Moral relativism is a term for the philosophical perspective that morality is the opposite of moral absolutism, based on the observation that moral codes vary greatly between groups and individuals. This is directly opposed to "universal morality" belief systems such as Christianity, which tend to move from "There is a Good TruthTM for everyone, everywhere and everywhen" to "what I say is the Good TruthTM is the Good TruthTM for everyone, everywhere and everywhen; indeed, those who disagree clearly deserve punishment."
Theists, specifically American Christian Fundamentalists, generally decry moral relativism, because it asserts that
- they, and not a single God, created the laws they live by
- they do not have an inherent, basic right to tell others how to live
- there is no justification for declaring gay activities, premarital sex, abortion, and eating nachos on Thursday universally "wrong"
 Descriptive vs. normative relativism
There are two branches of moral relativism, broadly speaking. One is "normative relativism," which is the way that the general phrase "moral relativism" is most often used (and abused). Normative relativism (sometimes called "meta-ethical moral relativism") rejects both the possibility and desirability of an absolute morality, holding that morality is and should be relative to individuals, groups, etc. Descriptive relativism, on the other hand, merely denotes the observation that an absolute moral system does not exist and has not been practiced throughout human history. In brief, descriptive relativism corresponds to the "is" of Hume's law and normative relativism to the "ought." There are philosophers who are descriptive relativists in that they reject the existence of absolute morality, but argue that some standard, universal system of morality and ethics should be constructed (e.g. J.L. Mackie[wp]). It is common for this position to be straw manned as "moral relativism."
 Religious moral relativism
It is interesting to note that religious values change with time and culture as well. Consider the question of slavery. Both the Old and New Testaments have specific instructions for both slaves and slave-holders, and never condemn the practice. Christian leaders used to have no qualms about using the Bible to support such positions. Nowadays, however, only Christian dominionists would dare voice such an attitude.
The Roman Catholic Church, for example, has had a history of being conflicted on whether slavery was compatible with natural law: first accepting it as compatible with natural law, accepting it as necessary and good utility-wise but outside of the realm of natural law, and finally rejecting it as an unnecessary evil that is against natural law by the time of the 20th century. Church apologists attempt to dance around the issue with various claims, such as certain types of slavery being bad (and the Church was, of course, always against whatever slavery was bad), that the Catholic who claimed it may have not been a real catholic (even if it was the Pope), or that the Church has really always been against all slavery and has just "clarified" its teachings on slavery when society finally figured out it was wrong.
 Other criticism
Another criticism of moral relativism is that its promoters do not actually believe it, and by asking an inconvenient question one could supposedly get such a promoter to reveal that he or she believes in some absolute morals.
In its purest form, moral relativism is an example of a self-refuting idea; the principle that "all moral frameworks are relative" being, itself, an absolute moral framework. In practice, though, it tends to be "All moral frameworks except mine are relative" which is instead an example of special pleading.
The above critique, however common, is rather weak as moral relativism should not be classified as a "moral framework" as it does not prescribe any moral values or principles. Moral relativism in it's purest form is the observation that all moral frameworks share a common trait - that they are not absolute.
 See also
- Entry in the Stanford Encyclopedia of Philosophy
- Jesse Prinz. Morality is a Culturally Conditioned Response. Philosophy Now, Jan/Feb 2012