Discuss the role of neural and hormonal mechanisms in human aggression. 25 marks
Neurochemistry provides a compelling argument for biological factors causing aggression. Both hormones and neurotransmitters seem to be involved in aggression.
Testosterone is an androgen, or male hormone, that seems to have a link with aggressive behaviour. There are two models of testosterone; the basal model states that aggression is a result of high levels of testosterone whilst, in contrast, the reciprocal model states aggressive behaviour brings about high levels of testosterone.
Evidence could be used to argue for the basal model of aggression because during puberty aggression increases when androgen levels are higher (especially in males).
Further evidence for the effect of testosterone was provided by Wagner et al who carried out a study in which mice were castrated. It was found that the level of their aggressive behaviour decreased after castration (testosterone is produced in the testes) and rose again once the mice were injected with testosterone. Wagner’s research gives good evidence for the basal model of testosterone which states that aggression is due to testosterone levels. However, aggressive behaviour did not completely disappear after castration which suggests that while testosterone is a contributing factor, there must be other influences involved. Evidence from castrating mice should be viewed with some caution as it is debatable whether this can directly be generalised to humans. While some researchers argue that the physiological processes of mice and humans is similar enough for research findings to be useful, it has to be acknowledged that humans have cognitive abilities which mice do not possess and these could modify the effects of hormone levels on aggressive behaviour.
Further evidence, this time from human studies, came from Dabbs who tested the saliva of twelve fraternities across two universities in the US. He found that those with lower levels of testosterone were more likely to show non-aggressive behaviours such as smiling or waving at fellow students. Dabbs then used this same method with prisoners and found that those with higher testosterone levels had committed more violent crimes (i.e. rape and murder).
However, many of these studies, both with animals and humans, were correlational and such studies cannot establish cause and effect as there could be many other environmental and psychological factors which contributed to the aggressive behaviour apart from testosterone levels. There is also the question in Dabbs’ fraternity study of how non-aggressive behaviour was operationalised; for example smiling may not necessarily be a sign of non-aggression and those who don’t smile aren’t necessarily aggressive. This could render the study invalid as the researcher may not have been measuring what he claimed to be measuring – i.e. aggressive behaviour.
It is too simplistic to attribute aggressive behaviour to testosterone alone and it is thought that the neurotransmitter, serotonin is also involved in an inhibitory role. Research has found that low levels of serotonin in vervet monkeys is associated with an increase in aggressive behaviour.
It is a mistake to assme that aggressive behaviour is simply the result of abnormal levels of brain neurotransmitters or fluctuating levels of hormones, as they are governed by brain structure. The amygdala is an important area of the limbic system and studies using cats have shown that stimulation of the amygdale leads to more vicious and predatorial attacks. Furthermore, it has been found that there is a density of receptor sites for testosterone around the amygdale which would also explain its role in aggressive behaviour.
The case of Charles Whitman, an infamous mass murderer provides some evidence for this. During autopsy, Whitman was found to have a tumour pressing on his amygdala. This was a possible explanation for why Whitman did something which those who knew him considered to be so out of keeping with his character.
Sometimes, abnormalities in the pre-frontal cortex also seem to be implicated in aggressive behaviour. This is understandable as the pre-frontal cortex plays an inhibiting role on impulsive behaviour, stopping us from doing stupid and irrational things. Phineas Gage suffered damage to his pre-frontal cortex when an iron bar pierced his skull in 1848. Phineas lived for twelve years after his accident, but underwent a personality change, was more aggressive and unable to interact with others in a positive way. Unfortunately, more cannot be learned from this case study as it occurred such a long time ago and the doctor’s handwritten notes produced with the more limited knowledge of the time are all we have to go by.