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.



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?


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/

  • 3
    Ask on an anthropology forum, not a philosophy forum May 22, 2022 at 5:04
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    @SwamiVishwananda please see philosophy.meta.stackexchange.com/q/5311/30545 May 22, 2022 at 6:48
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    Both the Internet Encyclopedia of Philosophy and the Stanford Encyclopedia of Philosophy have articles that directly mention incest here at Philosophy of Sexuality (IEP) and here at Sex and Sexuality, so don't sweat it your reasoning.
    – J D
    May 22, 2022 at 9:58
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    @SwamiVishwananda With all due respect, this clearly is a question that is discussed and answered in academic philosophy.
    – Philip Klöcking
    May 22, 2022 at 10:37
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    @JD Your IEP and Stanford links are interesting. Thanks for them May 22, 2022 at 11:47

6 Answers 6


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, 2022 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, 2022 at 12:24

This is my two cents.

Prohibition of incest is among prohibitions on sex activity itself (eg promiscuity, prostitution, homosexuality, etc). Nevertheless incest in various forms has indeed been culturally recognized in many areas, as well as other sexual activities have also been culturally recognized across human history (eg homosexuality, prostitution, etc).

Regulation of human sexual behaviour, sex revolution and emergence of AIDS: a historical perspective

No Society in the World permits fully free sexual behaviour. All societies utilize a variety of ways for regulation of sexual behaviour. This has been happening since antiquity.

Why is sex regulated in the first place is the main question.

Whatever way of answering above question (eg as a form of social control, as a religious/ideological thesis that pleasure or drives should be restrained, etc), there are some consequences. Since sex is regulated but sex is also needed for reproduction, that means that sex must be sanctioned when it leads to reproduction as desired.

Thus institutions like marriage are created in order to allow sex activity that is needed for reproduction purposes.

The above analysis exposes a historical social reality and does not necessarily imply that sexual activity should be regulated at such a degree (eg by the state).

Some references on why and how sexual regulation historically surfaced:

  1. The Origin of the Family, Private Property and the State by Engels
  2. "Civilized" Sexual Morality and Modern Nervous Illness by Freud and Totem and Taboo by Freud
  3. The History of Sexuality by Foucault
  4. Forbidden History: The State, Society, and the Regulation of Sexuality in Modern Europe
  5. Sex, Politics and Society: The regulation of sexuality since 1800
  • "Institutions like marriage are created in order to allow sex activity that is needed for reproduction purposes." - This is a religious/old fashioned notion. What about unmarried couples? Children out of wedlock have not been that stigmatised for a while now. Oct 24, 2022 at 7:06
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    @ChrisRogers of course this means that things progress and change, but does not change the basic observation of the answer that sex is regulated in the first place
    – Nikos M.
    Oct 24, 2022 at 7:07
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    @ChrisRogers my personal thesis is that sex is for those involved to decide and the state should have the least amount of say in this, but this does not change a social reality of what is happening
    – Nikos M.
    Oct 24, 2022 at 7:09
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    +1 from me. Especially with the 3 links to further reading. Oct 24, 2022 at 11:42
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    +1 "Totem and taboo" was the first thing that sprang to my mind when reading the OP, but your post goes much further.
    – Roger V.
    Oct 25, 2022 at 8:23

Your question presupposes that human cultural tendencies are justified by logic, when there is much evidence to suggest they are the result of innate conservatism. Alcohol and junk food kill more people each year than marijuana, and yet they tend to be considered socially acceptable while smoking dope isn't- that's a cultural phenomenon, not the result of logical assessments made by every member of society. The prevailing attitude to incest is just one of a long list of prejudices that have been common among humans for millennia. I find it ironic that your question is tagged 'logic', when logic is rarely the overriding factor determining societal trends.


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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    @LawrenceBragg Jargon. Buzzwords are another linguistic phenomenon entirely.
    – J D
    Jul 30, 2022 at 11:18

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 members with sexual indifference at least, or cause disgust at the thought. And there is no other likely theory of why family members typically do not sexually attract each other.

However even if such genetic programming of sexuality existed, those do not affect all humans in the same way, there are exceptions. As homosexuality shows, humans are not fully genetically programmed in a single way. However the typical genetic setup 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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    You said "There seems to be genetically encoded psychological mechanisms". Could you provide some further reading on genetically encoded psychological mechanisms? May 22, 2022 at 7:20
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    It's a hypothesis, see en.m.wikipedia.org/wiki/Westermarck_effect
    – tkruse
    May 22, 2022 at 13:30
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    -1 for the fact that your answer places genetics at the heart of all sexuality with no evidence. There is the hypothesis you put in the comments, but no real scientific evidence to prove your claims. If I could, I'd give another -1 for the homophobic references to genetic deficits causing "imperfect" sexualities. Oct 24, 2022 at 6:58
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    I tried to rephrase, i want to express the opposite. The science of sexuality is not always conclusive, but a strong genetic component to sexuality seems undeniable. That's why animal don't need sex education classes to reproduce.
    – tkruse
    Oct 24, 2022 at 7:14
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    Animals don't need sex education to reproduce, but there is homosexuality, bisexuality etc. in the animal world. Oct 24, 2022 at 7:16

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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    Jul 31, 2022 at 18:05
  • 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, 2022 at 8:58
  • This is not exactly true: e.g., in France acts of sexual violence committed against one's relatives (e.g., father coercing his daughter or brother his sister) are judged more harshly. Also, official unions between close relatives are prohibited and, by extension, children of unofficial incestuous unions may face legal difficulties - e.g., in having the names of both of their parents listed on their birth certificate. Similar restrictions exist in many of the countries that you listed.
    – Roger V.
    Oct 25, 2022 at 8:36

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