The use of twin studies helps explain how there could be genetic factors in aggressive behavior. There are two types of Twin, monozygotic (identical) and Dizygotic (non-identical). If both types are studied in terms of aggression and the M-Z twins are more similar in aggressive behaviour it would suggest there is a genetic influence on aggression. Coccaro et al found that when comparing MZ and DZ twins, over 50 % of variance in aggressive behaviours between MZ twins could be attributed to genetic factors.
Additionally the use of adoption studies can contribute to our understanding of genetic factors in aggression. If a positive correlation can be seen between adopted children and their biological parents ( where no environmental influenced has occurred) in aggressive behaviour then it would suggest a genetic influence. However if a positive correlation is found between adopted children and their foster parents, it shows there must be an environmental effect , a study of 14000 adoptions in Denmark show that a significant number of boys with criminal convictions had biological parents with criminal convictions. This provides evidence for the influence genetic factors has in aggression.
A gene called MAOA is associated with aggressive behaviour, it regulated the metabolism of serotonin, and additionally low levels of serotonin are associated with aggressive and impulsive behaviour. A study from a Dutch family with a history of violence including rape and arson found low levels of MAOA in their blood. It showed that the men had a defect in their MAOA. This study shows that the gene MAOA is associated with aggression, this means that some genes can influence the susceptibility to aggressive behaviour, however it is not certain that one gene can determine aggressive behaviour, however it is likely that many genes can contribute to human behaviour rather than one single gene.
A more integrated explanation is