The moral relativist believes that ethical truths are relative to groups smaller than humanity as a whole. That this is true of fundamental ethical truths, not merely derived moral rules, is essential to relativism. To allow just one fundamental universal ethical truth that is independent of the say so of individuals or groups is to abandon relativism in favor of a realist view of ethical truth. There are as many versions of moral relativism as there are groups or individuals ethical truth can be relative to.
A simple version of moral relativism, societal moral relativism, holds that a type of action is morally right relative to a society if it is deemed right by the majority of members of that society. Moral relativism is a view about what makes ethical truths true. It is not a view about how we come to hold moral beliefs. There is very good reason to think that in fact we often acquire our moral views from elements in our culture. What people come to believe depends on cultural influences, but this is not moral relativism. Moral relativism is the view that what determines the truth or falsity of moral beliefs is just what is endorsed by the prevailing culture. According to moral relativism, moral truths are made by the dominant view in a society, not merely propagated by the dominant culture.
Many would endorse some version of moral relativism on the grounds that it supports tolerance and respect for societies with differing moral views. Moral relativism seems to be a view that allows for different societies to embrace different moral standards that are right relative to the respective societies. Moral relativism rejects the notion that the moral standards of one society could be objectively correct. So if moral relativism is correct, no society could be justified in forcing its moral standards on other groups because its standards are objectively correct. This line of thought has lead many who value diversity and tolerance of diversity to embrace moral relativism. But this is a mistake. because moral relativism leaves open the possibility of intolerance justified by other means. In fact, the result of a straightforward application of societal moral relativism is that being intolerant towards other groups is right relative to a society if the majority of members of that society deem intolerance right. So for instance, assuming the majority of Germans in Nazi Germay approved of the persecution of Jews, then according to societal moral relativism, intolerance towards Jews was right relative to Nazi Germany. Those who value tolerance and respect of individuals or groups with differing moral views would do much better to endorse tolerance as an objective realist ethical value than to endorse moral relativism.
A strong argument against moral relativism is the argument from change. Sometimes our view about the moral status of some practice changes. A person might, for instance, think that eating meat is morally unproblematic at one time and then become convinced that animals deserve some kind of moral regard that speaks against eating meat. When a person’s moral views change in this fashion, the do not merely drop one moral belief in favor of another. Typically, they also hold that their previous moral view was mistaken. They take themselves to have discovered something new about what is morally right. Likewise, then the prevalent moral belief in a society undergoes a significant change, as in the civil rights movement, we are inclined to see this as a change for the better. But the relativist cannot account for changes in our moral beliefs being changes for the better. This is because the relativist recognizes no independent standard of goodness against which the new prevalent moral beliefs can be judged to be better than the old prevalent moral beliefs.
A closely related problem for moral relativism is the moral reformer’s dilemma. We recognize a few remarkable individuals as moral reformers, people who, we think, improved the moral condition of their society in some way. Common examples might include, Buddha, Jesus, Ghandi or Martin Luther King. While the relativist can allow that these individuals changed the moral views of their societies, none can be said to have changed their societies for the better according to the relativist. Again, this is because the societal moral relativist recognizes no standard of moral goodness independent of what is accepted in a society according to which a society that has changed can be judged to have changed for the better. A relativist that takes ethical truth to be relative to the dominant view in a society seems to be committed to taking institutionalized racism to be morally right relative to per-civil rights American society and wrong relative to post civil rights American society. But since standards of goodness are determined by the prevalent views in a society, there is no standard goodness to appeal to in judging that the change our society underwent in the civil rights movement is a change for the better. According to societal moral relativism, anyone who takes Martin Luther King to have improved American society by leading it to reject institutionalized racism is just mistaken about the nature of ethical truth.