Genetic factors of aggression

• Twin studies


  They suggest that heritability accounts for about 50% variance in aggressive behaviour.
• Coccardo et al twin studies 


  Coccardo et al studied men that were either monozygotic MZ or dizygotic DZ twins. MZ twins share 100% of their genes compared to DZ twins that only share 50% of there genes so if aggression is influenced by genetics MZ twins should have similar aggressive behaviour. This is because they are also raised in similar environments. For aggressive behaviour like direct physical assault concordance rate was 50% for MZ and 19% for DZ. For verbal it was 28% for MZ and 7% for DZ.
• Twin studies concordance rates

  For direct physical assault aggression it was 50% for MZ and 19% for DZ. For verbal aggression it was 28% for MZ and 7% for DZ.
• Adoption studies


  Similarities in aggression between adoptive children and their biological parents show that genetic factors are to blame this is because the environmental factors aren’t there.
• Adoption studies- Rhee and Waldman


  They did a meta analysis of adoption studies of direct aggression and antisocial behaviou. They found that genetic influences accounted for 41% of the variance in aggression, similar to the twin studies.
• The MAOA gene


  Controls the production of an enzyme in the brain called monoamine oxidise A. It regulates the neurotransmitter serotonin. The MAOA-L gene is associated with aggression because it has low MAOA activity.
• MAOA warrior gene- Lea and Chambers


  They found that the MAOA-L gene was possessed by 56% of New Zealand Māori men compared to 34% of caucasians. The Māori people have been associated with being warriors.
• MAOA-L and aggression- Brunner et al


  They studied 28 men from a large Dutch family who were involved with impulsively aggressive behaviour such as rape, assault and attempted murder. They had abnormally low levels of the MAOA enzyme and had the MAOA-L gene.
• Gene-environment interactions


  Genes don’t influence aggression in isolation from other factors. The MAOA-L gene activity is only related to adult aggression when combined with early traumatic life experience.
• Genes and diathesis stress model- Frazzeto et al


  Found an association between higher levels of antisocial aggression and MAOA-L gene variant in men, as expected. But this was only the case in those who had experienced childhood trauma like sexual or physical abuse during the first 15 years of life. Those who didn’t experience childhood trauma didn’t have high levels of aggression as adults, even if they had the MAOA-L variant. This is evidence of a diathesis stress gene-environment interaction.
• Evaluation- Research support strength

  One is strength is that there is evidence for the MAOA genes role in aggression. Research shows that MAOA-L gene is associated with greater aggression. Mertins et al found that the opposite is also true. Men with the high and low variant of MAOA took part in a money game. Men with MAOA-H were more cooperative and were less aggressive than MAOA-L participants. This supports the relationship between MAOA gene activity and aggression.
• Evaluation- non genetic factors like conformity is limitation

  However, Mertins et al also found that non genetic factors influenced aggression. They found that participants with MAOA-L gene behaved cooperatively when they saw that other participants were behaving like that. Knowledge of a social norm determined how aggressive or co operative MAOA-L participants were. Therefore genes are influenced by environmental factors like knowing about other people’s behaviour that are at least as important as aggression.