Another point I want to address. “The inclination, never mind the action, that one would like to engage in sexual intercourse with their direct sibling is a definite sign of inner confusion.” This claim seems to be unacceptably vague. Just what would the siblings be confused about? Are you referring to the transference of feelings for some non-related person to my sibling? In all fairness, this seems to be covered in the conditions of the situation where we specifically state that the attraction is genuine and mutual and in that case I fail to see any kind of “confusion” that could be taking place.

This does bring into focus the lengthy debate between rational morality (following Kant) and sentimental morality (following Hume), and in a rather interesting way. On rational grounds, incest with proper birth control, seems quite alright (although Kant was not a fan of sex in general) but we are quite willing to say nevertheless that there is something wrong with incest even in non-harm causing cases, which is where disgust comes in. But, it is also the case that not every one does think so. Even those, such as myself, who do not and would not practice incest are willing to say that even though they feel that incest is disgusting, it is not morally wrong. To put it plainly, if I were to witness an act of consensual and safe incest I would likely squirm but I would not feel compelled to denounce it.

Ultimately, from my discussions with others, I conclude that the wrongness of incest is not immediately obvious. Indeed, it very much depends on how you define wrongness itself. If wrong is just the kind of action that goes against certain plausible moral principles (do not harm, say), incest is not inherently immoral. If, however, wrong is just the kind of action that causes a sentiment of disapprobation (following Jesse Prinz’s constructive sentimentalism theory) in the observer, then incest is, largely, wrong. While I am sympathetic to the later view, I think it is, to put it simply, not good enough. I am far more inclined towards the former view.

Does this mean incest isn’t wrong anymore, and we should go against our (now useless) instincts? Depends on how you view morality. Let’s see what we’ve agreed on so far:
1. The Fritzl case is repulsive because it involves incest and a betrayal of familial trust (i.e. lack of consent).
2. However, it is wrong only because of the non-consensual aspect. We’re unsure whether the incest is wrong in itself.

I think that in drawing a line between what is “wrong” and what is merely “repugnant,” we are implicitly assuming that moral reasoning must be rational. If our “ick factor” in response to incest can’t be justified by a reasoned process, then we are hesitant to elevate that intuition to the status of a true “moral” value.

Haidt’s example about the brother and sister challenges us to question this assumption of rationality. As he points out elsewhere in the paper (pdf), even the “reasoned process” behind seemingly legitimate moral values can be reduced to a series of post-hoc rationalizations. We construct morality to bolster judgments that first formed as intuitions, not the other way around. If you’re a liberal on gay rights, you’ll see this in John Finnis’ condemnation of homosexuality (pdf, p.4). If you’re a conservative, chances are you’ll find his arguments the epitome of carefully reasoned moral philosophy.

One quick conclusion from all this is that our instincts matter just as much as supposedly “rational” moral values. Maybe “repugnant” and “wrong” aren’t separate qualities of an act, but both ways of tapping into the same deep-rooted adaptive instinct that we all have. Incest isn’t necessarily wrong because it is bad — it is wrong because it used to be bad, and that badness is still encoded in our psychology and communal values.

in other words, there’s a model that says that a daughter should be safer with her father than just about anyone else. obviously this is wrong in the example given; obviously it is wrong in other circumstances too. if nothing else it’s far more convenient and less subject to public scrutiny to hurt your own offspring than somebody else’s.

haidt’s question seems relevant b/c of the incest, but it lacks the aspects of rape and violation of safety within the family that i think really drive the horror of the fritzl case.