Describe and evaluate neural mechanisms in aggression. (8 marks + 16 marks)

Here you should discuss the roles of serotonin and dopamine in aggression.

Serotonin is a neurotransmitter that has an important role in social decision-making by inhibiting aggressive social responses. Serotonin usually reduces aggression by preventing stimulation of the amygdala, a structure in the brain's limbic system. Stimulation of the amygdala increases aggressive behaviour but serotonin prevents stimulation, thereby reducing aggression.

If there are low levels of serotonin in the brain, there is less inhibition of the amygdala. As a result it becomes more active when stimulated by external events, causing the person to act on their impulses and making aggression more likely. Therefore, low levels of serotonin have been associated with an increased susceptibility to impulsive and aggressive behaviour.

The relationship between low levels of serotonin and aggression has been supported by a study by Mann et al.. They administered the drug dexfenfluramine, which depletes levels of serotonin in the brain, to participants. The researchers then used a questionnaire to assess hostility and aggression levels, which were raised after taking dexfenfluramine in males but not in females.

There may be a gender bias in this research. Although a link was found between low levels of serotonin and aggression, this was not evident for the female participants. This suggests that the role of serotonin in aggression may be different for female compared to males, and therefore there is a gender bias in human studies in this area.

Further evidence 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.

The role of dopamine in aggression have also been demonstrated in studies which have used amphetamines, which increase levels of dopamine. Studies have found that when participants are given amphetamines, there is a corresponding increase in their levels of aggression.

Evidence supporting the importance of dopamine in aggression comes from studies using antipsychotics, which reduce dopamine levels in the brain. These studies have generally found that the use of antipsychotics reduces the incidence of aggression, thus suggesting that decreased dopamine levels lead to decreased aggression.

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.

A criticism of these links between neurotransmitters 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.

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