Many/most liberal countries have legalized homosexuality. Presumably one of the main reasons for doing so is that consensual sex between two adults ought not to be a crime, because there is no victim.

Given that, why do many of these countries criminalize incest? From the atlas, the countries for which incest is illegal include Australia, New Zealand, the UK, and the US; these countries all decriminalized homosexuality decades ago.

Only thing I can think of is that homosexuality cannot result in children, but incest can, with associated consequences of inbreeding. Still, this doesn't explain why these countries have not legalized protected incest. If contraceptive failure is a concern, that still doesn't explain why there can't be an exception if the woman is past menopause (and thereby biologically impossible to get pregnant). There are also only a few countries for which same-sex incestuous relationships are legal while opposite-sex ones are illegal.

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    Are you aware that incest is a taboo? Laws that enforce taboos usually don't get changed unless the behavior they address isn't taboo anymore. That's what happened with homosexuality in some societies. First it became accepted (by a majority), then the laws were changed.
    – Roland
    Commented Jun 12, 2023 at 7:14
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    As a practical matter, writing a law that has exceptions for menopausal women (and men with condoms) is not really feasible (and might even be unconstitutional e.g. in Germany, since it discriminates by age), and it would be barely enforceable. Commented Jun 12, 2023 at 8:46
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    I didn't know there were so many countries where incest isn't criminalised.
    – F1Krazy
    Commented Jun 12, 2023 at 9:04
  • @EikePierstorff The problem isn't really that you can't discriminate by age, like voting rights only apply past a certain age, drinking and smoking has age limits and so on. The problem is rather that you need a good explanation for why it's a necessity and not a discrimination. And arguing you don't want children with potential gene defects opens a whole other can of worms with regards to the rights of people with disabilities and the value of life and human dignity.
    – haxor789
    Commented Jun 12, 2023 at 10:03
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    @haxor789 we do not disagree :-) I find the question interesting and dug up a decision of the German supreme court, but since I am at work it will take a few hours until I can write an answer. Commented Jun 12, 2023 at 10:07

4 Answers 4


Ancient taboo
Most "liberal" modern societies were not created liberal, but rather emerged in the last couple of centuries from the societies that were very traditional and religious for millennia. Thus, liberalization of various aspects of life, especially those related to sexuality, is very gradual and depends on slowly evolving public opinion, rather than on whether this or that aspect of life corresponds to the formal liberal canon. In this sense, the emancipation of women, sex out of marriage, and legalizing homosexuality are all rather recent developments.

Interdiction against incest is arguably very ancient, perhaps even predating human culture and consciousness - because they are grounded in biological disadvantages. Sigmund Freud in Totem and taboo discusses complex caste and tribal systems preventing interbreeding with kinship dating several generations back, which exist in what one would consider otherwise primitive cultures - notably in terms of their lax attitudes towards homosexuality and marriage (especially monogamy.) One could even argue that many social animals avoid incest, since it confers a biological disadvantage on the progeny.

This interdiction only partially makes sense in the modern society, since incestuous couple may not necessarily wish to have children. (Moreover, they may be homosexual or simply in-laws - although such situations are usually not criminalized, they might still be negatively viewed by the society.) One could also cite more modern methods of avoiding incest, developed in societies where close kinship is a real problem, e.g., App to prevent 'accidental incest' proves a hit with Icelanders or genetic testing to prevent well-known hereditary anomalies, like Tay-Sachs disease.

Conflict with other liberal values
A big issue with incestuous relationships is that they happen among the family members, where the relationships are hidden from the rest of the society, and the power dynamics might be very different from that among strangers, or even supported by law - like the power of parents over their children. This opens multiple possibilities for abuse. While prohibiting incest doesn't address the core of the problem, it provides an easy way of punishing the abuse that is otherwise hard to prove.

The highly publicized incest stories often concern siblings who have grown separately and may not even be aware of their kinship, but decriminalizing incest potentially implies dismantling the corresponding unspoken interdictions for sexual relationships between parents and children or siblings growing together.

Is marriage liberal?
One could also ask a broader question: whether a liberal society needs ancient institutions like marriage, which are ultimately grounded in a religious practice. Notably, many socialist and communist utopias suggested abolishing marriage, in favor of freely intermixing of sexual partners, and raising children together. In this sense the fight for "gay marriage" might have been going in the wrong direction - by strengthening the very institution which is the root of the problem.

Civil union in Israel
An interesting example is Israel, where the marriage was outsources to the religious authorities from the moment when the state was established, which meant that only the recognized members of the same religious group could be married (by a representative of the same religious groups, such as a priest, rabbi, imam, etc.) This was seemingly discriminative of the couples with no religious affiliation or inter-religious couples. The state however had to deal with the reality of such couples, the couples married abroad, and the couples formally unmarried, but having children. This resulted in gradual emergence of legislation and court rulings that give rights to such couples in terms of taxation, visitation, inheritance, visas, etc. Today non-religious couples may get married abroad (with nearby Cyprus being the most common destination) or simply secure a proof of living in a common household (e.g., by having common property, paying bills together or arranging for an official declaration of such an intention at notary public.) In this context the legal recognition of the same sex couples was a seamless process, despite the religious marriage being impossible for them.

Civil union in France
Another example is the Civil solidarity pact (PACS) in France. Although the traditional marriage in France has been non-religious for a long time, until 2013 it had been restricted to heterosexual couples. As an early measure civil union was promulgated in 1999, giving same-sex couples a measure of legal recognition by the state. Surprisingly, the measure turned out to be hugely popular between heterosexual couples, unwilling to burden themselves with traditional expensive and time consuming marriage/divorce arrangements, but willing to guarantee a measure of legal protection to themselves and their children. As the results the number of couples entering PACS is nowadays nearly as big as the number of couples registering in an official marriage. (It is worth noting that PACS is open to any pair of individuals functioning as a common household - even if they are not engaged in a sexual relationship.)

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    Marriage isn't just a religious practice. Given the innate vulnerability of women due to being pregnant (fully occupied yet without income) stable relationships served a lot of purposes with regards to mitigate those risks. Previous to being able and allowed to work, it served as income and retirement plan for women. Though while obviously being ancient and sexist in large parts, it's still an attempt to solve problems, which would be required to be solved in other ways.
    – haxor789
    Commented Jun 12, 2023 at 10:38
  • Your first link (about the Icelandic "accidental incest" app) is incorrect and just links back to this question.
    – F1Krazy
    Commented Jun 12, 2023 at 12:34
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    @F1Krazy Thanks, fixed. I took a link at random though - it has been covered in many media at some point. What is more important is not the existence of the App, but that Iceland (and some other countries, if I am not mistaken) took an effort to compile the genetic information about their full population (though these are not full genomes - mostly SNPs relevant to possible genetic anomalies and/or person identification.)
    – Morisco
    Commented Jun 12, 2023 at 12:42
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    @haxor789 Well modern marriage is. The vulnerability thing was mostly solved by creating governments and many early humans were believed to have practiced polygamy (theguardian.com/science/blog/2015/may/19/…) for thousands of years, which would be a long time to survive without 'stable relationships' if protecting pregnant women was such a pressing issue and marriage was the solution
    – Tyler Mc
    Commented Jun 13, 2023 at 16:22
  • @TylerMc Modern marriage, if you want to be unromantic, is a legal union for tax reduction purposes, it's an entirely secular thing regardless of whether religions flesh out that rite by adding their own version. Also yes living in larger communities/herds/large families is also an option to solve that problem. Though ironically with modern societies the individualization rather reintroduced the problem.
    – haxor789
    Commented Jun 14, 2023 at 12:03

...associated consequences of inbreeding. Still, this doesn't explain why these countries have not legalized protected incest

Laws get created to address issues, with the most concerning issues being at the top of the priority list.

How big is the issue of brothers and sisters, fathers and daughters, mothers and sons eager to have protected sex but afraid of being caught and jailed? I don't recall any parades or demonstrations by the said category of people claiming their rights.

If you don't push something, more often than not it will not move by itself.

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    While there is truth to this, the largest group calling for decriminalization (at least here in Germany) are not people who want incestous sex, but people who think this should be dealt with in therapy, not in the court. That is not an argument that you can make in the streets (especially since it will draw ire from both sides of the fence). Commented Jun 12, 2023 at 6:38
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    @EikePierstorff Ire? Like LGBT parades were welcome from the beginning.
    – Greendrake
    Commented Jun 13, 2023 at 6:31

There's an implicit assumption to this question that needs some challenging.

This is the notion that liberal Western countries go for more ever more permissive sexual relations laws, that the doctrine of consenting adults trumps all other ethical considerations and that society as a whole demands this evolution.

A counterexample to that would be rules and regulations around teacher-student relations, even when both parties are adults (say in university). Over time, "power relationship" has been recognized as an ingredient making these relations likely to be toxic and legislation has evolved to regulate them more severely than in the past. Ditto boss-employee relations in the workplace.

Regardless of procreation risks, there is something fundamentally wrong, at a deep ethical level, with a parent engaging in sexual intercourse with their child. This goes so much deeper than the above student-teacher power imbalances and is wrong on too many levels for society to be supportive of liberalization due to non-discrimination considerations.

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    Looking through Wikipedia's list of laws, I didn't see any countries that prohibit incest between parents and children that don't also prohibit it between siblings. (A few prohibit it only when one participant is underage, but they're classified by the article as countries where incest is legal.) Doesn't that make your theory of the reason for these laws rather implausible?
    – benrg
    Commented Jun 12, 2023 at 19:06
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    I'm under the impression teacher-student relations are frowned upon because of conflict of interest - the teacher cannot give the student a fair grade. Once they cease to be teacher-student, they can do whatever they want. academia.stackexchange.com/questions/133607/…
    – Allure
    Commented Jun 13, 2023 at 0:06
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    @Allure That same line of reasoning does not apply to coach/athlete relationships however. Again, rather than arguing about the exact details of what motivates what, the notion that Western societies becomes ever more permissive in all sexual matters is an implicit part of your question and, while generally true overall it is not always true. With the case of incest being a good case in point. Also, wonder if you could replace the same-sex-marriage tag with ethics? Not your intent, I am sure, but associating incest with LGTBQ concerns isn't all that great a choice. Commented Jun 13, 2023 at 1:09
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    Are coach/athlete relationships prohibited though? A quick Google search indicates no; there are relationships where the partner of the athlete is also the coach, and the athlete is highly successful, e.g. en.wikipedia.org/wiki/Gunn-Rita_Dahle_Flesj%C3%A5. Also, I don't see why this is an ethical issue, or why LGBTQ isn't related.
    – Allure
    Commented Jun 13, 2023 at 1:44
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    #1, there have been a number of very $ settlements where apparently this was considered a problem. That said, I've answered, not all that interested in debating it all day, make of it what you want. #2 I think relating same sex marriage to incest is in very bad taste. Consider that a number of times LGBTQ preferences have been compared to sexually deviant acts such as pedophilia. cf. Rick Santorum. I see your Q as generally "given the drive to sexual freedoms, one of which examples is homosexuality, why is incest still illegal?". Same sex marriage is an example, not an equivalence. Commented Jun 13, 2023 at 1:53

Firstly, liberal societies do indeed prohibit incest. For example, in April of 2012, the European Court of Human Rights affirmed a three year incarceration sentence of a German court in a "consensual" brother-sister incest case. In this case, as is typical in brother-sister incest cases, the relationship happened, despite the innate natural aversion of children raised together to being romantically inclined towards each other, these siblings were raised apart during their childhoods.

But, to the point, several newly released think pieces attribute the economic rise of Europe and the West to the Christianity enforce ban on marrying people closely related to you, at a time when states were otherwise weak. See, e.g., Joe Henrich, "The WEIRDest People in the World: How the West Became Psychologically Peculiar and Particularly Prosperous".

High rates of cousin marriage and double cousin marriage are associated with corruption and nepotism that undermine the functioning of large bureaucratic modern state institutions. See, e.g., Mahsa Akbai, et al., "Kinship, Fractionalization and Corruption" (October 3, 2016) (with a 20th century Southern Italian data set).

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Cousin marriage rates. Image from Wikipedia.

Many political scientists see endogamy and cousin marriage as a serious barrier to the development of modern political institutions in the Middle East, North Africa, West Asia, and South Asia, where these practices are particularly common.

In a related point, there has been increasing recognition that the "state of nature" (e.g. in Afghanistan and Somalia) is not atomistic anarchistic freedom, but instead systems of clans and tribes with family ties to each other led by chiefs who force members of the clan and tribe to subordinate their personal freedoms those of the group. See, e.g., Mark S. Weiner, "The Rule of the Clan".

Thus, the cause and effect relationship between liberal societies and incest prohibitions may be backward. It may actually be incest prohibitions that contribute to societies being liberal.

Of course, incestuous relationships are frequently also arranged marriages, in practice, so the notion of free choice of a spouse, on balance, mitigates against it. Other incestuous relationships (e.g. uncle-niece, father-daughter) frequently involve abuses of positions of trust that amount to rape.

Truly consensual incestuous relationships between very closely related people are rare exceptions. And, easing proof of misconduct in the predominant cases involving these delicate situations often outweighs the harm of quashing a very small minority of legitimately consensual relationships.

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