What's wrong with relativism
A lot of commenters on previous relativism threads describe themselves as relativists. About half of these self-identified relativists seem to be arguing for moral skepticism or moral nihilism.
Moral skeptics deny that anyone knows anything about morality. The skeptic notes that different cultures have different moral beliefs, and that moral disagreements seem to endure within cultures. If universal moral truths are knowable, the skeptic wonders, why hasn't everyone converged on them already? Moral nihilists reject the whole concept of morality. They argue that so-called moral principles are just norms or customs with no special authority. Most people think the fact that stealing is wrong is a prima facie reason not to steal, regardless of the practical reasons for or against. Nihilists argue that moral principles don't provide reasons to do anything.
By contrast, moral relativists argue that the morality of an action depends on (is relative to) prevailing cultural norms (or, to each individual's beliefs and values). According to the relativist, claims about morality are analogous to claims about legality. It doesn't make sense to ask whether radar detectors are illegal, simpliciter. There are facts about whether radar detectors are legal in Utah, or California, or Finland, but something can only be illegal relative to a particular legal code. The relativist claims that the same is true of questions about morality. According to the relativist, you can't just ask whether slavery is wrong, you have to ask whether a particular culture considers it to be wrong at a particular time.
Unlike skeptics and nihilists, moral relativists agree that there are moral facts and good reasons to be moral. Unlike moral skeptics, relativists think we know a lot about morality. Ironically, the moral relativist is committed to much stronger claims about the state of our moral knowledge than most non-relativists (hereafter, "objectivists"). If morality is relative to culture or individual values, then everyone must already know a lot about morality.
Many self-described relativists make some version of the argument from ontological queerness against objectivists. They argue that it's absurd to suppose that there are any abstract universal moral principles. Where do these standards come from? How do we learn about them? How could such metaphysically weird things fit into a scientific worldview? Queerness is a profound objection to all theories of morality, including moral relativism. But it's even harder to explain why right and wrong should depend on the prevailing attitudes of the culture in which the act takes place. It seems unlikely that the truth of moral claims should be indexed to the ebb and flow of popular opinion, that is, if we assume that morality is the sort of thing we should take seriously.
One of the most serious problems with cultural relativism is that "culture" is a nebulous abstraction. We all belong to multiple social groups, and the moral codes of these groups often conflict. The choice of reference group is critical for relativist ethics. If the rightness of my actions is relative to the norms of my culture, I have to know which group constitutes my culture. Is is culture defined by race, nationality, geographic proximity, religion, gender, class, or what? If a draftee is both a Quaker and a citizen, is she both required and forbidden to serve in the army? Any moral theory that allows for the same act to be simulatneously required and forbidden should be rejected as incoherent. Even if we suppose that there is some non-arbitrary criterion for cultural identity, relativism must still contend with ideological diversity within cultures. How do we know what Our Culture thinks about anything? Inevitably, there will be moral disagreements within any group. Who's to say which opinons define the moral standards of a culture. Powerful factions within cultures have always asserted their values as definitive of the culture as a whole. When we say that slavery was acceptable in a particular culture, we're probably going by the slaveholders' opinions rather than those of the slaves.
Cultural change raises additional complications for relativist ethics. If you want to say that morality is relative to culture, you have to specify what time-slice of the culture you're talking about. At one time, homosexuality was all but universally condemned in our society, but this consensus has dissolved. It's odd to think that homophobia was right until 1970 and became increasingly morally problematic thereafter. Likewise, slavery used to be a universally accepted human institution. The relativist has to explain at what point slavery became wrong. According to relativism, the early abolitionists were incorrect because they started protesting slavery at a time when it was still widely accepted. It would be absurd to argue that all dissenters and iconoclasts are necessarily wrong, that is until they gain some critical mass of public opinion.
Reformers change public opinion by offering reasons and evidence to support their position. If they are successful, they will change people's minds. They may well change the culture itself. Relativists might argue that reformers are really appealing to some deeper, shared set of cultural values to resolve an apparent disagreement. But how are we to separate a culture's "deep" values from whatever people happen to believe at the moment?
It is telling that most self-identified relativists don't take arguments from popular opinion or tradition seriously in everyday contexts. Appeals to traditional values tend to leave them cold. Most would agree that it's fallacious to defend torture on the grounds that most people support the practice. Likewise, most relativists don't think that mere tradition is is a good argument against gay marriage.
Finally, it is odd that many cultural relativists are drawn to this position because they are atheists or agnostics. It's puzzling that they do so, knowing how many widespread moral beliefs are based on dubious religious beliefs. Many Americans disapprove of homosexuality because they believe in vengeful fag-hating God. The atheist rejects this factual claim. It's odd to argue that cultural norms determine right and wrong while acknowledging that these norms are predicated largely on delusional beliefs. It's perverse to argue that homosexuals actually deserve to be punished because a majority of the population believes some sick folktale.Update: I want to emphasize I'm not arguing for objectivism here. For the moment, I'm just arguing against relativism. Relative moral truths are at least as difficult to reconcile with naturalism as absolute ones. If anything, relative moral truths are queer twice over because relativism doesn't give a good explanation for the alleged metaphysical fact that first-person opinion or cultural norms make moral claims true or motivating. I think skepticism and nihilism are more serious challenges to absolutism than relativism.