The role of genetic factors in aggression can be tested in twin studies. Genes are identical in sets of monozygotic twins but are different between sets of dizygotic twins, meaning that if monozygotic twins are more alike in terms of aggression than dizygotic twins are, then this should be due to genes rather than environment. These twin studies have generally found that almost 50% of variance in aggressive behaviour can be attributed to genetic factors.
Adoption studies can help determine the relative contributions of environment and heredity in aggression. An adoption study by Mednick, Gabrielli & Hutchings (1987) shows an interaction between genes and the environment. The researchers studied the criminal records of all Danish children adopted outside their biological family between 1924 and 1947. 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 which 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. Thus, low levels of MAOA are also associated with aggression. Studies of violent criminals have found that they often have a defect in the gene that produces MAOA.
Research on humans has supported the notion that genes are an important factor in aggressiveness. Rutter et al. (1990) carried out a meta-analysis of twin studies on criminality and found that Dizygotic Twins have concordance rates between 13…