genetic factors in aggression

• researchers studied adult male monozygotic and dizygotic twins. For direct physical aggression, the researchers found concordance rates of 50% for MZ twins and 19% for DZ twins. For verbal aggression the figures were 28% for MZ twins and 7% for DZ
• similarities in aggressive behaviour between an adopted child and biological parents suggest genetic influences are operating. However, similarities with adoptive parents suggest environmental factors. A meta-analysis of adopted studies found genetic influences accounted for 41% of the variance in aggression
• the MAOA gene controls production of enzyme MAO-A which regulates serotonin. Serotonin is thought to play a role in impulsive aggression. The low variant of MAOA (MAOA-L) is linked to increased aggression
• researchers studied 28 male family members repeatedly involved in impulsively violent criminal behaviours like rape and assault. These men had both abnormally low levels of MAO-A enzyme in their brains and MAOA-L variant
• One strength of the role of MAOA gene is researich support. Researchers also found that the MAOA-L variant is linked with high aggression. Male participants with the high activity MAOA gene variant were more co-operative and less aggressive in a money distributing game. This finding supports the relationship between MAOA gene activity and aggression, increasing the validity of this genetic theory of aggression
• One limitation is that the twin studies in genetic factors lack validity. Both individuals in a twin pair share the same environment (as they are raised together). However, DZ twins may not share environments to the same extent that MZ share environments. The equal environments assumption is wrong because MZ twins are treated very similarly but DZ twins are treated less similarly. This means that concordance rates are inflated and genetic influences on aggression may not be as great as twin studies suggest