More than 100 studies of twins and adopted children have confirmed that about half of the variance in aggressive and antisocial behavior can be attributed to genetics

Numerous studies have found that identical twins, who have all of their genes in common, are much more similar to each other in terms of crime and aggression than are fraternal twins, who share only 50% of their genes. In a landmark 1984 study, Sarnoff Mednick found that children in Denmark who had been adopted from parents with a criminal record were more likely to become criminals in adulthood than were other adopted kids.