Genetic factors

• several twin studies have suggested that heritability accounts for 50% of the variance in aggressive behaviours
• Coccarro et al investigated the levels of aggression in MZ and DZ twins and found physical aggression concordance rates of 50% for MZ twins
• researchers of adoption studies found that genes account for 41% of variance in aggression
• the MAOA gene is associated with low serotonin activity which can lead to increased impulsivity, aggressiveness and violence
• Rhee and Waldman used a meta-analysis to investigate levels of aggression in adopted children
• MAOA-L variant has been linked to high levels of aggression
• low levels of MAOA enzyme is caused by the MAOA-L variant gene
• The MAOA gene regulates the production of monoamine oxidase A (an enzyme)
• Brunner et al studied 28 male members of a dutch family who were repeatedly involved in aggressive, violent crimes and found they had MAOA-L variant gene and low activity of MAOA enzyme
• Aggression can be explained by the disruption of MAOA gene and stress caused by experiences such as childhood trauma
• Frazetto et al found that men without childhood trauma didn't have high levels of aggression even if they possessed the MAOA-L variant, compared to the men that did go through trauma
• MAOA enzyme is used to break down or recycle neurotransmitters (including serotonin) in the synapse after a nerve impulse has been transmitted