Aggression: Evaluating the Role of the Limbic System, Neurotransmitters & Hormones

These study notes evaluate the following explanations of aggression: 1) Limbic System, 2) Neurotransmitters, and 3) Hormones.

Kluver and Bucy (1939) were early researchers who, using Rhesus monkeys, removed the main areas of the limbic system including the amygdala, hippocampus and surrounding cortical areas. They found that the monkeys displayed an absence of emotional, motor and vocal reactions normally associated with stimuli or situations eliciting fear and anger. Lesioned monkeys also lost the social understanding of group hierarchies and would try to fight the more dominant and larger members of the group. This research demonstrates the importance of the limbic system in regulating aggressive responses. One of the main criticisms of research using animals to provide evidence for aggression in humans is the differences between animal and human physiology and the question of whether we can extrapolate research findings from animals to human aggressive behaviour. Despite humans and monkeys both possessing similar neural structures, we cannot be sure that the processes involved in mediating aggression in humans are the same as those shown in animals such as the Rhesus monkey.

More recent technological advances have allowed neuroimaging techniques such as MRI scans to investigate the relationship between neural structures such as the amygdala and aggressive behaviour. Wong et al. (1997) undertook MRI scans of 19 violent male criminals in Broadmoor hospital and compared the size of the amygdala with 20 ‘normal’ control subjects. He found that the volume of the amygdala was significantly smaller in the 19 violent criminals, thus supporting the role of the amygdala and limbic system in aggression. One of the problems with this research is a lack of population validity: the sample was relatively small, and thus the issue of whether these findings can be generalised to the wider population can be raised. The research can be accused of gender bias, as their research was confined to males; therefore, caution must be taken in using this research to explain aggression in females. When research is used to explain behaviour is both males and females yet only undertaken on males, this is called beta bias.

Ferrari et al. (2003) provide support for the role of serotonin in aggressive behaviour. They allowed adult male rats to fight with another rat at a specific time for ten days. On the eleventh day, the rat wasn’t allowed to fight. However, researchers found that the rat’s dopamine levels had raised by 65%, and his serotonin levels were reduced by 35%. Despite the fact that the rat was not fighting, the experience had changed the rat’s brain chemistry. Ferrari et al.’s (2003) research raises the question of whether lower levels of serotonin cause aggression or whether they are a response to aggression being carried out. The issue of cause and effect is a key factor in the explanation of aggressive behaviour. The aim of any science is to establish the cause by measuring the effect. However, if the cause, i.e. lower levels of aggression, are the effect, this substantially lowers the validity of the explanation as to the causes of aggression. Nevertheless, this research does demonstrate the complexity of the role of serotonin in aggressive behaviour.

Mann et al. (1990) administered the drug dexfenfluramine (which depletes serotonin in the brain) to 35 healthy adults. The researchers then used a questionnaire to assess hostility and aggression levels, which rose following administration of dexfenfluramine amongst males, but interestingly not amongst females. The research by Mann et al. (1990) demonstrates the issue of beta bias that is inherent in neural explanations of aggression and shows that males and females may not be subject to the same physiological factors when explaining aggression.

Psychologists interested in finding out if testosterone was implicated in aggression decided to test out this theory by castrating animals, thereby removing their testes. Wagner (1979) castrated mice and aggression levels went down, thus providing support to the theory that testosterone is implicated in aggression. Wagner’s (1979) research however only provides correlational support for the cause of aggression, as the research only demonstrates a relationship between lowered testosterone and lowered aggression. However, Wagner’s (1979) research does demonstrate that testosterone plays a crucial role in aggressive responses.

Exam Hint: It is important to ensure you focus on answering the question. If you are asked to consider the role of neural and hormonal explanations in aggression, make sure you specifically refer to the role.

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