Genetic Explanations of Aggression; The MAOA Gene

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AO1 - The MAOA Gene

  • The gene Monoamine Oxidase A (the MAOA gene) has recently been thought to be a cause of aggression.
  •  Found on the X chromorsome, this gene's role is to regulate the enzyme also known as monoamine oxidase A (MAOA) which breaks down excess neurotransmitters in the brain such as dopamine and serotonin after they have transmitted an impulse from one nerve cell to another.
  • When the excess is broken down, the neurons in the brain are ablke to communicate more efficiently. 
  • There are two abnormalities that can occur on this gene one of which is thought to cause increased aggression; MAOA-L.
  • MAOA-L causes there to be too little of the MAOA enzyme in the brain, this low activity means that the excess neurotransmitters cannot be broken down. Studies have indicated that there is a link between aggression and the MAOA gene, especially this variant.
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Research Evidence

  • There is a large amount of support for the link between a defective MAOA gene (MAOA-L) and increased aggression
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Family Studies

  • Brunner et al (1993) - Brunner found that the 5 male members of the family with a record of aggression were all found to have excess levels of neurotransmitters usually broken down by the MAOA gene in their urine samples, showing that they all have a low levels of MAOA enzyme activity. Brunner concluded that this could have possibly been caused by the MAOA-L abnormality on their genes. This suggests that types of behaviour, including impulsive aggression may have a genetic cause.  
    • Limited gene influence- Although this study is informative, the assumption that the MAOA gene deficiency causes aggression is not widely supported. Brunner himself said that it was unlikely that there was a direct causal relationship between a single gene and specific behaviours. A more intergrated conclusion would be that genes have some influence on behaviour but arent the sole cause.
    • Environmental factors- In the family studied by Brunner, the frequency of AB was higher than what you would find in a 'normal' family, maybe the AB is caused by shared environmental factors such as bad parenting/role modelling, making results less generalisable to a general population
      • These issues with the study weaken the amount of support that it gives to the theory that AB is caused by genotypal abnormalities.
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Animal Studies

  • Cases et al (1995) - Found that upon disabling the MAOA gene in the X chromosome of mice, the males became highly aggressive whereas females were unaffected by the procedure. Whilst the males lacked the MAOA enzyme to break down the neurotransmitters, causing their levels to rise to an excess, females still had a functioning gene on their other X xhromosome to continue the function. Upon restoring the males gene, the mice returned to their normal state.
    • This clearly supports the theory that AB is caused by abnormalities on the MAOA gene, and that it is found on the X chromosome.
      • Simplistic- Because the findings of the study are based on simple animal models, we cannot draw the oversimplistic causal conclusion that abnormalities on the MAOA gene cause aggression in humans, human behaviour is much more complex.
    • This major issue weakens the amount of support that the study can lend to the gene-hypothesis of aggression.
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fMRI Studies

  • Meyer-Lindberg et al (2006)- fMRI scans of healthy pps found significant reductions in volume of several brain areas found to be involved in emotion such as the amygdala/pre-frontal cortex, in MAOA-L pps compared to MAOA-H pps. These structures are often impaired in anti-social individuals.
    • This study gives direct support to the idea of the link between genetic abnormalities and an increased tendency towards aggression.
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Response to Provocation

  • McDermott et al (2008) - found that MAOA-L pps showed higher levels of aggression in response to provocation, compared to those with the MAOA-H variation. As well as this, evidence for a gene/environment interaction was also found, in that aggressive reponses were modulated by environmental factors. When the money taken from the MAOA-L pps was a high amount, they were more likely to be aggressive (forcing the other person to eat chilli sauce) than when provocation was low (less money was taken.)
    • This study clearly supports the idea that people with the MAOA-L gene variant are more likely to exhibit aggression (when provoked) and so supports the theory.
      • Not representative- Although this study does provide a link, the findings cannot be generalised to all examples of aggression as the measure of aggression used (eating chilli sauce) is not a reflection of real-world aggression. 
      • Individual differences- Factors such as whether or not the person values money, or wanted to keep the money won in the task could affect the liklihood that they would pay to punish the other person, if this is the case then the 'aggressive behaviour' may not just be due to their gene abnormalities.
    • The issues raised from this study weaken the amount of support it gives to the theory
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  • The MAOA hypothesis is clearly on the side of nature in the nature/nurture debate, as it does not consider the role of nurture e.g. environmental factors) when explaining AB, which can also influence aggression. A more valid explanation of the causes of this kind of behaviour would be to use an interaction between nature and nurture, for instance, if males did have a genetic predisposition towards aggressive behaviour, this may lead them to actively seek out aggressive environments.
  • Another issue is that the explanation is biologically determinist, therefore suggesting that behaviour is caused by factors outside of the individuals control. This raises problems when rsponsibilty of behaviour is trying to be determined, such as the legal system. An example of this is the trial of Jon Hall, whilst although evidence was given to suggest that the low levels of serotonin found in his cerebrospinal fluid (CSF) was what caused him to commit the violent act of murdering his estranged wife. But, the state reached the conclusion that Hall was responsible for his actions, as measuring serotonin in the CSF did not give an accurate indication of serotonin at the synapse (where it operates), and also there was no general consensus of what constituted as a 'normal' level of serotonin. Hall was convicted and was assumed to have free will over his actions, making him responsible.
  • This overly simplistic approach to explaining a complex behaviour such as impulsive AB is useful to establish cause&effect, but is limited as it does not consider other influencing factors. A more holistic approach would be an explanation that included many different influences, but then a causal relationship becomes difficult to draw. 
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