Genetic Explanations

• What are genetic factors?

  • Genes consist of DNA which decides hair colour, height, and size of brain structures etc.
  • Genes are inherited from parents to offspring and may impact on psychological factors
• What is MAOA?

  • Gene responsible for the activity of the enzyme monoamine oxidase in the brain
  • Low activity variant of gene is closely associated to aggressive behaviours
• What is the concordance rate?

  A measure of similarity expressed as a % between two individuals on a given trait
• Twin Studies - Coccoro et al

  • Studied adult male monozygotic and dizygotic twins
  • For direct physical aggression he found a concordance rate 50% for MZ and 19% for DZ
  • For verbal aggression he found a concordance rate of 28% for MZ and 7% for DZ
• Adoption Studies Summary:

  • Similarities in aggressive behaviour between an adopted child and biological parents suggest genetic influences are operating
  • Similarities between adopted parents = Environmental influences
• Adoption Study - Rhee and Waldman:

  • Meta Analysis
  • Study found genetic influences accounted for 41% of variance in aggression
• MAOA and Serotonin:

  • MAOA = Enzyme

  -'mops up' neurotransmitters after a nerve impulse has been transmitted

  • Breaks down neurotransmitters into constituent chemicals to be recycled or excreted
  • Production of enzyme is determined by MAOA gene
  • A dysfunction of this gene may lead to abnormal activity of enzyme

  -Affects levels of serotonin (low levels = aggressive behaviour)
• Warrior Gene:

  • Variant of MAOA gene
  • Leads to low MAOA activity in the brain
  • Associated with aggressive behaviour
• Brumer et al:

  • Studied 28 male members of a Dutch family involved in impulsively aggressive violent criminal behaviours
  • e.g. rape, attempted murder, and assault
  • These men had abnormally low levels of MAOA in the brain and low activity version of MAOA gene
• Limitation - Genetic factors are difficult to isolate:

  • Difficult to separate genetic and environmental factors
  • Someone with an aggression-associated gene may only behave aggressively if the environment conditions are suitable
  • McDermott et al - Participants with the low activity MAOA gene behaved aggressively in a lab based task but only when provoked
• Limitation - Findings depend on how aggression is measured:

  • Methods of measuring aggression differ significantly between studies and include self reports, parent and teacher reports and direct observations
  • In Rhee and Waldman's meta analysis genetic factors had a greater influence on aggression in studies using self reports rather than parent or teacher reports
• Strength - MAOA explanation is support from prosocial behaviour research:

  • The low activity variant of the MAOA gene is associated with greater aggression so presumably the opposite is true
  • Mertins et al - found males with the high activity MAOA variant were more cooperative in a lab based test
• Strength - Support from animal studies:

  • Genetic deletion techniques allow researchers to remove single genes in mice - an experimental manipulation that lets them observe the effects on aggression
  • Godar et al - Showed MAOA removal mice have increased brain serotonin and are hyperaggressive
  • When serotonin is blocked by the drug fluoxetine the mice reverted to non aggressive