Introduction to Philosophical Inquiry
Plato, "Are Human Beings Selfish?"
Glaucon says that if you look at what people really are, then you will see that they believe to do wrong is desirable and to suffer wrong is undesirable. Since we do not want to suffer wrong, we compromise with others and form a compact (a social contract) not to harm each other. These agreements are the origination of justice in society.
Human beings practice justice in order to avoid the harm that would come to them if they disobeyed the laws of the society. Thus, it is in our self-interest to obey the law because we fear the consequences if we were to get caught disobeying the law.
Glaucon's account is in accordance with the cluster of ethical theories such as psychological and ethical egoism, psychological and ethical hedonism, and ethical relativism. Essentially, he believes all persons are selfish, self-interested, and egoistic.
Glaucon proposes a mind-experiment: the myth of the magic ring of Gyges. Note how his account relates to the ad populum fallacy.
If anyone had a ring that would make him invisible, then that person would be a fool not to use it for personal advantage. Whether one were just or unjust, with such a ring, that person could do almost anything he wanted without fear of being caught. Both the unjust and the just person would use its magic powers because one would be a fool not to do what personally pays him much better.
It is important to notice that in this essay from the Republic, Plato is using Glaucon's account as a challenge for Socrates to overcome.