Relativism is the view that states that moral principles vary by culture (conventionalism) or by individuals (subjectivism). Conventionalists like Ruth Benedict argue that since different cultures hold different principles, one culture has no basis to judge another culture's morals. She uses the argument of normality: each culture defines what behavior is normal to fit the behavior of the majority. The majority of that population then defines normality and lives by it, and only a small minority deviates from that normality. According to Benedict morality is just term that we've come to use for socially approved habits, and normal is a variant of the concept of good. Subjectivism is the extreme end of relativism. This view holds that morality is determined at the individual level, not a social or universal level. Therefore, the only moral principles that are valid are the ones you believe in, and basically all principles are equally valid.
Criticism of these arguments starts with the judgment question: how can a society or individual judge the behavior of another if all socially accepted behaviors or personal moral principles are valid? The answer is that it can't, but a few examples will show what tolerance can allow. From a historical standpoint slavery was considered normal by those who held slaves. Since slaveholders were the dominant culture in that area, the normal and therefore, the good behavior was to own slaves. According to conventionalism slavery was a morally right act at the time that it was popular, and only