Rather than explaining with religious restrictions, how could you explain why incest relationship is wrong? How can you explain with morality? Yes, I know scientifically, incest generations are more susceptible to the diseases and more. I don't dispute these facts. However, my question is this; Why it is wrong when you think beyond science and religion.

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    1. If in doubt about whether your question is on topic or not, ask on Philosophy Meta first. 2. 'science' and 'religion' both give us information that we need in order to base our morality on something. Without that something, your morality is completely random. 3. That 'incest generations are more susceptible to the diseases and more' is a fact, not a moral claim. It can be used to make a moral claim: 'Incest is bad, because ...'; but others may use the fact to claim the exact opposite. Science is not a moral framework.
    – user2953
    Commented Feb 9, 2016 at 8:30
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    You say science is not a moral framework. However, before this sentence you say; scientific facts can lead us moral results. I didn't understand your answer.
    – sergerde
    Commented Feb 9, 2016 at 8:32
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    Science itself doesn't make moral claims. It doesn't say "It is good that ..." but "It is the case that ...". You can use these truth claims in a moral framework, i.e. making moral claims (e.g. "Because [it is the case that ...], it is good that ..."), but science doesn't do that itself.
    – user2953
    Commented Feb 9, 2016 at 8:34
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    Incest is defined differently in different cultures. It is a cultural taboo, no science behind it. Cleopatra was the result of 29 generations of brother/ sister marriages. Different cultures define differently exactly what is taboo. Commented Feb 9, 2016 at 11:04
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    @nocomprende Not really a philosophy question. There have been a lot of studies done in anthropology on it. Lot in anthropology literature, but can't give specific references as haven't read about it in well over 40 years. Commented Feb 12, 2016 at 5:08

9 Answers 9


First of all, you need to specify which system of ethics you want to base your judgement of incest on. People tend to think that "right" and "wrong" are absolutes (even when removed from religious considerations), when in fact "right" and "wrong" can only be specified with regards to specific systems of ethics, of which there are many.

Second, you need to define incest more precisely: In some cultures, incest includes relations between cousins, and between relatives by marriage, etc, while others consider only relations between direct blood relatives to be incest. Henceforth, I will consider only relations between siblings and parents to be incest.

In your question, you add the additional twist of seeking reasons "beyond science and religion". This would rule out any consequentialist ethics like utilitarianism, since you are looking for something other than the negative consequences of incest, typically based on genetics and medical science. It would also rule out virtue ethics and some deontological ethics based on religious considerations.

This would leave us with non-religious deontological ethical systems as our only system for figuring out whether incest is wrong or not. I can only think of two such approaches:

  • Kant's categorical imperative: An action is morally acceptable if it can be universalized, meaning that it should be allowed in every situation, not just the current one. So can incest be universalized? The answer is most likely "No", and incest is wrong. There are far too many situations where allowing incest would disrupt family structures and relationships. In particular sexual partners can break up whenever the relationship no longer satisfies them, whereas the parent-child and sibling relationships are permanent ones that underly the very way our society is structured. Can you imagine a family holiday dinner where the a brother and sister had just broken up after a months long relationship? A man may divorce his wife to be with another woman, but what would happen to our social structure if men started divorcing their wives to be with their daughters?
  • Natural rights: Alternatively, one could argue from a point of view of natural freedom. As long as you are not harming others, you can do what you please. In this case, one has to admit that there is nothing wrong with incest per se. Presumably, those engaging in incestuous relationships would have to make sure that this never leads to procreation, since then they would be harming the eventual children by giving them a higher chance of genetics defects, but otherwise they are free, as consenting adults, to engage in whatever relationship they want to. Indeed, there are countries where incest between consenting adults is legal.
  • I like the explanation based on relationships. For adults, this is the reason: we resist being connected with others in "too many" deep ways. It is just too problematic.
    – user16869
    Commented Feb 11, 2016 at 13:55
  • Since you mention cousins: In some places this was dependent on who the parents of the cousins are (brothers, sisters, or mixed). The reason: If we believe that two people are cousins, they might actually be half siblings because some of the parents cheated.
    – gnasher729
    Commented Feb 12, 2016 at 23:37
  • This is a pretty iffy application of the categorical imperative. Certainly you haven't shown that willing incest entails willing a contradiction, just an inconvenience.
    – Canyon
    Commented Apr 20, 2017 at 20:39
  • In most western countries incest is legal, AFAIK. Just marriage between close relatives is prohibited.
    – rus9384
    Commented Sep 28, 2018 at 11:13
  • "whereas the parent-child and sibling relationships are permanent ones that underly the very way our society is structured" Maybe your society is.
    – Alice
    Commented Sep 28, 2018 at 14:23

This looks like a philosophical question - but it isn't. It's touched upon in the anthropological literature, for example, by Claude Levi-Strauss, who using elements of Mauss's The Gift, explained the universal incest taboo via a theory of kinship networks.

He focused specific attention to the fieldwork of Margaret Mead amongst the Arapesh, where she reported when asked does a man ever sleep with his sister, the Arapesh replied:

No we don't sleep with our sisters. We give our sisters to other men, and other men give us their sisters.

When she further pressed the point repeatedly, they said:

What, you would like to marry your sister? What is the matter with you anyway? Don't you want a brother-in-law? Don't you realize that if you marry another man's sister and another man marries your sister, you will have at least two brothers-in-law, while if you marry your own sister you will have none? With whom will you hunt, with whom will you garden, who will you visit?

The point here is kinship. It's also worth noticing that in Levi-Strauss' considered opinion the taboo is universal.

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    It sounds like a made-up explanation by the people involved to cover their discomfort with the idea of being "too close" to someone psychically in too many ways. The psyche tends to resist being enmeshed or engulfed, lest it be overpowered.
    – user16869
    Commented Feb 11, 2016 at 13:51
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    @no comprendre: all explanations then are 'made up' - including your 'psyche'; the point is this is backed up by field-research; Levi-Strauss is a highly respected anthropologist; the more important point, which perhaps you missed is that this isn't a philosophical question. Commented Feb 11, 2016 at 16:14
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    "Ain't got no money in my pocket but I'm already here." Hic Rhodus, hic salta. Did you know that 67% of statistics are made up on the spot? Research can prove anything. Personally, I don't much care if it is Philosophy or Psychology. But - You are Right On that all explanations are made up. Congratulations! Now maybe we can study Nonduality.
    – user16869
    Commented Feb 11, 2016 at 23:19
  • But this is only true for monogamy. And sex is not marriage. Siblings can't marry in eitger way in many countries.
    – rus9384
    Commented Sep 29, 2018 at 11:28

Moral revulsion ?

One reason advanced for the immorality of incest is the supposed universal revulsion which the practice inspires. In general I think that the moral emotions are important and need to be taken into acccount. But since incest is a contextual concept, historically and socially specific, there can hardly be a universal revulsion against it since 'it' means different things in different cultures. Father/ daughter, mother/ son, step mother/ step-son, step-father and step-daughter, sister/ brother, grandmother/ grandson/ first cousin/ first cousin : the concept can elastically include all of these and more or exclude some of them. All we can safely say is that incest is widely taken to apply to sexual intercourse between people forming a familial core - it is as vague and imprecise as that. Do you - whoever is reading this - feel equal revulsion to all the relationships listed ? Do you feel anything so strong as revulsion in all cases ? That between first cousins, for instance.

Incest - or incest-plus ?

Sexual relations between consenting adult first cousins is seldom harmful as such in any sense I can think of. (How ?) The problem with incest, or the major moral problem, comes when the conditions of incest are damagingly different.

Incest often causes harm; it can do so, physically and psychologically, especially when it occurs between adults and minors within the family unit. It can represent an abuse of power relationships, again especially when it occurs between adults and minors. It often links with the repression of, and disrespect for, women : part of a syndrome which has the crude and casual use of female children for sexual gratification at the far end.

It is the combination of incest with such conditions as these that makes the main moral case against incest so far as I can see.

Incest and autonomy

If personal autonomy is a value, it develops with a loosening of ties with the familial core. It entails 'becoming one's own person', moving out from the core to the risky, unpredictable, varied and enriching experience - including sexual experience - of non-familial relationships. This isn't a matter of abandoning the family but of enlarging one's experience beyond it. Incest, which closes the door on this enlargement of experience, is not a gateway to autonomy.

  • (+1) for "abuse of power relationship", that would be my answer. I would argue that non-abusive consensual incest between siblings (if that's possible) is ok, if contraceptives are used. Parent/child incest is inherently abusive, and thus never ok. Commented Dec 31, 2023 at 7:27

Assuming that a society is structured in families, the absence of the taboo would mean that those responsible for infants would legally have sexual access to those they should protect. Thence sexual abuse would be rampant - more than it already is.

So the taboo protects a moral good: relative sexual freedom for children.

  • This fails to address the situation where everyone is over the age of consent.
    – CriglCragl
    Commented Mar 11, 2019 at 0:11
  • @CriglCragl - No, "the situation where everyone is over the age of consent" is just collateral damage. Commented Mar 12, 2019 at 12:47

I really like the note mentioned Alexander, that is in an incest relationship there is no concept of divorce. I mean it would eventually lead to a divorce between siblings. But we all know that family is a great support and blessing for us and there's already enough internal struggle in between so let's not add anymore :)

Same can be said for a 4 year old girl. You can touch her, so why not have non-penetrative sex with her in her sleep? There is a natural shame in doing it (There may also be a desire, but still the shame is there). Same shame exists for your siblings. The only difference with siblings is it's consensual and with an adult.


  • It would make inheritance complicated. A sister may also be a mother, etc.
  • Families would be in contact with a smaller population and learn less from others. It's like them living in their own village. (Normally people/groups who only marry their own kind/group have a lower level of tolerance for othering. The more diverse you more you're adaptable you become and more you can understand others. (I don't mean accept everyone, but some differences make total sense, some make zero sense) If you move to a new city you learn something new, if you learn a new language you learn something new, if you marry a different person you learn something new. But with with incest you're planting a seed of social restriction onto yourself and your children.)
  • There are genetic concerns. Inbreeding which is very similar creates many genetic issues. See here and here Now imagine if this happens 2-3 generations in. The effect would be magnified. From the article it says:

However it is estimated that 55 per cent of British Pakistanis are married to first cousins, and a third of children with recessive genetic disorders are born to that community.

So to conclude it's to be avoided for complications in inheritance, creating social restrictions, genetic issues.


I mainly just want to defend this as being a philosophical question. In particular, it is an exemplary case to distinguish between consequentualist and virtue ethics, and to examine the roles of intuitions and biology in ethical decision making. Hume used incest to reject appeals to naturalism for moral universalism:

"But to choose an instance, still more resembling; I would fain ask any one, why incest in the human species is criminal, and why the very same action, and the same relations in animals have not the smallest moral turpitude and deformity? If it be answered, that this action is innocent in animals, because they have not reason sufficient to discover its turpitude; but that man, being endowed with that faculty which ought to restrain him to his duty, the same action instantly becomes criminal to him; should this be said, I would reply, that this is evidently arguing in a circle. For before reason can perceive this turpitude, the turpitude must exist; and consequently is independent of the decisions of our reason, and is their object more properly than their effect. According to this system, then, every animal, that has sense, and appetite, and will; that is, every animal must be susceptible of all the same virtues and vices, for which we ascribe praise and blame to human creatures." A Treatise On Human Understanding, Book 3 part 1 section1.

What are people saying, they know what is philosophy better than Hume?

There is a really good discussion between Richard Dawkins, who limits his consequentialism in this case to biology, and Sam Harris who widens the analysis of consequences, here.

This topic is important for philosophy exactly because it crosses these various demains, and so highlights the conflicts we can face with simplistic moral theories.


I agree with you that the only reason against incest are genetics.

I assume that societies who have detected the resulting genetic diseases have incorporated the incest tabu into their moral values, sometimes even amplified by religious prescriptions.

I consider the moral of a society a set of rules and values determined and established by the society in question. According to this view the incest tabu is a moral rule.

In my opinion moral laws are set by society, not detected like laws of nature.

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    It's a rather non-philosophical tangent, but I'd like to point out that there no need for societies to detect genetic diseases or to associate them with incestuous relationships. Being disgusted by incest is simply adaptive because you're more likely to have healthy offspring. The moral and legal proscriptions against incest are derived directly from the disgust response, and not from any reasoned consideration of the consequences.
    – Era
    Commented Feb 9, 2016 at 16:33
  • @Era yes, and the disgust response shows in animals who are too closely related also, it is how they try to avoid too-close mate choice. Disgust in this case (animals and people) is cued by scent: if you are too closely related to someone, they smell bad to you (for mysterious reasons). Of course, anyone with a disease or genetic problem could smell bad also, so there is a one-size-fits-all detector for that. I think that any proscriptions would more likely be based on long experience of how these relationship choices would destabilize other relationships - psychology.
    – user16869
    Commented Feb 11, 2016 at 23:29
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    @nocomprende: I read (but cannot remember where) that will kids grow up together, they can form strong feelings similar to friendship, but become sexually unattractive to each other. But that siblings who were separated and meet as adults often find each other very attractive due to their similarity. Whether that is true I don't know.
    – gnasher729
    Commented Feb 12, 2016 at 23:20
  • @Era Why then there are so many people who see nothing objectively wrong with consensual protected incestous sex? Objectively means they still might not want to be involved.
    – rus9384
    Commented Sep 28, 2018 at 17:44
  • @rus9384 One reason might be that we now understand the (reproductive) health risks associated with incest, so having the understanding that protection prevents this risk reduces the innate inhibition for some people (note that there may or may not be psychological health risks, I've heard this asserted but I don't know any evidence for it myself). Also, I don't think that any disgust response or moral belief is absolutely universal. Moral beliefs about sexuality, in particular, have been evolving to become more permissive over the recent centuries in Western culture.
    – Era
    Commented Aug 6, 2019 at 23:20

Under a naturalistic worldview there is off course nothing wrong with incest. If two consenting adults would have sex then why should being blood relatives stop them? After all what adults do behind closed doors has nothing to do with us. Atheism defends the right of sexual freedom as long as it is not hurting anyone and no one can deny this is a beautiful coming together of two souls.

Really it would be hypocrisy from those same people who advocate gay marriage to somehow now grow a conscience when it comes to brothers and sisters. What if two brothers want to get married? I thought a marriage certificate is everyones right?

I wish that one day we can escape religious oppression and then brotherly love can take its rightful place in society.

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    Atheism is about the (in)existence of god(s). It does not defend sexual freedom - though of course many atheists will do so. Commented Apr 21, 2017 at 9:57
  • 'Not hurting anyone'. Someone under the age of consent may not be hurt by having sex, they may even initiate it. It is classed as statutory rape because these cases are so hard to prosecute, and it lowers the burden of proof to just showing it happened. And so to protect young people from exploitation. Law will always be about more than 'enabling freedom'
    – CriglCragl
    Commented Mar 11, 2019 at 0:18

The only other reason, if you exclude science and religion, is that the love is the same between close family members, so no real love can be made.

To made new love requires a distance between two opposite poles (like positive and negative on a battery). Even in the Garden of Genesis, Adam and Eve couldn't have made (i.e. created) much love. There just wasn't that much experience to separate the two individuals in order to come back with something new.

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