Genetic theories

• What are 3 genetic theories?

  Twin studies

  adoption studies

  jacobs XYY study
• Explain adoption studies 


  compared adopted children to their biological parents and adopted parents

  Argues that if the adopted child is adopted soon after birth and shares the same environment as adopted parents but still shows similar criminal traits as the biological parents who are criminal it would support genetic explanation that crime is hereditary
• What is the evidence for adoption studies?

  Mednick examined data on 14,000 adopted sons

  found sons more likely to have a criminal record if their birth parents have one - 20% - supports genetic explanation

  compared to if their adoptive parents have a criminal record - 14.7%
• What are the strengths of adoption studies?


  Support genetic explanations as they show adoptees were more likely to have a criminal record if their biological parents had one

  Research design is logical. Allows us to see relative importance of ”nature” (genes) Vs “nurture” environment
• What are the limitations of adoption studies?


  Research shows and argues that adoption studies show genes have little effect on criminality

  Many children are not adopted immediately after birth so remain with their biological parents

  • this early environment maybe the true cause of the criminality
• Explain twin studies


  Genetic theorist say that because family members who are blood relatives often share the same genes

  • which is why criminal have relatives who are criminals

  So they tested this theory on MZ twins as they both developed from the same egg - so if one is criminal then the other must be too
• What is the evidence on twin studies?


  Christiansens studied 3,586 pairs of twins

  • found a 52% concordance rate between MZ
  • found a 22% concordance rate between DZ
• What are the limitations of twin studies?

  If genes were the only cause of criminality for identical twins than concordance rate would be 100%

  High concordance rate between identical twins maybe due to sharing the same environment leading to criminal behaviour

  In early studies there was no way of testing if twins where genetically identical since DNA testing did not exist
• What are the strengths of twin studies?

  As MZ twins are genetically identical - logical to examine whether the offending behaviour is also identical

  give some support for genetic explanation
• Explain Jacobs XYY study?


  Abnormality of the sex chromosomes is another possible cause of criminality

  • mothers chromosome = XX
  • fathers = XY
  • we inherit one chromosome from each parent so will either have XX or XY

  One abnormality is an extra Y chromosome is inherited

  • they are tall, well built and have low intelligence
  • labelled as super male syndrome
  • men who have it are more aggressive and violent than others
• What is the evidence for Jacobs XYY study?


  based on studies of imprisoned criminals in psychiatric hospitals

  higher proportion had XYY and had history of aggression and violent assault

  • found to be immature and unstable
  • strong tendency to commit motiveless property crime
• What are the strengths of the XYY syndrome?

  Jacob et al found an association between XYY and offenders imprisoned for violent behaviour

  Price and Whatmore found some linkes between the syndrome and property crime
• What are the limitations of the XYY syndrome study? 


  Even if some violent offenders have the syndrome

  • doesn’t prove they will commit crime

  Syndrome is very rare 1/1,000 men have it so cannot explain that much crime
• Outline the case study of Jim Fallon

  He had the MAOA Gene - violent gene makes him a psychopath

  • gene only gets activated when watching violent events
  • He hasn’t witnessed any violence so no criminal behaviour has been produced