AO3

• Twin studies suggest genes must play a role in criminal behaviour as concordance rate is higher when they are genetically more similar
• Could equally argue that as MZ twins are treated more similarly to eachother than DZ, this could account for increased rate of concordance and twins have been reared in shared environments which may have increased concordance rates, so cannot be sure whether genetics or environment were more influential in determining criminal behaviour 
• Bartol used a small sample size so difficult to generalise
• 2 candidate genes have been revealed that are believed to be linked to criminal behaviour, one linked to aggressive behaviour and the other substance abuse by adoption studies
• Results from family studies merely illustrate that offending behaviour runs in families, and does not automatically mean that it is due to genetic transmission: cannot show how the results occured, could equally be social learning or other environmental influences
• Research for diathesis stress model - Mednick et al checked police records of adopted people and whether their adoptive and biological parents had a criminal record. They found 20% of those whose biological parents had criminal records were found to later become involved in criminal behaviours compared to 24.5% who had both biological and adoptive parens with a background of crime