A lot of research has been conducted into how aggression is affected by the structure of and hormone levels in the brain.
Androgens, and in particular testosterone, has been linked with higher levels of aggression in both males and females. NELSON conducted a meta-analysis of research into levels of androgens in male and female prisoners and found that there tended to be a positive correlation between high levels of androgens and aggressive behaviour. However these levels were not known at the time of the aggressive acts committed and so it cannot be certain that this is the prevailing factor in affecting behaviour. As this sample, as with many studies into hormonal mechanisms related to aggression, uses only prison inmates, it may be that the data collected could be ungeneralisable to the general population.
DABBS looked at the relationships between testosterone, crime and prison behaviour. Testosterone levels were measured in the saliva of 692 adult male prisoners and found that those who has been convicted of sexual and violent crimes had higher levels of testosterone than inmates who had committed less physically aggressive acts, such as theft and burglary. This study could be accused of showing beta bias as the sample used only males and therefore it is unclear if the results can be generalised to females as well.
DABBS has conducted another study which did look at female prison inmates with 84 participants. It was found that testosterone levels was highest in those who had committed unprovoked violence but lowest in those who had committed violence in defence. This raises as issue in aggression research as links found between hormones and aggressive behaviour can be very different depending on how aggression is operationalised.
In males, androgens encourage not just aggression but also social dominance, impulsiveness and competition. PILLAY noted that testosterone is often associated with athletic qualities which can include aggressive behaviour. Saliva samples obtained from 94 athletes indicated that males and females in aggressive sports had the highest levels of testosterone. However, the methodology of this study can be questioned as saliva samples may not always be reliable due to the fact that hormone levels in saliva fluctuate.
HUSTON ET AL also looked at the effects of high testosterone and found that men with high levels performed well in competitive tasks but not as well on cooperative tasks. This could be because testosterone causes a person to be competitive and hence dominant.
However it must be noted that there are individual differences as HARRISON ET AL conducted a study with 56 male participants aged 20-25 who were given testosterone and then asked to play a frustration-inducting computer game. Aggression responses increased by not for everyone.
CONNOR studied rats that had been castrated early, soon after birth, and found that as adults there showed low levels of aggression that did not get higher when they were injected with testosterone. As well as this, research into testosterone in animals may not be generalisable to humans as it presumes that we respond to hormones in the same way which may not be the case. Although research tends to agree that castration can reduce aggression, it affects several hormone systems as well as testosterone and these other hormones may also play a part.
Another factor that is thought to influence aggressive behaviour is the role of neural mechanisms, which are either structures within the brain or the influence of certain chemicals such as neurotransmitters. A neurotransmitter is a chemical messenger which allows impulses to be passed from one brain cell to another. One neurotransmitter thought to be involved with aggressive behaviour is Serotonin. Normal levels of serotonin exhibit an inhibitory effect to emotional stimuli that may lead to aggression. Low serotonin levels, therefore, remove the inhibitory effect and individuals become more prone to impulsive and aggressive behaviour. This was demonstrated by research carried out by MANN ET AL (1990) who gave 35 healthy participants dexfenfluramine (which reduces serotonin in the brain) and a questionnaire. The findings showed that males reported an increase in aggressive behaviour after taking the drug. This therefore supports the link between low serotonin and aggression. However, the self-report method used is unreliable due to a tendency of social-desirability bias. This means the answers given may not have been 100% accurate, to fit into a perceived general trend. For example, women may have had the same effect as men but did not state this in their answers. This is a problem as the research may lack reliability.
The neural/hormonal explanation of aggression is reductionist as it over-simplifies complex behaviour and focuses solely on the cause of aggression being a structural or chemical problem. Aggression may also depend on social situations and events and cannot be attributed to only biological factors. It also fails to explain why there are cross cultural differences; if neural and hormonal mechanisms were the sole cause we would see a world-wide distribution of aggression. In conclusion, it may be observed that a broader range of approaches and factors needs to be assessed before implementing a conclusive explanation for aggressive behaviour.
Research is also deterministic; as it assumes that everyone will act aggressively to the hormonal shift and does not take into account individual differences of exerting free will. A deterministic view is usually more applicable to non-human research, which is a problem as this means that much of the human research surrounding aggression lacks validity.
The hormone cortisol is thought to inhibit aggression. It is thought to do this by having a mediating effect on other hormones related to aggression such as testosterone. High levels of cortisol inhibits testosterone and so inhibits aggression. This may be due to the fact that cortisol increases anxiety and the likelihood of social withdrawal.
The moderating effect of cortisol on aggressive behaviour is supported by a four-year study of boys with behavioural problem. The boys with consistently low cortisol levels began antisocial acts at a younger age and exhibited three times the number of aggressive symptoms with boys with higher or fluctuating cortisol levels. This demonstrates that cortisol levels are strongly and inversely related to aggression.
Research has shown that simple exposure to certain stimuli can increase testosterone levels and thus aggression. KLINESMITH ET AL found that when participants assembled a gun, their testosterone levels increased and thus became more aggressive towards other participants. This has an important real-world implication, as some countries’ governments are debating whether gun ownership increases or decreases violence. This study would suggest that more guns would lead to more violent behaviour.