Moral psychology aims to find the psychological factors involved in ethical thinking and behavior. The degree to which we act ethically is governed by our willpower, moral judgments, social norms and situations, and moral self-image. Although we like to think that our moral judgements are based on rational thought, they are more often based on subconscious thinking heuristics and biases. When someone behaves in a way that seems wrong, but doesn't cause anyone harm, we have an automatic reaction of disgust or dislike. After having this gut reaction, we try to rationalize why the behavior is wrong. Long held traditions and perceived social norms also inform our moral behavior.
Our ability to make moral judgments develops over the course of our lives. One theory that attempts to map out the steps of this growth is Kohlberg's Theory of Moral Development, which can be found in the table below. Most people typically reach the fourth stage: Law and Order.
Although Kohlberg places "universal ethical principle" higher than "social contract," this hierarchy may be arbitrary and partly biased. In general, women tend to use a morality based on social relationships, while men are more likely to use a morality based strictly on what is "right" and "wrong."
The clips below, from Mulan (1998), show two people who have both reached the "Principled Level" of moral development. In the clip on the left, Mulan's father demonstrates a commitment to honor and an ethical principle. In the clip on the right, the narration describes how Mulan has broken all the rules, but served her family and country well. This is reflective of the "social contract" level.