Describe and evaluate the role of genes in the causation of aggressive behaviour

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Psychology Unit 3

Describe and evaluate the role of genes in the causation of aggressive behaviour


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Twin studies

McGuffin and Gottesman found a concordance rate of 87% for aggressive behaviour in Monozygotic (MZ) twins and 72% in Dizygotic (DZ) twins.

It was assumed that MZ twins would share 100% similar aggressive behaviours and DZ twins 50%

This finding indicates that family environment exerts an important role.

Studies support the importance of genetic factors in aggression but also highlight the significance of environment.

Mason and Frick looked at 12 twin studies using 3795 twins and 3 adoption studies.

They found that the 50% difference between anti-social behaviour and non-anti-social behaviours could attribute to genetic factors.

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Evaluation of twin studies

In twin studies assumptions ar made:

  • Twins are usually raised in the same family environment
  • MZ twins are more alike and the extra likeness should be due to genetic factors
  • Mz and DZ twins are raised in the same home experience and environment bu MZ twins may be treated more alike as they look and behave more similarly.


Many twin studies dont take gene-environment interaction into consideration, causing a lot of the studies to become reductionist

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Adoption studies

If researchers find a greater similarlity in levels of aggression between adopted children and biological parents it would suggest an important genetic influence

If children are more similar to their adoptive parents than their biological prents it would suggest environment has more of an influence

Hutchinsgs and Mednick reviewed 14000 adoptions. They found significant positive correlations betwen the number of convictions for violent criminals among biological parents.

Suggesting that biological influence is greater than environmental.

Rhee and Waldman carried out a meta-analysis of 51 twins and adoption studies using 87000 individuals.

They calculated that the genetic influence over the studies was approximately 40% and 60% was environment contribution.

There were little gender differences found.

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MZ twins reared apart is rare, so researchers have to compare MZ and DZ twins to assess which type is more similar

Twins and adoption studies are natural experiments where the IV already occurs so cannot be manipulated, making it impossible to control confounding variables

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It has been discovered that the gene producing Monoamine Oxide A (MAOA) may be associated with aggressive behaviour.

  • In the 70s a family was discovered to have many of the men with histories of extreme violence
  • One family member tracked this condition back 100 years and found 9 others with similar behavioural characteristics.
    • It was concluded that these men were suffering a defective MAOA gene.

Evaluation of research:

  • Reductionist:
    • Doesnt take into account the fact that these men shared similar family environments.
    • The assumption that this gene is resposible for aggresion isnt widely recognised.
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Evaluation of genes

  • Morely and Hall criticised that gene association is deterministic and only poorly predicts the likelihood that an individual will display aggressive behaviour.
    • They also stated that the information obtained from genetic studies may be useful in helping the development of new treatment for personality disorders.


  • The presence and absence of enviornmental risk factors cannot be idenfied by genetic testing, making accuracy of the predicition of specific behaviour less likely.
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