Genetic factors in aggression

MZ twins - share all of their genes

DZ twins - share max, 50%

If MZ twins are more alike = strong genetic influence

Studies suggest at least 50% variance can be explained in terms of genetic factors

Adoption studies can help determine the relative contributions of environment and heredity in aggression.

Mednick and Hutchings

• interaction between genes and their environment
• criminal records of adopted Danish children
• criminal biological father - increased criminality
• criminal biological and adopted father - highest risk

Gene coding for the production of MAOA.

MAOA regulates the metabolism of serotonin 

Brunner

• studied a violent dutch family 
• many of the men haad abnormally low levels of MAOA
• defective gene for the production of MAOA

Miles and Carey

• meta analysis of 24 studies
• evidence of a strong genetic influence
• younger individuals - genetic and environmental influences
• older - genetic influences

Genes are influential in ALL individuals 

Rutter 

• meta analysis of twin studies on criminality
• DZ twins have concordance rates between 13-22%
• MZ twins 26-51%
• difference likely due to genetics

consider MZ twins do not show 100%

Caspi

• 500 male children
• low levels of MAOA significantly more likely to grow up to exhibit antisocial behaviour
• ONLY if they had been maltreated as a child

• focused on individuals convicted of violent crime
• represent a small minority of those who regularly engage in aggressive behaviour
• single offence e.g. murder is not necessarily a consistently violent criminal

Validity may be suspect 

• Real life applications
• genetic engineering to prevent crime
• using chemical castration to treat potential violent offenders
• Ethical concerns about labelling someone a threat

Methodological issues

• self reports and observations
• meta analysis - file drawer effect - support have used parental or self reports