Twin Studies

• What are twin studies?

  Where researchers assess hereditary influence by comparing the resemblance of identical twins and fraternal twins with respect to a trait
• What is likely to happen to the other identical twin if one has 'criminal genes'?

  It is likely that his or her blood relatives will have as well
• Identical twins are also known as

  Monozygotic (MZ)
• Non-identical twins are also known as

  Dizygotic
• Why do MZ share exactly the same genes?

  They are both from the same fertilized egg
• What is it known as when both twins share a characteristic such as being aggressive?

  Concordance rate
• Is crime more likely in monozygotic or dizygotic?

  Monozygotic
• Lange (1929) found a much higher degree of concordance in...

  ...monozygotic twinns
• 10 out of 13...

  ...MZ twins had served time in prison
• 2 out of 17...

  ...DZ twins had served time in prison
• Christiansen (1977)


  Studied 3,586 pairs of twins from Denmark and found evidence to support the view that crime can be explained by genetics
• In MZ twins what was the percentage of the concordance rate

  52%
• In DZ twins there was a much lower concordance rate of?

  2%
• Strength - Ishwikawa and Raine (2001) found evidence that crime can be explained genetically. Their researcher showed...


  ...44% concordance rate for MZ twins compared to 21% rate for DZ twins
• Strength - The results of twin studies have helped in the prevention of...

  ...vulnerable disorders
• Limit - If genes were the cause of criminality then...

  ...identical twins would show 100% concordance, but studies show much less
• Limit - The small samples involved in many twin studies are...

  ...unrepresentative and said not to be typical of the wider population
• Limit - If twins are brought up in the same environment...


  ...crime could be due to nurture rather than nature
• Limit - Early twin studies such as that of Lange were poorly controlled as...


  ...MZ or DZ twins were based on appearance as DNA did not exist at the time
• Limit - Higher concordance rates among MZ twins may be due to them sharing the same home, school, etc...


  ...their shared environment may cause similarities in their behavior rather than identical genes