Psychology - Aggression Genetics

• Twin studies:

  • Monozygotic twins share 100% of their genes
  • dizygotic twins only share 50% of genes
  • in twin studies researchers compare the degree of similiarity for aggression between sets of both twin types
  • if the monozygotic twins are more like in terms of aggression then this is due to their genes
• Miles and Carey

  carried out meta analysis of 24 twin and adoption studies investigating the genetic basis of aggression.

  Most studies relied on participants self report of aggression but some also involved observation

  results suggest a strong genetic influence that could explain 50% of variance in aggression
• Adoption studies

  • help to explain the environmental and hereditary explanations for aggression
  • if positive correlation found between aggressive behaviour in adopted children and their biological parents a genetic effect is implied
  • if a positive correlation is found between the adopted child and the adopting family aggressive behaviour an environmental effect is implied
• Hutchings and Mednick

  P: conducted adoption study on over 14000 adoptions in Denmark

  F: positive correlation between aggressive criminal behaviour in adopted boys and their biological parents

  C: evidence to support genetic effect
• MAOA gene

  • controls the production of enzyme MAO-A
  • enzyme regulates serotonin by mopping this up in the brain after nerve impulses have been transmitted between neurons
  • breaks neurotransmitters down to chemical constituents they can be recycled or excreted
  • dysfunction in the gene can affect the enzyme function leading to abnormalities in seretonin
• Caspi et al

  discovered variants of the MAOA gene

  • MAOA-H is associated with high levels of the MAO-A enzyme
  • MAOA-L is associated with low levels of the enzyme
  • MAOA-L leads to enzyme deficiency leading to high aggression
• AO3: Limitation - complex link

  relationship between the MAOA gene, serotonin and aggression is complex

  VIRKUNNEN et al

  P: compared levels of the serotonin breakdown product in violent impulsive and non-impulsive offenders

  F: levels higher in impulsive offenders

  BUT the MAOA-L gene causes low activity of the enzyme which should lead to higher serotonin levels as doesnt deactivate serotonin.

  shows the relationship between gene and serotonin and aggression not fully understand
• Gene-environment interactions:

  • genes influence aggression but do not function in isolation
  • low MAOA gene activity only relevant to adult aggression when combined with early trauma
  • twin and adoption studies suggest genes are cause of aggression
  • also evidence of environment influencing aggression
  • environmental factors interact with genes and influence how genes are expressed
• Frazetto et al

  F: association between higher levels of antisocial aggression and low activity gene in adult males. only the case for those with trauma during first 15 years of living. Those who had no trauma did not have high levels of aggression even if they had low variant

  C: suggests gene environment interaction
• AO3: methodology

  twin studies can lack validity

  MZ twins share the same environment but DZ twins may not to the same extent however it is assumed they all share the same environment

  inaccurate as MZ twins are treated similarly whereas DZ twins are not
• AO3: Gender bias

  many studies used to support genetic explantion conducted on male samples. Beta bias assumes that findings from males can also be used to explain aggression in females.

  minimises the differences between men and women. women have higher levels of oxytocin so respond to stress differently . this hormonal difference suggest men cannot represent women