What, precisely, is so bad about sex between adult siblings, bestiality, and the eating of corpses? Most people insist such acts are morally wrong, but when psychologists ask why, the answers make little sense. For instance, people often say incestuous sex is immoral because it runs the risk of begetting a deformed child, but if this was their real reason, they should be happy if the siblings were to use birth control -- and most people are not. One finds what the social psychologist Jonathan Haidt called "moral dumbfounding," a gut feeling that something is wrong combined with an inability to explain why.
Haidt suggests we are dumbfounded because, despite what we might say to others and perhaps believe ourselves, our moral responses are not based on reason. They are instead rooted in revulsion: incest, bestiality and cannibalism disgust us, and our disgust gives rise to moral outrage.
You can test your own moral intuitions with the Taboo game at Butterflies & Wheels. (Yeah, I know I've been linking to them a lot recently, but what can I say - it's just a damn good site!)
The game is followed by an excellent essay by Ophelia Benson:
But this business of repugnance we can't quite articulate should give us pause - should make us come to a screeching halt, in fact. Why can't we articulate it? Could it be because there is nothing to articulate? [...]
Especially since people have always 'just somehow known' in their guts or their hearts or their gluteus maximus, without being able to say why, all sorts of things that the world would be better off if they hadn't just known. That Africans should be slaves, that Jews were polluting Germany, that women should be kept under house arrest at all times, that witches should be burnt, that the races must never mix. We're all too adept at thinking what we're not used to is inherently disgusting.
I suspect that this is a major cause of conservative opposition to gay marriage (and homosexuality generally). I find it rather ironic how they accuse liberals of 'relativism', when much of conservative "morality" is little more than knee-jerk emotivism, followed by feeble rationalisations ("Uh, it's unnatural! Spreads disease! Is bad for families!", etc.). Of course, the loony left is no better, especially when it comes to biotechnology. But I digress.
Moral facts are determined by reasons, not emotions. Still, it is undeniable that emotion plays a huge role in our moral lives. Sometimes this can lead to unreflective prejudice, as mentioned above. But it can also help motivate us in a way that pure reason cannot. Although disgust can cause us to dismiss others from moral consideration, sympathy and empathy can play the opposite role, encouraging us to reach out to those we might otherwise have ignored.
I think that morality is fundamentally about character. It's not just about what you do, but rather, who you are. Thus, I think that emotion is important not as an 'input' to the moral calculus, but as an 'output'. That is, our emotional responses do not determine morality. Rather, morality informs us what our emotional responses ought to be.
An ideal moral agent, by this view, would always have an appropriate emotional response to any given situation. He would thus be able to rely upon his emotions as a sort of 'moral barometer' - he would feel appalled by immoral acts, but unconcerned by permissible ones. But it is important to note the direction of fit: he is good because his emotions correlate well with the moral facts of the situation. It is not the case that the situation is good/bad merely because of his (or our) emotional response to it.
We are hopefully good enough that our consciences reliably correlate with the moral facts, as a general rule. So our emotions are a useful shortcut which can serve us well in our daily lives, when we haven't the time to reason carefully. But in controversial cases, we should remember that these heuristics are far from infallible. Ultimately, if something is wrong, then it's wrong for a reason. And "yuk" is not a reason.