By their very nature, moral laws are both universal and objective. This may not seem obvious upon first consideration. Don’t the moral practices of the different cultures of the world vary widely? Isn’t there moral diversity within our own society? It seems there is no universal, objective morality. Such a conclusion, however, arises from an error of fact and an error of logic. It is certainly true that the moral behavior of the world’s cultures shows enormous variation. Carl Sagan writes that there are cultures like the Ik of Uganda, “where all the Ten Commandments seem to be systematically, institutionally ignored.” My own anthropological work on the Ik is incomplete, so I cannot say whether he is right. But let’s assume that he is. What does this show? That the Ik are radically different from us? But we too live in a culture where the Ten Commandments are systematically and institutionally ignored. Sagan’s example seems to establish not diversity but unity of practice.

But even better examples fail to establish Sagan’s point. Let’s say that anthropological investigation reveals that the Ik routinely beat their wives. Would this prove that beating your wife is the right thing to do? Of course not. The presence of moral disagreement does not indicate the absence of universal morality. How can the fact of behavior, however eccentric and diverse, invalidate the norm of what is right?

It would be interesting to conduct a global survey to see if all the cultures of the world accept the heliocentric theory. Let’s say that such a study shows there are people in South America and Southeast Asia who, even when shown the reasoning of Copernicus, supplemented by photographs taken by modern spacecraft, emphatically reject the idea thatthe earth goes around the sun. These people are delighted to discover that a fellow named Ptolemy held a rival view, and they decide to go with Ptolemy. Would anyone conclude from this that the heliocentric theory had been refuted? The suggestion is absurd. I am highly confident that there are many cultures in the world, even today, that would emphatically reject Darwin’s theory of evolution and Einstein’s theory of relativity. None of this would show that scientific laws are relative, only that the people who reject them happen to be wrong. Thus the testimony of a hundred quarreling tribes and widespread differences of opinion about morality do nothing to undermine the notion of universal morality.

Over the last several decades, anthropologists have been comparing the norms and practices of the various cultures of the world. Two of their findings are relevant for our purpose. First, morality is universal.