Is it rational to have the moral belief that there should be slaves. A rational person would look at all the supporting beliefs and see if they are themselves rational. For example, are slaves inferior beings. The answer, as we know, is no. In terms of the mass slavery of large portions of people, this has often been due to some characteristic like high levels of melanin for the slaves in America. These characteristics don't make people inferior and they sure don't make people inhuman.

On the other hand, high levels of melanin were correlated with lower intelligence.