Discuss neural and hormonal mechanisms in aggression. (8 marks + 16 marks)

 You should outline and evaluate the link between aggression and neurotransmitters such as serotonin and dopamine, and hormones such as testosterone and cortisol.

Low levels of the neurotransmitter serotonin have been associated with an increased susceptibility to impulsive and aggressive behaviour. Serotonin usually prevents stimulation of the amygdala, so if there are low levels of serotonin in the brain, the amygdala will be more active when stimulated by external events, causing the person to act on their impulses and making aggression more likely.

Evidence for the role of serotonin comes from the use of drugs that raise levels of serotonin in the brain, such as antidepressants. In clinical studies,  antidepressant drugs which elevate serotonin levels (e.g. SSRIs) also tend to reduce irritability and impulsive aggression. This suggests that  increased serotonin levels do lead to reduced aggression. However, this relationship may not be causal.

Some evidence for the importance of serotonin in aggression comes from studies of non-human animals. Popova et al. found that among dogs that have been selectively bred for domestication and for increasingly docile temperament, there has been a corresponding increase (over generations) of brain serotonin. This suggests that there is a link between low levels of serotonin and high levels of aggression. However, these results may not apply to humans due to the differences between our brain structures and those of dogs.

There appears to be a link between the neurotransmitter dopamine and aggression, in that increased dopamine levels can produce increased levels of aggressive behaviour. For example, the increased rates of aggressive behaviour found in the schizophrenic population are believed to be due to the raised levels of dopamine in the brain.

However, evidence for the causal role of dopamine in aggression is inconclusive. A study by Couppis and Kennedy found that dopamine may be a consequence of aggressive behaviour rather than a cause.  They suggest that people may seek out aggressive encounters because dopamine is released as a positive reinforcer whenever they engage in aggressive behaviour.

There is also research that challenges this link altogether. A meta-analysis of studies that had examines neurotransmitter levels in antisocial children and adults found lower levels in those individuals described as 'aggressive' but no difference in dopamine levels for these individuals when compared to 'normal' individuals.

The male sex hormone testosterone is thought to increase levels of aggression from young adulthood onwards. It is thought that testosterone may influence areas of the brain that control behavioural reactions, such as the amygdala and the hypothalamus. Testosterone also influences the levels of other hormones which are thought to be involved in aggression, such as vasopressin.

Some researchers suggest that we should distinguish aggression from dominance. Aggression refers to the intent to inflict injury, while dominance is the desire to achieve or maintain status over another. Aggression may be just one form of dominant behaviour. The influence of testosterone on dominance is likely to be expressed in more varied and subtle ways. According to this view, much of the research into aggression was actually measuring dominance, making this research invalid.

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 they became more aggressive towards other participants. This has an important real-world implication, as some countries' governments (e.g. that of the USA) are debating whether gun ownership increases or decreases violence. This study would suggest that more guns would lead to more violent behaviour.

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 than boys with higher or fluctuating cortisol levels. This demonstrates that cortisol levels are strongly and inversely related to aggression.

A criticism of these links between neurotransmitters, hormones and aggression is that they can be described as reductionist. The complexity of human behaviour means that biological explanations are insufficient on their own to explain the many different aspects of human aggression. For example, research by Bandura et al. found that social learning can be a powerful influence on the aggressive behaviour of children.


  1. Very well put and extremely helpful.

  2. how many marks would each of your essays be?

  3. @Niya Javed

    It would be a 24 marker split into 8+16. The 8 is A01 and 16 marks for the A02/3/IDA.