The role of genetic factors in aggression can be tested in twin studies. MZ twins share all of their genes, while DZ twins share a maximum of 50%. If MZ twins are more alike than DZ twins in terms of their aggressive behaviour than this suggests a strong genetic influence rather than environmental. Studies of adult twins suggests that at least 50% of the variance in aggression can be explained in terms of genetic factors.
Adoption studies can help determine the relative contributions of environment and heredity in aggression. A study by Mednick and Hutchings shows an interaction between genes and their environment. They studied criminal records of adopted Danish children, the results showed that having a criminal biological father increased the risk of criminality, but the highest risk was for those with a criminal biological father and a criminal adoptive father.
One gene that has been linked to aggression is the gene coding for the production of MAOA. MAOA regulates the metabolism of serotonin in the brain, and low levels of serotonin are associated with aggressive behaviour. Brunner studied a violent Dutch family and found that many of the men had abnormally low levels of MAOA and also had a defective gene for the production of MAOA.
There is research support for twin studies. A meta analysis of 24 twin and adoption studies (Miles and Carey) found evidence of a strong genetic influence. In younger individuals, both…