Aggression biological approach - Neural

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The role of neural mechanisms in aggression

Amygdala

  • The Amygdala forms part of the limbic system. The limbic system is a part of the brain located on top of the brain stem andwithin the temporal lobes.
  • One of the functions of the limbic system is to control emotions. It therefore makes sense to see how this area of the brain could be linked to aggressive outbursts.

Kluver and Bucy (1939)

  • Removed the entire temporal lobe much of the limbic system from monkeys. They found that monkeys became much more docile showing that aggression may be caused by this part of the brain.

Further stuides narrowed down the part of the limbic system that was respossible for aggression and settled on the amygdala. This is because removal of the amygdala alone causes a decrease in aggresion.

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The role of neural mechanisms in aggression

Conversely, stimulation of the amygdala seems to cause an increase in aggression.

A02 - Charles Whitman in 1996

  • Killed his mother and wife then also killed and injured some students from the univeriaty of Texs in Austin. A postmortem showed that he had a cancerous tumour on his amygdala. This tumour would have stimulated the amyglada due to the pressure it would have applied to it thus showing a link between the amyglada and aggression.
  • Limitation of this research, is that is a case study and therefore it can not be assumed that these findings would be found in another person with a tumour on the amyglada as Charles Whitman may be unique.
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The role of neural mechanisms in aggression

Commentary on the role of the amygdala in aggression

In addition to the studies mentioned alrerady there are a number of other studies which suggest that the amygdala is responsible for aggressive behaviour.

Narabayashi 1961

  • Developed a procedure called 'Amygdalotomy' which involves destroying the amygdala. This procedure was used on patients with extreme levels of aggression. Narabayashi 1972 went on to report great success for the procedure with 43/51 patients showing a decrease in aggressive behaviour.
  • Why is this revelvant? The participants were less aggressive following the removal of the amygdala. This supports the theory that the amygdala is responsible for aggressive behaviour.
  • However, the amygdala is responsible for things other than aggression e.g. fear, pleasure, memory and so the reduction in aggression could be attributed to general brain damage following the removal of a significant neural structure. This weakens the conclusions somewhat.
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The role of neural mechanisms in aggression

Raine et al 1997

  • Also support the role of the amygdala in aggression. They carried out PET scans on a group of murderers (a measure of aggression) and a group of controls. They found that the murderers did have abnormalities in the amygdala.  
  • Why is this relevant? This shows a direct limk between the amygdala and aggressive behaviour.
  • However, this was one of many brain abnormalities thgey discovered in the group of murderers. This makes it difficult to narrow down exactly which neural structure was responsible for the aggressive acts.
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The role of neural mechanisms in aggression

Serotonin and Dopamine

Serotonin is a chemical in the brain that keeps us calm. It stops us overreacting to emotional stimuli. It is suggested that if levells of serotoin became too low this inhibitory effect is reduced and aggression is more likley to result. Specifically low levels of serotonin have been experimentally linked to impulsive behaviour, aggression and violent suicide.

It has been found that major metabolite of serotonin is low in the cerebrospinal fluid of people who displaying impulsive and aggressive behaviour. This suggests that these people do not have much serotonin in their system and is consistent with the theory.

Dopamine is a chemical that is responsible for many things from movemnet to motivation. It is suggested that high levels of dopamine lead to aggressive behaviour, however, it should be noted that the link between dopamine and aggression is not as strong as the link between serotonin and aggression.

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The role of neural mechanisms in aggression

Commentary on the role of serotonin and dopamine on aggressive behaviour

Raleigh et al 1991,

  • Used an independent groups design on two groups of monkeys. One group was fed diets high in tryptophan, and one group was fed diets low in tryptophan. Tryptophan is abuilding block for serotonin. they found that the first group showed a decrease in aggression, whereas the second group showed an increase.
  • Why is this relevant? This study shows a direct link between low levels of serotonin and aggression so supports the theory.
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The role of neural mechanisms on aggression

Mann et al 1990,

  • Gave 35 healthy participants a drug called 'dexfenfluramine'. This drug is known to decrease serotonin. Using a questionnaire to assess hostility and aggression they found that this drug caused an increase in hostility and aggression in their male participants but not their female pateints.
  • Why is this relevant? This study shows a direct link between decreased serotonin and increased aggression and therefore supports this theory.
  • However, this study can only provide partial support for the theory as the female participants did not show an increase in aggression following a reduction in serotonin. This raises interesting questions a to whether aggression is caused by different factors for men and women and is cautionary tale for any researchers wishing to generalise conclusions from all male samples when studing this area.
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The role of neural mechanisms in aggression

Ferrari et al 2003

  • Found support for both the role of serotonin and dopamine in aggression. They allowed a rat to fight every day for 10 days at the same time. on the 11th day the animal was not allowed to fight but they measured its serotonin and dopamine levels which were llow ans and high respectively.
  • Why is this relevant? this study shows that an animal who is expecting to behave aggressively has low levels of serotonin and high levels of dopamine and so supports both theories.
  • However, this study does raise the issue of cause ans effect. It could be the case that being aggressive or expecting to causes the neurotransmitter levels to chnage rather than the other way around
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The role of neural mechanisms in aggression

Scerbo and Raine 1993

  • Carried out a meta-analysis which showed a role for serotonin in aggression but not dopamine. They looked at 29 studies published prior to 1992 looking at neurotransmitter levels in antisocial children and adults. They found lower levels of serotonin in such people but found their levels of dopamine to be stable.
  • Why is this relevant? this study provides strong support for the theory linking low levels of serotonin to aggression but goes against the theory linking high levels of dopamine to aggression and so suggests that serotonin is a more important factor to look at when trying to explain aggression.
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The role of neural mechanisms in aggression

Couppis ans Kennedy 2008

  • Investigated the complex relationship between dopamine and aggression using as a starting point the idea that dopamine levels increasing causes a natural sense of pleasure. They found that in mice, performimg an aggressive act led to the reward pathway in the brain being activated and dopmaine levels increasing. Dopamine acted as a positive reinforcer and so may have been  the factor that encouraged these rats to act aggressively in the future.
  • Why is this revelant? This study shows us that increased dopamine cant explain why aggression begins but can explain why aggression continues once it has occurred.
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The role of neural mechanisms in aggression

Issues and Debates Limitations

  • Nature = The neural theories take the 'nature' side of the nature/nurture debate as they look soley on the role of biology in trying in trying to explain the orgin of aggressive behaviour. This is a limitation as the theories ignore the quite compelling evidence from the 'nurture' side of the debate such as the SLT which provides strong evidence to the contrart- that a person's environment can lead to them showing increased aggression.
  • Reductionist = The neural theories are reductionist as they attempt to explain the acuse of aggression in very limited terms. This is a limitation as the act of aggression is so complex that it is arguably not possible to narrow down its cause to a biological factor such as decreased serotonin. This is illustrated by Raine et al's 1997 study which showed that many neural factors were linked to the aggressive behaviour, each part of the brain and each neurotransmitter is responsible for a number of behvaiours and so it is not possible to narrow down the cause of aggression to one factor alone. It is more realistic to suggest that several factors work together to cause aggression.
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The role of neural mechanisms on aggression

Issues and debates Limitations

  • Socially sensitive = The neural theories can be seen to excuse aggressive acts in humans as the theories believe that any aggression shown is outside of our conscious control. For example, Charles Whitman is thought to have shown aggressive behaviour as a result of the tumour on his amygdala. It is not his fault that he had a tumour and this raises interesting questions about the legal responsibilities of such individuals. If a person acts in a way that they can't control as a result of faulty brain structures/ chemicals should they be accountable for their actions there could be an increase in aggressive crime in society and victims may not receive the justice that they need.
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