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When talking about sexual relationships between brothers and sisters, our initial intuition might be, “Well that’s just wrong. There’s something immoral about that”. Now, to examine the logic, I want to discount that intuition; but we need to probe that intuition.

To move away a little from any discomfort some may have with consensual sex between an adult brother and adult sister, let's use the following example.

A brother and sister, unknowingly separated at birth through adoption, over the legal age of consent, are in a sexual relationship on a consensual basis. They love each other as anyone else does in such a relationship and everyone is fine with it. Suddenly, the information about who they are comes to light. They now know they are brother and sister and suddenly, their sexual relationship has to end according to society, and the law.

Now, they cannot just switch off their feelings just like that. What if they had a healthy baby out of their relationship? It is very possible.

Now, if they continue having a sexual relationship, whether it is consensual or not, they can find themselves imprisoned.

What we are not talking about here

When talking about the incest prohibition laws, typically, people think about incest as about adult sex with children (e.g. Anderson, 2006; Gordon, 1986). I am not talking about this, as we need to be very clear that this scenario cannot justify the incest prohibition. That is because we have other laws that prohibit sex with children. It is illegal for an adult to have sex with children and that would be true, even if incest were permitted. So, we need to get the adult sex with children out of our minds when we think about incest prohibition.

Logic

Consent

Consent isn't an issue either, because if an incestuous relationship was not consensual it would be rape or sexual assault. We have laws covering that too.

The Genetic Abnormality Argument

With the genetic abnormality argument, about 7.9 million infants (6% of worldwide births) are born every year with serious birth defects, and the causes of over 50% of birth defects are unknown (Lobo & Zhaurova, 2008). And, do we prohibit non-incestuous marriage due to the potential for genetic abnormalities? That would be illegal. For example, you can’t say to a couple who both carry sickle cell genes that they cannot marry because it is likely their child would have sickle cell anaemia.

In some jurisdictions, even stepchildren cannot marry or have sexual relationships due to the incest laws. If the genetic abnormality argument is the reason for the abolition of incestuous relationships, why stop relationships between step siblings?

The crux of the question

As you'll probably see by the format of my question, I am not looking for pure opinions here. I would like any facts supported by links/references for further reading.

Overall, what is the reason for incest prohibition laws?

References

Anderson, K. M. (2006). Surviving Incest: The Art of Resistance. Families in Society, 87(3), 409–416. https://doi.org/10.1606/1044-3894.3546

Gordon, L. (1986). Incest and Resistance: Patterns of Father-Daughter Incest, 1880-1930, Social Problems, 33(4), 253–267. https://doi.org/10.2307/800718

Lobo, I. & Zhaurova, K. (2008) Birth defects: causes and statistics. Nature Education 1(1):18 https://www.nature.com/scitable/topicpage/birth-defects-causes-and-statistics-863/

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Jonathan Haidt uses exactly this example in his research on what he calls 'moral dumbfounding', discussed here: What’s the Matter With a Little Brother/Sister Action? in Psychology Today.

Here are a couple more examples, that we have discussed here:

How do ethicists tackle the question "Is it immoral to have sex in public places?" Is it possible to use rational and empirical ideas to answer?

Is artificially generating images of minors in sexual positions unethical?

The TLDR is, we hijacked fear and disgust for social purposes, and they allow rapid reprogramming of societies by feedback about living successfully, that we call: culture.

The sex-in-private thing is a really clear example. We so take it for granted that it's an act that needs privacy, that it's fairly shocking to find we are one of only two species on Earth known to have this preference. But it's fairly easy to see from game theory arguments, that it greatly helps humans to cooperate to have the preference. So, socially programmed intuition.

We know from Hume that you can't 'get an ought from an is'. We don't reason our way to our morality. We have feelings what is right, then we use reason to justify and tune that, and decide how to act. Research on cognitive biases and post-hoc reasoning shows that this is how we do all our reasoning, unless we take special steps or systems we have developed to correct ourselves.

Haidt's research into how humans actually undertake moral reasoning in practice, led to Moral Foundations Theory. Lots of interesting conclusions follow, like feeling under threat as a teenager eg by pandemics or border conflicts, makes people tend to be more conservative for life, and less tolerant of ambiguities.

To get at the problems with legalising consensual incest, I'd draw comparison to the issues around legalising euthanasia. Every broadly agrees on what the risks of harms are, hardly anyone agrees about how or whether they can be managed, and public policy around the world varies wildly. Consider the wide variation in penalties for incest, and even definition such as whether cousin marriage is legal, as comparison.

Edited in response to comment

I hadn't properly understood the rating system of the Wikipedia page I linked to about variation in prosecutions. Same sex consensual adult incest is legal in Germany & Ireland. Legal full stop in Belgium, Argentina, Brazil, China, Israel, Ivory Coast, Latvia, Japan, Lithuania, Luxembourg, Philippines, Portugal, Russia, Serbia, Slovenia, Spain, South Korea, Thailand, & some US states. 20 out of 192 countries. That covers jurisdictions with approximately 1/3 of the world's population. Many more like India don't have specific prohibitions, likely leading to uneven persecutions, and casting doubt on the idea it is a universal cultural taboo or surely it would have been codified into law early on. Honestly I think this challenges the basis of my argument. It would be interesting to find research about any tensions between social pressure/condemnation and the law. Same-sex consensual adult incest is an even clearer example for discussion, because there is zero risk of offspring - it perhaps also challenges reasoning from intuition even more directly, adding unease many feel but can't justify logically about homosexuality. Bigger than the questions about moral behaviour here, are questions about the role of government and when intervention in private lives is permissible.

Edit ends

I would look to the history of prohibition of incest. Greece has some of the oldest records regarding it. Many stories in Greek mythology are about kings who could literally do anything they wanted, like Midas, and Sisyphus. Oedipus was blamed for a pandemic that struck Thebes, which the Oracle at Delphi said resulted from the killer of the previous king - leading Oedipus to find that was him. Greek mythology has the Erinyes or Furies, deities of vengeance that punished 'the crimes that most offended the gods', regicide, killing family, and murder, by hounding those that committed them to madness. We can understand this now as guilt, and maybe PTSD. Orestes was a just king, but his crimes to get there meant he could not be free of the Furies. Incest was likely to result in abnormal births or stillbirths, interpreted as judgement by the gods (the Minotaur is another example of a probable abnormal birth, interpreted as a judgement). We should see that judgement as not on a single case, but as on a social tolerance of incest, because multiple generations committing incest gigantically increase the risks. We can see this with the results of Habsburg cousin-marriage, especially in Charles II of Spain, child of an uncle and niece marriage.

The powerful have many times denied the hold of laws about incest over them. But social condemnation, and conscience, and the feeling they and their lineage were facing the wrath of the gods led to a cross-cultural sense that incest was worse than a crime, it would lead to being cursed.

We begin our moral reasoning with our intuitions, that is inevitable. Our intuitions come from game-theory dynamics, encoded into culture. We have a deep intuition incest is wrong, and cultural guards against beginning to tolerate it, because the impacts are cumulative, eg one case might be consensual, but tolerating incest in general makes cases with bad power dynamics more likely, and compounds genetic damage. Our societies are not ordered by accident, we can look to the idea of Moral Foundations Theory to understand that holding certain core values is key to humans collaborating. And I would say tolerating incest, even consensual sibling incest, directly challenges these sources of social cohesion (specifically Care & Sanctity).

Final edit

Given wide cultural variation in prohibition of consensual adult incest, it seems wrong to say 'deep intuition incest is wrong, and cultural guards against' is a cultural universal. There is cultural variation in whether natural disasters are a judgement, so perhaps cultural framing is key. Maybe also how hierarchical a society is, with Habsburg gene-flow into the wider population being pervasive and very widespread, the impact of their inbreeding preference was more damaging. Maybe other factors about experiences of negative impacts of incest should be considered, like in Judaism where marrying within the religion was strictly enforced in relatively small communities, the Torah prohibitions against incest only cover Jews not gentiles (and men not women, Sodom was about abuse of strangers through explicitly treating sodomy as a violation of male power, a widespread concern only in highly patriarchal cultures). Looking at these together, it would be interesting to look at whether incest prohibitions relate to social inequality when the law was passed, as in unequal patriarchal societies the impacts of incest would be concentrated and affect populations more.

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    You have given an interesting answer within the field of psychology and I thank you for that. However, there are a couple of pointers I'd like to make. You "don't think anywhere has legalised consensual sibling incest." But, @tkruse interestingly pointed in an answer that it is legal in many countries in Europe and the Psychology Today article said it is legal in France. Plus, there is a good Q&A on sex in private at psychology.stackexchange.com/q/1567/7604 May 22 at 7:05
  • @ChrisRogers: Thanks for pointing that out, I had misread the Wikipedia page. Have edited my post, and my argument.
    – CriglCragl
    May 22 at 12:24
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The rational reasons against incest are weak, and legal action against adult love relationships between kin are always highly controversial.

It's imaginable that incest may become decriminalized at least in the future, actually it is not illegal in many European countries, see https://en.m.wikipedia.org/wiki/Legality_of_incest

The rate of birth defects in other closed groups like the Ashkenazi Jews also does not lead to immorality or laws (though it leads to research on preventing such defects via genetic medicine). Other examples are populations of remote islands.

There seems to be genetically encoded psychological mechanisms that commonly cause humans to regard close family pets with sexual indifference at least, or cause disgust at the thought. However those do not affect so humans in the same way, there are exceptions, similar to homosexuality and other gender/sexuality-related phenomena. It explains why a majority in society does not question the immorality "following their heart", while rarely some people nevertheless feel attraction to family members.

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I want to mention three reasons, the strength of which I leave to the reader's evaluation. But first, some clarification:

(a): I take it that parent -child incest is always wrong, due to power dynamics involved it constitutes rape.

(b): I will take it as scientific fact that inbreeding, practiced over generations, clearly decreases fecundity and fitness. it is likely the catholics have some argument against it then via natural law ethics, since natural law ethics tends to strongly link all normative issues together (the relevant normative issues being health and morality). I leave it to those more versed in natural law to expound.

(c): IN the same vein as above, if one sees the purpose of state as strengthening the welfare of its own people, there might be laws to be had against incestual relations. This has little to do with morality, but everything to do with law.

(d): lastly, there is likely an argument from some axiology: merely place the value of goods that cannot be obtained where incest is moral/legal/etc over those obtained via incest. likely this constitutes some bonds of family vs some sexual relation that one could otherwise obtain elsewhere.

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  • When a person reaches adulthood, sex with one of their parents is not Statutory rape. There is likely genetic predisposition to shun close relatives, same as for same-sex relationships. But nature makes exceptions. Strengthening the welfare of the people by genetic control is what the Nazis did, eugenics. Not popular in mainstream ethics.
    – tkruse
    May 21 at 23:22
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    @tkruse thanks for the engagement. by law perhaps. one of the primary reasons for the prohibition of sex with minors is due to power dynamics involved. this extends to bosses sleeping with coworkers, professors with their students, and almost certainly to parents with their children, regardless of age. of course, if you don't think mismatched power dyanamics can constitute rape I have little to say
    – Papuseme
    May 21 at 23:37
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    perhaps I am being unclear. i take it as given that power impacts consent. if this is the case, mismatches in power dynamics truly can constitute rape- although not always. but in the case of parents, almost certainly.
    – Papuseme
    May 21 at 23:47
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    It's fine to post something that is not consensus when it's quoted. Like saying "According to Ptolemy, sun revolves around earth". But it's not fine to just say "sun revolves around earth". It's worse in forums where people say "homosexuality is always wrong" or "abortion is always wrong" or "democracy is always wrong", but your quote "parent -child incest is always wrong" is in the same league. You're not god, don't judge others like that without at least quoting who in philosophy made such claims.
    – tkruse
    May 23 at 0:13
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    This board strives for the highest quality, as in academia. There are larger "schools of thought" in philosophy, and when they disagree, no answer here should pretend there is consensus. The rational reasons against incest (genetics) are not of philosophic nature. I don't engage much because they are for biology forums. I gave the link showing incest is not against the law in many developed countries as sufficient evidence of the risks being small.
    – tkruse
    May 24 at 6:34
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We in Argentina, Armenia, Azerbaijan, Belgium, Brazil, People Republic of China, Estonia, France, Georgia, India, Israel, Italy ( if no scandal is caused) Ivory Coast, Japan, North Korea, South Korea,Latvia, Lithuania, Luxembourg, Malta, Monaco, Montenegro,Netherlands,New Jersey (US), Pakistan, Portugal, Rhode Island ((US), Russia, Serbia, Slovenia, Spain, Thailand,Turkey, Ukraine, don't prohibit consensual incestuous relationships.

If you think that ciaosexuality is wrong FOR YOU, you're not an incestophobe.

If you think that ciaosexuality is wrong FOR EVERYONE, then you are an incestophobe. I think the main reason consensual adult incest oriented (CIAO) relationships are still illegal in some countries is incestophobia. As with homophobia, incestophobia is caused by disinformation and ignorance, influenced by such things as the pseudo-science of eugenics and irrational bigoted religious beliefs.

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  • Is it really a sexuality or more like a fetish, like there is no "sex" of blood related relatives. Also gene defects in children of incestuous relationships are a thing or do you have evidence to the contrary and I'm not talking about anecdotal evidence but larger studies. And lastly you have the problem of age differences and grooming, as well as the dependency relations which can make it harder for it to be considered consensual even if it were. So it's not as easy as you make it or it would be up to you to prove that it is.
    – haxor789
    Aug 4 at 8:58
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You've touched on some of the logical shortcomings in arguing for banning incest, but from a philosophical position, I think you've underestimated psychologism. In Western philosophy, there is a great amount of resistance of seeing moral constructivism as the best description of the consensus and codification of morality.

I think in a clear-cut affirmation of non-cognitivism from meta-ethical discussions, the prohibition of incest is primarily driven, from a naturalized epistemology, by a psychological aversion to inbreeding. This would be an affirmation of Frans de Waal's phrase "thin veneer of civilization" which he used in his book Primates and Philosophers: How Morality Evolved.

Essentially, the meta-ethical argument goes that given the dangers of inbreeding, by way of the evolutionary psychology (which I see as a partial endorsement of moral rationalism), there is a psychological disposition of revulsion to incest. Thus, there is a motivation to use reason practically to justify prohibition. And practical reason is tolerant of contradiction. In fact, one might suspect that the embrace of dialetheism is a hallmark of practical reason. According to the ideas of non-cognitivists like A.J. Ayer, the ethical argumentation surrounding the issue is largely psychological rationalization. Another prime example of this sort of philosophy draped over psychology is the resistance to the animal rights movement.

The very famous Peter Singer has made a number of compelling arguments for animal rights, most of which I and my friends personally find logically persuasive. But when it boils down to it, I like bacon, and consuming bacon is a cultural phenomenon. I have dogs, pigs are more intelligent than dogs, and yet I'd only eat my dog in the apocalypse, but happily engage in eating meat otherwise despite it's poor economic, health, and environmental side-effects. Boo to incest! Yay to bacon!

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    what a lot of buzz words. Jul 30 at 5:26
  • @LawrenceBragg Jargon. Buzzwords are another linguistic phenomenon entirely.
    – J D
    Jul 30 at 11:18

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