Genetic Factors in Aggression

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Twin and Adoption Studies - A01


 Monozygotic (MZ) twins share the same genes - IDENTICAL 

Dyzogotic (DZ) twins share 50% of the same genes- NON-IDENTICAL 

  • If MZ twins are more similar in aggressive behaviour than DZ twins, this would suggest that aggressive behaviour has a STRONG GENETIC INFLUENCE. 


-If a POSITIVE CORRELATION is found between an adopted child's aggressive behaviour and that of its biological parent then this implies a GENETIC EFFECT. If the correlation is STRONGER between the adopted child and their REARING FAMILY, then this suggests that ENVIRONMENTAL influences are STRONGER.

  •  MILES AND CAREY: 1400 adoptions in DENMARK: A study on adopted boys with CRIMINAL CONVICTIONS, and found that a significant number had BIOLOGICAL PARENTS who had criminal convictions, suggesting a strong genetic influence. 
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Twin and Adoption Studies - A02


  • A Meta-analysis of 24 twin and adoption studies. Found evidence of a STRONG GENETIC influence in the development of aggressive behaviour. 
  • In younger individuals, BOTH genetic and environmental influences contributed to aggression, but in OLDER individuals environment influeneces were LESS important than GENETIC influences. 

Rhee and Waldman supported miles and carey in a later META-ANAYLSIS of 51 twins and adoption stuides. They concluded that aggressive and ANTI-SOCIAL behaviour was largely the product of genetic contributions. 

THERE ARE PROBLEMS IN ASSESSING AGGRESSION: As most studies have relied on either PARENTAL or SELF-REPORTS of aggreeive behaviour. In the Miles Carey study those who had a stronger genetic influence on aggressive behaviour had used PARENTAL AND SELF REPORTS, where those studies that used OBSERVATIONAL techniques found significantly LESS GENETIC influence. This suggests that the method used to asses aggression is a significant influence on findings. 

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A Genes For Aggression? A01

The Role of MAOA

  • One of the genes reponsible for producing MAOA is associated with aggression. 
  • MAOA regulates the metabolism of SEROTONIN in the brain, and LOW LEVELS of serotonin are associated with aggression
  • BRUNNER: A study of a DUTCH FAMILY found that many of the MEN had abnormally LOW LEVELS of MAOA and also had a defective gene for the production of MAOA.

Gene - Environment Interactions 

  • CASPI ET AL: 500 male children: discovered TWO variants of the gene MAOA 
  • One associated with HIGH LEVELS of MAOA and one with LOW LEVELS. 
  • Those with LOW LEVELS were significantly MORE LIKELY to display AGGRESSIVE behaviour. 
  • This suggests thst it is the INTERACTION between genes AND the environment that determines aggression, NOT GENES ALONE 
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A Gene For Aggression - A02


  • Most studies have focused on individuals convicted of violent crime. There are two problems with this 
  • First: These indivudals represent a very small MINORITY of thise who regulary engage in aggressive behaviour 
  • Second: A person who is imprisoned for a SINGLE VIOLENT OFFENCE is not necessarily a consistently a violent individual.
  • This means that the VALIDITY of conclusions drawn from these studies may be suspect.

THERE ARE DIFFICULTIES IN DETERMINING THE ROLE OF GENETIC FACTORS... As its diffcult to establish contributions to aggressive behaviour for the following reason: 

  • More than ONE gene usually contributes to a specific behaviour 
  • As well as genetic factors there are many non-genetic influences e.g. Social learning and the environment 
  • These influences may interact with each other (gene- environment) 
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A03 - Nature - Nurture Debate

S: The role of genetic factors in aggression can be explained through the nature - nurture debate. 

E: McGue et al completed a twin study and found a correlation of +0.43 on MZ twins and +0.30 on DZ twins. This suggests that genes (nature) plays a bigger role in aggression as MZ twins share more genes than DZ twins.

E: However the reason why MZ twins are so similar may be due to environmental factors. As MZ twins are more likely to be treated the same in their home environment as they LOOK MORE SIMILAR. So CONCORDENCE RATES may be due to aggression being treated the same (Nurture) 

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A03 - Determinism

S: the role of genetic factors in aggression can be said to be deterministic as it suggests that we are pre-programmed to be aggressive, when this could be due to free will.

E: If people are biologically determined to be aggressive then its not possible to hold them responsiable for their actions, and this may have implications on legal systems.

E: Therefore this theory does not take into account the fact that individuals may be aggression because they WANT to be, not just because of genetics. This has implications of the validity of the biological approach as it cannot be generalised to all people, as it lacks support to suggest our genes only play a part in our aggression.

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