"Legal != immoral != socially acceptable" looks like a whole other thread. — fdrake

Then why did you drag the legality into this one? The OP question was whether incest between consenting adults is immoral - not whether it should be forbidden by law.
I was merely answering

Never being immoral" isn't the same thing as "being required not to". It's never immoral to eat ice cream, but you are not required not to. Separate ideas. — fdrake

It's a question of cultural norms. Incest has historically been widely practiced to varying degrees, especially among ruling classes. Is morality a cultural phenomenon? Or is culture a moral phenomenon? You decide.

There's been plenty of historic instances of royalty wanting to keep their blood pure through what we now consider incestuous relations. It's quite a sociological condemnation. That said, you are obviously hinting at parent-kid relations, or uncle-niece, which comes with huge issues with regards to influence and power imbalance. That's generally why they are wrong for the range reasons as "teacher-pupil".

Plus it's icky... I don't know how natural that reaction is but fear of snakes, for instance, is innate. And if it's innate disgust, I think we should trust nature.

Everyone is aware that first cousin marriage is legal outright in 20 states and DC, right?

The prohibition on incest is a form of eugenics, and that's okay. — Leontiskos

I think it's a proxy against molestation as well. Obviously it's theoretically possible to consent to incest, but it so rarely occurs between two consenting adults that it's used as an identifier something is terribly amiss.

But if you favor eugenics, why limit based upon consanguinity? Why not use more accurate genetic testing?

And I win the argument here for knowing the word "consanguinity."

It's willful engagement in behavior that is likely to produce an unsafe condition of elevated likelihood for birth defects. "Life is better than no life" would not be a way to justify drinking alcohol during pregnancy or competing in a boxing competition while pregnant. Why would it be any different in this scenario? — Outlander

Adding to this, I would also argue that there is a psychological dimension to this as well; we evaluate the mental health of people's decisions. Outside of religious and elitist ideals of pure blood delusions, when people live close to each other, such as within a family, and form sexual attraction, it generally arise out of issues in forming social normality.

The Westermarck Effect shows that people growing up together forms aversion towards attraction. It's seen both between biological siblings and those who aren't biologically linked. But genetical similarity can also produce attraction, seen in relatives who never grew up together and meet as adults.

So, human's seem to form aversion of incest through the Westermarck Effect, a socially formed programming of their attraction mechanism that prevents incest. And incest that is occurring may happen due to a problem or issue with that process forming properly.

A good explanation for it might be that humans were generally living closer together as a family and the Westermarck Effect formed properly because of it. But civilisation broke up these structures faster than evolution could keep up, and so distance between family members screwed up that programming to properly form. Either families split up, family members were too distant to each other, or other psychological traumas prevented it from forming.

From an evolutionary perspective, incest does not make sense, and so nature has a lot of functions to avoid incest from happening. Many animals have strong scent cues to avoid it, but human's generally form it through social structures and processes programming our brain and chemistry.

But just like many things in modern society, we break against the norms of our species evolution through culturally formed behaviors, and thus we have broken up family structures of our species into a culturally formed structure, dependent on societal behaviors rather than what we developed as animals.

We have less social programming in our modern world to avoid incest. And the latest findings that incest is far more common than previously thought, support that conclusion.

So is it morally wrong? If we're applying our behavior to the conditions of our species, many animals will perform incest if there's no other mating partners available, but seen as how many people there are in the world, we can only conclude that incestual behavior is a psychological defect of failed social programming among relatives; primarily by our modern society standards not aligning with our natural state of evolutionary programming.

It then comes down to if we can apply morality to such a psychological defect, or which defective behavior that we would consider immoral. It could be said that every psychologically deviant behavior that is destructive in society is immoral as every one of them are formed as psychological defects, and in that case incest is immoral. But if we aren't considering psychological deviant behavior as immoral and more of an involuntary mental illness, then it is a form of mental illness formed out of a failure to form our natural avoidances of incest. Just like we have other mental illnesses that's formed in modern society because we're not aligning with what is natural for us as a species, for example how modern society increase our stress levels to such dangerous levels that it produces brain damage.

It is rather worrying that incest is so common in society as it is. The 1 out of 7000 is very telling, but that's only counting the times when incest leads to childbirth. There's such an obvious obscured number in those statistics seen as incest are more common without producing a child. So the statistical number might be a lot higher. But in my conclusion, not that surprising.

, I completely agree. There are very few instances where there isn't some imbalanced power dynamic that would cause it to be a toxic relationship.

, I'd agree.

every one of them are formed as psychological defects, and in that case incest is immoral — Christoffer

That is just eugenics for the disabled.

That is just eugenics for the disabled. — Hyper

That's why I wrote out both paths, not to propose it is, but as a form of question. You either attribute acts in society that happens out of the psychological problems people have as immoral, and in that case you use moral values to judge acts out of the psychological state they're in.

Or you make no moral value apply and accept that there are only different psychological states which produce certain behaviors.

The problem with morality overall is that people want to talk about if an act is moral or not, but all acts comes from the psychology of a person, and that psychology can be defective.

Where do you draw the line between a decision that is psychologically affected and one that is not? Because the fact is, there are no acts that aren't psychological.

So, are you calling psychopaths disabled? ADHD? People suffering from trauma? PTSD? Alcoholism? Stress syndromes?

What about what we call "normal states of mind"? What is a normal state of mind?

If all acts are psychological, then what is a disability? And what can be judged as moral and immoral?

It's a question of cultural norms. Incest has historically been widely practiced to varying degrees, especially among ruling classes. Is morality a cultural phenomenon? Or is culture a moral phenomenon? You decide.

Might the bolded part not assume the answer here a bit?

Across history, there has been a wide amount of disagreement about the shape of the Earth (e.g. flat, spherical, cylindrical, etc.). Such beliefs tied into cultural norms, religion, philosophy, mythology, etc. Likewise, there has been widespread disagreement about the sources of infectious diseases, and these explanations have often been given culturally constructed moral dimensions (e.g. plague as God's vengeance). What people believed about these examples has varied by time and place, and people raised in one culture have tended to believe what others around them believed.

And yet, I don't think we would want to say that the shape of the Earth or the nature of infectious diseases is just about cultural norms. To be sure, our understanding of these is bound up in and filtered through such norms, since education, science, etc. are social practices, and the findings of science can fit into a metaphysical framework. But presumably we'd like to say that there is a "fact of the matter" about the shape of the planet or germ theory, and that this has been what has driven the evolution of cultural norms on this topic.

We don't generally think that "people have believed many different things about this at different times," represents a good reason to assume that most facts are primarily "cultural construction," so I think this appeal can be a red herring. It only makes sense if we assume that "what is good" is obvious. But what is "good" in many senses is often far from obvious, as the history of medicine can attest (e.g. treatments that do more harm than good).

I would tend to go with something like: "culture is a moral phenomenon," in the sense that Hegel lays out. E.g.:

• Institutions develop due to principles at work in the world, principles which are external to the thoughts or feelings of any individual. There is a certain logic to processes at work in the world, and this logic inexorably drives on the development of social institutions, shaping their essential structure even as historical contingencies also shape their specific actualization. (For example, there is an essence that all criminal justice systems share, but contingency also shapes the specific ways in which that essence is actualized.)
  
• When we talk about “the logic of development,” we can make a rough analogy to “the logic of natural selection,” which drives organic evolution. “Survival of the fittest,” is a description that “maps onto” the external world, and describes, in part, the process by which biological evolution occurs. Hegel believed he had identified a similar process at work in how institutions develop. (Note: Hegel is writing before Darwin, the comparison here is mine, not his).
  
• The logic that spurs on institutional development is deeply tied to our moral values. (This shouldn’t be surprising. If Hegel is correct, said institutions will turn out to be what actually shaped our individual morality in the first place!) This is Hegel’s foothold across the “is-ought gap.” He can now identify the origins of moral principles extrinsically in the logic of institutional development, in a process that is causally prior to any one individual’s development of their moral sense.
  
• Institutions survive and thrive because they promote justice and human happiness. In a sort of organic analogy, we could also say that they “survive and reproduce” because human beings find such institutions — free markets, courts, marriage, the state, etc. — to be beneficial and act to sustain and spread them. Moreover, this dynamic is intrinsic to states as entities made up of citizens. This is akin to how traits that facilitate the general health of an organisms’ cells will not be selected against.
  
• Human morality is clearly quite malleable. If one goes back far enough, the people of the past almost universally supported wars of conquest, slavery, etc. That humans today generally find these things abhorrent is itself the result of cultural conditioning. It is an obvious fact that most people tend to embrace the morality of their particular society.
  
• But — and this is the crucial point — it is institutions, the state key among them, that control the type of morality that a people comes to embrace. Thus, the morality embraced by a given people is not random, but tied to the development of their institutions. And these institutions themselves develop according to a specific, extrinsic logic (see above). They are also sometimes consciously reformed according to the moral preferences of individuals.
  
• Together, these points allow Hegel to ground the origins of morality in the external logic of the world. Such a logic is not “cut off from the world,” like a deontological logic that is born of pure, abstract reason, but neither does it collapse into relativism. Relativism is not a problem if we are able to recognize the essential principles at work in the development of our institutions and separate these principles from the merely contingent factors that shaped current institutions. These contingent factors will eventually be swept away by the progress of history if they contradict the essence that underlies the institution in question.
  
• And indeed, Hegel does think he can identify the core principle driving the evolution of human institutions: the promotion of human freedom!

For example:

A better reason for claiming that incest should not be considered as permissible is that the conditions for consent to it don't make that much sense, the hypothetical scenario in the OP is not representative of the scenarios where incest occurs. It's a bit like saying that murder is permissible since there are conditions in which killing is permissible.

This right here is an example of progressing from the particular prohibition, the "letter of the law," to the more general essence in question, "the spirit of the law." This is something people have an easier time doing if they live in a society that fosters human flourishing through education, etc.

We might disagree here that "consent" is the only issue at play, but either way we will be moving beyond "what norms are" towards "why they are," which in turn informs "what they should be," and our conception of "what they should be," in the aggregate, shapes what norms actually become.

And yet, I don't think we would want to say that the shape of the Earth or the nature of infectious diseases is just about cultural norms. To be sure, our understanding of these is bound up in and filtered through such norms, since education, science, etc. are social practices, and the findings of science can fit into a metaphysical framework. But presumably we'd like to say that there is a "fact of the matter" about the shape of the planet or germ theory, and that this has been what has driven the evolution of cultural norms on this topic. — Count Timothy von Icarus

And I would agree in general with this argument. Except to say that, at a fundamental level, there may indeed be "facts of the matter" which cultural norms - or metaphysical presuppositions - both proscribe and prescribe. So there may be things about reality - facts of the matter - that our presuppositions (of whatever kind) don't allow us to grasp. And also, yes, perhaps there are facts of the matter when it comes to incest-prohibitions, but these would be more like "genetic imperatives," for which there is no trivial translation into a moral vocabulary.

Life created in that way is morally and genetically wrong.

At a moral standpoint, having a child with someone that you are related to will cause the child to get many unfortunate traits that will just cause the child to suffer. Even if life is created, it would not be right as it would live in constant pain as well as the child would not live for very long.

Genetically speaking, having a child through incest is completely against the point of procreating. Sexual reproduction is done so that recessive diseases would be taken out of the lineage and the DNA to become better. It is even makes people want to not have any sort of sexual relationship with family members.

In both cases it would be wrong, but let me know why you think it isn't.