Kohlberg was influenced by Piaget’s theory of moral development. He believed, like Piaget, that the moral reasoning of children changes over time. He also used moral dilemmas to gather insight into the types of reasoning used by people as they mature. The most famous of Kohlberg’s dilemmas concerns whether Heinz was right to steal a drug to help his sick wife. Kohlberg (1984) gave the dilemmas to 72 boys from Chicago when they were aged between 10 and 16. It was a longitudinal study; Kohlberg tested participants' reasoning every three years for 20 years.

Piaget's theory of moral understanding was formulated using similar research methods to his Cognitive Development theory: he used open ended clinical interviews to question 5 to 13 year old Swiss children on the rules of marbles and posed moral dilemmas so that he could examine their moral reasoning. Here is an example of one of Piaget's dilemmas: