An excerpt from Mob Morality: The Dangers of Repugnance as Moral Authority by Tauriq Moosa on 3QD:
"Jonathan Haidt famously provided the following example in a study.
Julie and Mark are brother and sister. They are travelling together in France on summer vacation from college. One night they are staying alone in a cabin near the beach. They decide that it would be interesting and fun if they tried making love. At the very least, it would be a new experience for each of them. Julie was already taking birth control pills, but Mark uses a condom too, just to be safe. They both enjoy making love, but they decide never to do it again. They keep that night as a special secret, which makes them feel even closer to each other. What do you think about that? Was it ok for them to make love?
Haidt, in an interview, explained the responses of subjects reaching ‘moral dumbfounding’:
People almost always start out by saying it’s wrong. Then they start to give reasons. The most common reasons involve genetic abnormalities or that it will somehow damage their relationship. But we say in the story that they use two forms of birth control, and we say in the story that they keep that night as a special secret and that it makes them even closer. So people seem to want to disregard certain facts about the story. When the experimenter points out these facts and says “Oh, well, sure, if they were going to have kids, that would cause problems, but they are using birth control, so would you say that it’s OK?” And people never say “Ooooh, right, I forgot about the birth control. So then it is OK.” Instead, they say, “Oh, yeah. Huh. Well, OK, let me think.”
So what’s really clear, you can see it in the videotapes of the experiment, is: people give a reason. When that reason is stripped from them, they give another reason. When the new reason is stripped from them, they reach for another reason. And it’s only when they reach deep into their pocket for another reason, and come up empty-handed, that they enter the state we call “moral dumbfounding.” Because they fully expect to find reasons. They’re surprised when they don’t find reasons. And so in some of the videotapes you can see, they start laughing. But it’s not an “it’s so funny” laugh. It’s more of a nervous-embarrassment puzzled laugh. So it’s a cognitive state where you “know” that something is morally wrong, but you can’t find reasons to justify your belief. Instead of changing your mind about what’s wrong, you just say: “I don’t know, I can’t explain it. I just know it’s wrong.” So the fact that this state exists indicates that people hold beliefs separate from, or with no need of support from, the justifications that they give. Or another way of saying it is that the knowing that something is wrong and the explaining why are completely separate processes."