Like Sohpie, I want to congratulate you for you excellent idea. And thank you and Sophie for the references. I want to suggest a wonderful chapter by Lawrence Kohlberg, entitled, From is to ought: How to commit the naturalistic fallacy and get way with it in the study of moral development. In T. Mischel (Ed.), Cognitive development and epistemology (pp. 151-235). New York: Academic Press.
I cannot share a link because the chapter is not in the Internet. It is a superb chapter for one or two lectures. In this chapter, Kohlberg cogently argues against the idea that we cannot erect as moral ideals (e.g., the principle of justice) those ideals that are not followed in factual or practical terms. To do so -- Kohlberg observes -- is tantamount to committing the naturalistic fallacy, that is, to go from the "is" domain (a given moral principle, such as the golden rule is not followed at many times and places) to the "ought" domain (because the golden rule is not followed at many places and times, we cannot erect it as a universalizable and prescriptive moral principle). In this chapter, Kohlberg opposed to what is often called ethical relativism.