Genetic

• Twin studies (genetic)


  A genetic link may increase the risk of criminal behaviour. Identical twins are monozygotic and share 100% of their DNA as they come from the same fertilised egg. Dizygotic twins come from 2 separate eggs so only share 50% of their DNA. Johannes Lange found that MZ twins showed a much higher concordance rate than DZ twins for criminal behaviour. 10/13 MZ twins both served time in prison whereas 2/17 DZ twins both served time in prison. Overall showing in MZ twins that if one served time in prison, there was an increased chance the other twin would too.
• XYY (genetic)


  Men who have an extra Y chromosome also produce more testosterone than normal XY men. High levels of testosterone are linked with aggressive behaviour which could explain violent crimes committed by men. Jacob found that 9/315 men had XYY at a maximum security prison. Other studies showed that men with XYY were over represented in prisons 15 per 1000 whereas 1 per 1000 in the general population. This study does not account for why men with XYY do not commit crimes, it assumes that all men in prison are guilty and does not explain why women commit crimes.
• Evaluate XYY (genetic)


  Jacob found that they were over represented in the prison population. Studies have found that genetic abnormalities are widespread throughout the general population and therefore do not explain aggression. Not all men with XYY commit crime or act aggressively. The theory ignorea environmental factor e.g upbringing. Having XYY can make you appear different which could lead to bullying. This theory can’t explain why men who show aggression or commit crime that are XY. It also can’t explain female criminality as it’s a gene only found in men.