Much of the 20th century's empirical research in the social sciences has been devoted to understanding the causes and contributing factors of antisocial behaviour. In studies of children's moral reasoning and conduct, developmental psychologists have probed the cognitive and social bases of aggression, conflict delinquency, and prejudice. In contrast to psychology's lengthy preoccupation with negative behaviour in children, the study of children's altruistic, cooperative, and sharing behaviour has a relatively short history.