Neural Mechanisms

It is debated whether aggression is controlled by top-down neural mechanisms (cerebral cortex) or bottom-up mechanisms (from the limbic system and reptilian brain).

The top-down process consists of one main neural mechanism that it involved in aggression. This is the pre-frontal cortex which is a specialised part of the neomammalian cortex and is responsible for the decision to move. The neomammalian cerebral cortex is the more intelligent part of the brain which is responsible for high order thinking. It is thought that this part of the brain inhibits aggression. 

Limited evidence for the top-down process and showing evidence that the pre-frontal cortex is linked to aggression comes from Macmillan who describe Phineas Gage's damage to his pre-frontal cortex. After his injury he showed increased levels of aggressive behaviour and appeared drunk at times, swore, and showed inappropriate behaviour that was out of character before the damage. This research suggests aggression is caused by a top-down process, as damage to his pre-frontal cortex is caused his increased aggression.  

Further convincing evidence to support the top-down process comes from Anderson, who found that individuals with damaged pre-frontal cortex during childhood were more likely to show aggression as adult, again suggesting the role of this brain structure in aggression.

Opposing this top-down process, the reptilian brain and limbic system are responsible for simple processes which represent basic human instincts. Parts of these systems include the hypothalamus and amygdala, which control instincts such as hunger/thirst, as well as sexual behaviour and aggression (amygdala), whilst the hypothalamus is thought to be responsible for attack behaviour in aggression (fight or flight). Other bottom-up processes include serotonin, a neurotransmitter that carrier chemical messages from the pre-synaptic neurone to the 5ST receptor sits in the post-synaptic neurone. 5ST receptors regulate mood, sleep and aggression. Therefore, normal levels of serotonin have a calming influence, whilst low levels are linked with aggression. Equally, the hormone testosterone (responsible for male sexual characteristics) is linked with aggression which high levels are present, which perhaps is why men tend to be more aggressive. All of these parts work together in a feedback system.

Convincing evidence to support the bottom-up process and the role of the amygdala in aggression comes from the case of Charles Whitman, who shot and killed 15 people after complaining of strong feeling of aggression. His autopsy found a tumour compressing his amygdala (part of the limbic system and reptilian brain responsible for the bottom-up process), suggesting this brain structure may have been responsible for his aggressive acts. 

Similarly, evidence for the bottom-up process and the role of the hypothalamus in aggression comes from Summer, who reported a patient with a tumour pressing on their hypothalamus in the limbic system. After it was removed, the patient showed far less aggression, perhaps suggesting the bottom-up process must be more involved in aggression than the top-down process.  

Finally reliable evidence to support the bottom-up role of testosterone comes from Dabbs who found prisoners convicted of more violent crimes had more testosterone in their saliva than prisoners who committed less violent crimes.

However, much of this research may be criticised for having difficulty establishing cause and effect, which happens when there is no control over the independent variable. This is because (particularly in the case of Charles Whitman) it is impossible to know if a tumour or brain damage to a particular area actually causes aggression, or if damage to this area interacts with another part of the brain ultimately responsible for the aggressive act.

Beyond these methodological evaluations, the role of neural mechanisms in aggression can be criticised for being simplistic, because the explanation unfairly simplifies aggression by ignoing other important influences. This is because it ignores external, environmental factors that may influence aggression such as the importance of cues (Berkowitz) which are necessary to spark aggression. In the same way, neural mechanisms also ignore Social Learning Theory's suggestion that aggression is learn (children are more likely to behave aggressively if they have learnt vicariously from adult role models; Bandura). As a result, neural mechanisms in aggression can be criticised for ignoring nurture. This is when an explanation explains behaviour using internal, genetic factors alone, ignoring the role of learning or the environment (nurture).

In conclusion, it is clear that bottom-up processes have a greater overall influence in aggression that the top-down process. Simply due to their number they are likely to be more active in aggression, which (due to its biological nature) must be partly instinctive. However, the pre-frontal cortex must have evolved to suppress the natural instincts of the bottom-up processes until such time we are forces to show aggression (e.g. if attacked).