Genetic factors

• Twin Studies


  If aggressive behaviour is influenced by genes, there would be high concordance rates between closer relatives than those distantly related.
• Who studied concordance rates for aggressive actions and verbal aggression?

  Coccaro et al
• Coccaro et al - aggressive actions

  Found concordance rates for aggressive actions to be:

  - 50% for MZ twins

  - 19% for DZ twins
• Coccaro et al - verbal aggressive

  Found concordance rates for verbal aggression to be:

  - 28% for MZ twins

  - 7% for DZ twins
• Adoption studies


  If aggression was under genetic influence, you'd expect the adopted child to show more similarity to their biological parents

  
  

  If aggression was under environmental influence, you'd expect the adopted child to show more similarity to their adoptive parents
• Rhee and Waldman (2002)


  Meta-analysis of adoption studies of direct aggression and antisocial behaviour

  
  

  Found that genetic influences accounted for 41% of the variance in aggression
• Hutchins and Mednick (1975)


  Found a significant number of adopted boys with criminal convictions had biological parents with convictions for criminal violence

  
  

  Suggests there is a genetic component to aggressive behaviour
• What environmental factors might increase aggression in adopted children - regardless of hereditary factors?

  Trauma of adoption, bullying or exclusion by other children
• MAOA gene


  The gene responsible for the activity of the enzyme monoamine oxidase in the brain.

  This protein allows for the metabolising of the neurotransmitters - noradrenaline, serotonin and dopamine

  
  

  The low-activity variant of the gene is closely associated with aggressive behaviour - 'warrior gene
• High dopamine levels...


  increase the likelihood of feelings of reward when aggression is carried out.
• Low serotonin levels...


  cause a lack of inhibition over impulsive behaviour
• High levels of noradrenaline...

  cause overreaction to perceived threats
• Bruner et al


  Studied 28 males of a large Dutch family who were repeatedly involved in impulsively aggressive violent criminal behaviour

  
  

  Found they had abnormally low levels of MAOA in their brains, and had the low-activity version of the MAOA gene
• Frazzetto et al


  Found the association between the low-activity MAOA gene and higher levels of anti-social behaviour only occurred in those who had experienced significant trauma during the first 15 years of life

  
  

  Those with low MAOA gene activity but no early trauma weren't seen to engage in particularly high levels of aggressive behaviour
• Diathesis-stress model


  The low-activity MAOA gene leads to a higher probability of aggression

  The environment can be a risk factor or a protective factor - child's brain is easily malleable early on

  
  

  Underlying genetic vulnerability coupled with environmental experiences