2. Genetic Theories - Twin Studies

• The twin method is used to estimate the relative importance of genetic and environmental factors.
• Twin studies involve comparing monozygotic twins (MZ), identical, share 100% DNA, and dizygotic twins (DZ), non-identical, share 50% DNA. If criminality is inherited, MZ twins should be criminals.
• Christiansen et al - Found out that there is a 52% concordance rate between MZ twins (if one identical twin had a conviction, the other also will. There is a 22% chance between DZ twins. This results in the idea that if one twin commits violent crimes then it is likely for the other twin to also commit this crime.
• UNSUPPORTED - If genes were the only cause of criminality, identical twins should have 100% concordance rate across all twin studies.
• INFLUENCED - It is difficult to only ever examine genes (separately from the environment) but Christiansen's study has shown it is possible and it further develops our understanding.
• IGNORES - Too simplistic - fails to recognize other important factors as to why people turn to crime.

  Glueck & Glueck - criminality cannot be explained by biology alone, environmental and psychological factors also need to be considered.
• NOT ALL CRIMES/ PEOPLE - Not every crime is driven by biological reasons, it suggests people do not have free will to make decisions.
• Parents treat identical twins more alike than non - identical twins, influencing them to behave the same way.