he believed that there was a great deal of category overlap in Moyer's classification system. Brain proposed an alternative classification, which focused on the utility (the usefulness) of the aggressive behavior to the animal and NOT (unlike Moyer) on the evoking stimuli/environment. Brain's classification was as follows:

1. Self-defensive behavior - when aggression results from threats to the individual, is accompanied by fear and is generally preceded by escape/avoidance attempts.

2. Social conflict - a variety of activities, all of which relate to inter-specific competition for resources that are important for reproductive success.

3. Predatory attack - attack of an object which approximates to a natural prey species

4. Parental defense - when parents react aggressively in order to protect their young ones.

5. Reproductive termination - refers to infanticide - killing of the young.

In conclusion, the difficulties of defining aggression lie in the fact that there are many different kinds of aggressive behavior. Animal aggression has obvious parallels to the aggressive behaviors exhibited by humans. Which type of behavior that will be exhibited therefore depends on the environmental circumstances, the active neural circuits and the specific, 'triggering' stimuli (Moyer) and/or how useful the behavior is to the animal/human exhibiting the aggression (Brain). All of these factors should therefore be considered when attempting to explain aggressive behavior - it is not enough to simply say he/she is 'aggressive'.