Genetic explanations

• Twin and adoption studies


  • Christiansen studied over 3500 twin pairs in Denmark and found concordance rates of criminal behaviour
  • Male - MZ: 35%, DZ: 13%
  • Female - MZ: 21%, DZ: 8%
  • Crowe found that the adopted children with bio mothers with criminal records had a 50% risk of having a criminal record by 18
  • Adopted children whos bio mother didn't have a criminal record only had a 5% risk
• Candidate genes


  • Tiihonen et al. analysed 800 Finnish offenders and suggested that the MAOA and CDH13 genes may be associated with violent crimes
  • MAOA regulates serotonin and has been linked to aggression
  • CDH13 has been linked to substance abuse and ADHD
  • 5-10% of all severe violent crime in Finland has been attributable to these genes
• MAOA gene


  1. Mutated, shortened allele of MAOA gene is inherited
  2. Fewer MAOA enzymes which break down excess neurotransmitters (monoamines)
  3. There is an excess of dopamine, noradrenaline and serotonin
  4. The person is less able to regulate mood and are more impulsive and more likely to behave aggressively
• Diathesis-stress model


  • Offending behaviour is a combination of:
  • Genetic predisposition
  • A biological or psychological stressor or 'trigger' e.g. trauma
• The counter-intuitive relationship between serotonin and aggression


  • Can be explained by the development of the brain in utero
  • The foetal brain is bathed in extra serotonin, therefore the brain is effectively desensitized to its calming effect in later life