Genetic theories

• Twin studies -this theory is about proving the criminal gene that’s passed down , theorists believe this is why criminals have criminal relatives . Monozygotic twins share the same DNA so if there was a ‘criminal gene’ both twins in theory should be criminals if there is a genetic link in criminality .
• Twin studies research :

  Christiansen studies 3586 twins , found 52% concordance between MZ twins - meaning if one twin had a conviction the other was a 52% the other twin would have one as well. Among DZ twins there was only 22% concordance

  -the research suggests a genetic link
• Twin studies strengths :

  • Natural experiments as biological relationships are naturally occurring
  • supporting evidence (52% concordance)
• Twin studies weaknesses:

  • Early studies weren’t controlled as they Guessed if the twins were MZ or DZ by appearance not genetics (1920s)
  • Samples were small , lack population validity
  • twins were brought up in the same environment so criminality could be nurture not nature
• Adoption studies - compare adopted children to birth parents and adopted parents . This is to discover whether genetics or environment have more of an effect on criminality .
• Adoption studies research:

  Mendick et al examined 14,000 adopted sons . They found sons were more likely to have a criminal record if a birth parent has a criminal record (20% concordance) with adopted parents (14.7% concordance)

  -suggests genetics have more of an affect on criminality but environment still has a part to play as genetics isn’t’t 100% concordant.
• Adoption studies strengths:

  • children are exposed to different environment to birth parents , it’s much easier to separate environmental and genetic factors -increases validity as research is clearer
  • studies conclude a correlation between adopted child and birth parent eg: if birth mother had record 50% children would have one before their 18.
• Adoption studies weaknesses :

  • children can be adopted at any age before 18 - they could’ve already learnt criminal behaviour from bparents
  • info about biological family isn’t always available
  • the adoption process isn’t always random , children can be placed with adopted parents similar to biological - they’d have similar values so could influence criminal behaviour in the same way.
• Jacob’s XYY - abnormality of the sex hormones cause criminality , and extra Y chromosome is names ‘super male syndrome’ . Men with this tend to be well-built , tall ,low intelligence .

  Jacob et al claimed XYY men are more aggressive
• XYY research :

  Jacob suggested XYY men were more aggressive/violent . Jacob found 7 cases of XYY men in a study of 197 individuals in a mental hospital.

  -some issues with this as we’d expected a higher chance of XYY in mental hospitals than in the population - sample is small so lack population validity
• XYY strengths :

  • support - Jacob found more XYY men in prison than XY , suggests link between it and criminality
  • Alder et al indicated aggressive behaviour is at least partially determined by genetic factors
• XYY weaknesses :

  • studies found genetic abnormalities are widespread throughout population , not confined to prisoners
  • theory focuses too heavily on genetics , ignores environment - studies show both cause criminality
  • Theilgaard researched traits of XYY men and XY men , found aggression isn’t just associated with XYY men.