Genetic Factors in Aggression (A01)

Genetic Factors in Aggression (A01)

1. Is Aggression Inherited?

   • Twin studies and adoption studies are used to find out if genes are involved in aggression
   • Monozygotic twins are identical so have the same genes
   • Dizygotic twins are unidentical so share 50% of genes
   • Researchers compare similarity for a trait between MZ and DZ twins. -If MZ twins are more alike in aggression for example this should be more due to genes than environment.
   • Coccaro et al
     • found nearly 50% of variance in direct aggressive behaviour could be due to gentic factors (studied adult twin pairs)

2. Adoption Studies

   • Hutching and Mednick
     • Study of over 14,000 adoptions in Denmark. Found that a significant number of adopted boys with criminal convictions had biological parents with criminal convictions (mainly fathers) suggesting evidence for a genetic effect.
   • Positive correlation between aggressive behaviour in adopted children and aggressive behaviour in biological parents=GENETIC EFFECT
   • Positive correlation between aggressive behaviour in adopted children and aggressive behaviour in adopted families=ENVIRONMENTAL EFFECT

3. A Gene for Aggression?

   • THE ROLE OF MAOA
     • MAOA-enzyme
     • no individual gene for aggression but a gene responsible for production of a protein called monoamine oxidase A has been associated with aggressive behaviours.
     • MAOA regulates the metabolism of serotonin in the brain. Low levels of serotonin--> aggression
     • Brunner et al
       • Study of a dutch family. Found many male members behaved in a particularly violent and aggressive manner and a large proportion had been involved in serious crimes (eg rape and arson). These men had abnormally low levels of MAOA and a defect in the gene.
   • GENE-ENVIRONMENT INTERACTION
     • Caspi et al links MAOA to aggressive behaviour
       • 500 male children
       • found variant of the gene associated with low levels of MAOA and a variant associated with high levels of MAOA
       • Those with low levels of MAOA were more likely to grow up to have anti social behaviours if they had also been maltreated as children
       • Those with high levels of MAOA who were maltreated as children did NOT grow up to show antisocial behaviours
       • Shows that it is the interaction between genes and environment that determines behaviour such as aggression

4. Genetics and Violent Crime

   • Brennan and Mednick
     • series of adoption studies
     • criminal history of an adopted male was compared to criminal history of his biological and adoptive fathers
     • found genetic influences were significant in cases of property crime but not in cases of violent crime.
   • Researchers don't suggest there is a single gene for violent crime. They think it may be inherited personality traits that place some individuals more at risk of committing violent crimes.
   • Adoption studies shown that highest criminal violence in adopted children occurs when both biological and adoptive parents have a history of violent crime. This is clear evidence of a gene-environment interaction.