E.O. Wilson in Consilience: The Unity of Knowledge:

Where cultural relativism had been initiated to negate belief in hereditary behavioral differences among ethnic groups – undeniably an unproven and ideologically dangerous conception – it was then turned against the idea of a unified human nature grounded in heredity.  A great conundrum of the human condition was created: If neither culture nor a hereditary human nature, what unites humanity? The question cannot be just left hanging, for if ethical standards are molded by culture, and cultures are endlessly diverse and equivalent, what disqualifies theocracy, for example, or colonialism? Or child labor, torture, and slavery?