MZ - identical genes, DZ - 50%. Coccaro - 50% of aggressive behaviour could be due to genetics

 Hutchings & Mednick, 14,000 adoptions - boys with criminal convictions had biological parents with criminal convictions

LOW MAOA ENZYME - BUILD UP OF SEROTONIN - LEADS TO AGGRESSION

 aggressive family - had low levels of MAOA due to defect in gene (MAOA-L)

MAOA gene linked to X chromosome. Defective X chromosome is more likely to affect men and not women

2Multiple genes usually contribute to a behaviour. Environmental factors to consider. Genetics influence which environmental factors have an influence

 Nature and nurture. Caspi - people with low MAOA only aggressive if maltreated. Genetic predisposition based on environment

 215 non-violent prisoners, 538 violent - 84 extremely violent who were more likely to have MAOA-L and CDH13

1Tihonen studied Finnish prisoners - MAOA-L combined with CDH13 = violent behaviour - wasn't found in non-violent offenders

 Walters, meta-analysis - low correlation between heredity and crime