Genetic factors in aggression

AO1

Twin studies: Indicate that heritability accounts for 50% of variance in aggression. Coccaro studed MZ and DZ adult males. For aggressive behaviour defined as physical assault, there were concordance rates of 50% for MZs and 19% for DZs.

Adoption studies: Similarity with biological parents indicate genetic influences are operating and similarity with adoptive parents indicate environmental influences are operating. Rhee and Waldman carried out meta-analysis of adoption studies of direct aggression and anti-social behaviour and found 41% of variance was due to genetic factors.

MAOA gene: This is an enzyme that mops up neurotransmitters in brain after nerve impulse has gone to another neuron. It breaks it down into chemicals that can be recycled (catabolism). The MAOA gene determines the production of the enzyme. One variation is low MAOA activity which is linked to aggression. Brunner studied 28 males of a Dutch family who had a history of impulsively violent criminal behaviour - they had the low MAOA activity variant. Stuart studied 97 men who used IPV. Those with the low-activity variant were the most violent to their partner. 

GxE interactions/Diathesis stress: Genes don't work…