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The two neurotransmitters that are believed to be the most associated with aggression are low levels of serotonin and high levels of dopamine. These two chemicals allow impulses to be transmitted to another area; therefore all behaviours are influenced by neurotransmitters. There is also the influence of the amygdala, which controls the emotional responses, the hypothalamus, which coordinates both the autonomic nervous system and the activity of the pituitary and the frontal cortex. Normal levels of serotonin produce a calming effect on the individual.

However, low levels remove that calming effect leaving the individual less able to control their impulsive and aggressive behaviour. Serotonin usually keeps the amygdala under control so that the emotional responses are disciplined. If there is less serotonin, there will be less inhibition and so when the individual’s amygdala is stimulate by external events, it becomes more active causing the person to become more aggressive. Flynn and other psychologists conducted research in the 60s to test the effect that the amygdala has on aggression. There was a study where cats had the amygdala removed.

After being removed, the cats were described as tame and significantly reduced aggression. This studied showed that the amygdala did cause aggression, but when amygdalectomy was performed on humans, although the aggression was reduced, they were left with what was observed as no emotion at all. And so the amygdala is necessary for emotions and therefore cannot really be removed to prevent aggression. However, not all low serotonin sufferers are violent. Booij et al conducted a longitudinal study measuring aggression from parental and self-reports and PET scans.

They found higher levels of aggression in children with low levels of serotonin. This study supports the idea that serotonin levels affect aggression. Dopamine may influence aggression as the brain appears to see it as a reward and so whenever we do something that we find rewarding the brain releases higher levels of dopamine. The chemical creates a pleasure circuit, and therefore we repeat the cycle, which could results in the individual becoming aggressive more often. Van Ero and Miczek measured dopamine levels in the prefrontal cortex of male rats before, during and after conflict with another rat.

They found that there was an increase in dopamine levels after the confrontation suggesting that this chemical imbalancement is the result of aggressive behaviour and not the cause. The frontal cortex deals with impulsivity, social interactions, irritability and short temperedness. It modifies our emotions and interprets social situations. It also has pathways to the amygdala and the hypothalamus, and so it is linked to aggression. The case study of Phineas Gage in 1848 supports this idea. Gage suffered an injury in an accident when a tamping iron, 3cm in diameter, passed through his jaw behind his eye and out of the top of his head.

He survived for eleven years after the incident. However, it was recorded that Gage’s persona completely changed. As before he had a stable job and was an average and kind man, he was now unable to hold down a job and was constantly aggressive. However, this is a case study and therefore only depicts one individual therefore results cannot be generalised as he may have completely unique. The results also cannot be repeated and so are not entirely reliable. There were no MRI scans available at this time and so it is still unknown exactly what part of the brain the iron damaged.

The aggression could also have been to do with the blindness he suffered, and the frustration of his state. He may have attracted unwanted attention and other people’s reactions to him could have increased his aggression. All these factors mean that this case study lacks internal validity and so cannot be completely reliable. There are also a few hormones that could account for aggressive behaviour. These include testosterone and cortisol. Testosterone is a male hormone that is often considered and important factor of aggressive behaviour.

Hutchinson conducted a psychological experiment in 1979 to test this idea. They castrated mice and found that they displayed lower levels of aggression then they had before: the average bites per day by the mice dropped from 800 to 100. When the testosterone was then put back into the mice, they began to show their normal levels of aggression again. This experiment showed that testosterone does indeed cause aggressive behaviour. Of course the fact that mice were used in this experiment means that generalising and applying the results to humans is impossible, therefore lacks validity.

There is also the ethical issue of using animals where there is a lack of consent, and cruelty to the animals. However, there was a study by Dabbs in 1987 that did research on men and so holds more credibility. Testosterone was measured in the saliva of 89 male prison inmates. The results showed that the inmates with higher testosterone concentrations had been convicted of violent crimes more frequently. 9 out of 11 inmates with the lowest testosterone were the ones who had committed nonviolent crimes, and 10 out of 11 inmates with the highest testosterone levels had committed violent crimes.

This study showed that higher levels of testosterone was the common occurrence in most of the violent criminals, therefore suggesting levels of testosterone do have an influence on aggression. However, a weakness of this study is that it is a small sample size and is of criminals, therefore cannot not be representative of the population. A study that criticises the idea that hormones can have an effect on aggressive behaviour was conducted by Capsi in 2002. He looked at 500 male children and found that a variant of the gene for low levels of MOMA and a variant of the gene for high levels of MOMA.

They found that people with low levels and who experienced maltreatment as a child were highly likely to become aggressive later. Those who had high levels of MOMA or who were abused in some way as children did no show significant levels of aggression and most importantly neither did those children with low levels of MOMA who experienced no maltreatment. This study shows that low levels of MOMA is not the sole cause of aggressive behaviour, but rather experiencing abuse in childhood. Unless there is a trigger from the environment (such as abuse) levels of MOMA will not dictate aggression.

An issue with much of the research done on hormonal reasons for aggression were samples from prisons, so are not representative of the population. This theory is also highly deterministic as it says that aggressive behaviour is inevitable if problems in neurons or hormones occur. This may lead to injustice such as in the example of a series of cases in the 1980s where hormonal fluctuations around pre-menstrual tension was used as grounds for temporary insanity. This resulted in some murder charges being reduced to manslaughter as PMT was taken as a contributory factor behind the killings.

Idea of hormonal and neural mechanisms in aggression may also be socially sensitive as it labels people that may have neural problems as being aggressive, and this raises issues for ethical reasons (discrimination). A theory that disagrees with the idea that hormone and neuron mechanisms are the cause of aggression is social learning theory. This theory introduced by Bandura states that aggression is the result of behaviour witnessed by a role model and then copied by the individual.

Another theory that also criticises neural and hormonal mechanisms is deindividuation. This theory says that people become aggressive due to anonymity, suggestibility and social contagion. There are many studies and evidence that has been collected to support the idea of hormonal and neural mechanisms in aggression, but there are also many issues with this suggestion, and so cannot be universalised. Other theories such as social learning theory and deindividuation may also be causes of aggression.

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