The Incest Argument & Same-Sex Marriage

Marriage March 2013

(Photo credit: American Life League)

One of the stock fallacious arguments against same sex-marriage is the slippery slope argument in which it is contended that allowing same sex-marriage will lead to allowing incestuous marriage. The mistake being made is, of course, that the link between the two is not actually made. Since the slippery slope fallacy is a fallacy, this is obviously a bad argument.

A non-fallacious argument that is also presented against same sex-marriage involves the contention that allowing same-sex marriage on the basis of a certain principle would require that, on the pain of inconsistency, we also accept incestuous marriage. This principle is typically some variant of the principle that a person should be able to marry any other person. Given that incestuous marriage is bad, this would seem to entail that we should not allow same-sex marriage.

My first standard reply to this argument is that if different-sex marriage does not require us to accept incestuous marriage, then neither does accepting same-sex marriage. But, if accepting same-sex marriage entails that we have to accept incestuous marriage, the same would also apply to different-sex marriage. That this is so is shown by the following argument. If same-sex marriage is based on the principle that a person should be allowed to marry the person they wish to marry, then it would seem that different-sex marriage is based on the principle that a person should be allowed to marry the person of the opposite sex they wish to marry. By analogy, if allowing a person to marry any person they want to marry allows incestuous marriage, then allowing a person to marry a member of the opposite sex would also allow incestuous marriage-albeit only to a member of the opposite sex. But, if the slide to incest can be stopped in the case of different-sex marriage, then the same stopping mechanism can be used in the case of same-sex marriage.

In the case of different-sex marriage, there is generally an injunction against people marrying close relatives. This same injunction would certainly seem to be applicable in the case of same-sex marriage. After all, there is nothing about accepting same-sex marriage that inherently requires accepting incestuous marriage.

One possible objection to my reply is that incestuous different-sex marriage is forbidden on the grounds that such relationships could produce children. More specifically, incestuous reproduction tends to be more likely to produce genetic defects which would provide a basis for a utilitarian moral argument against allowing incestuous marriage.  Obviously, same-sex marriages have no possibility of producing children naturally. This would be a relevant difference between same-sex marriage and different-sex marriage. Thus, it could be claimed that while different-sex marriage can be defended from incestuous marriage on these grounds, the same can not be said for same-sex marriage. Once it is allowed, then it would be unprincipled to deny same-sex-incestuous marriage.

There are four obvious replies here.

First, if the only moral problem with incestuous marriage is the higher  possibility of producing children with genetic defects, then incestuous same-sex marriage would not be morally problematic. Ironically, the relevant difference between the two that prevents denying same-sex-incestuous marriage would also make it morally acceptable.

Second, if a different-sex incestuous couple could not reproduce (due to natural or artificial sterility), then this principle would allow them to get married. After all, they are no more capable of producing children than a same-sex couple.

Third, if it could be shown that a different-sex incestuous couple would have the same chance of having healthy children as a non-incestuous couple, then this would allow them to get married. After all, they are no more likely to produce children with genetic defects than a non-incestuous couple.

Fourth, given that the principle is based on genetic defects being more likely than normal, it would follow that unrelated couples who are lkely to produce offspring with genetic defects should not be allowed to be married. After all, the principle is that couples who are likely to produce genetically defective offspring cannot be married. Thanks to advances in genetics, it is (or soon will be) possible (and affordable) to check the “genetic odds” for couples. As such, if incestuous marriage is wrong because of the higher possibility (whatever the level of unnacceptle risk might be) of genetic defects, then the union of unrelated people who have a higher possibiity of genetically defective children would also be wrong. This would seem to entail that if incestuous marriage should be illegal on these grounds, then so too should the union of unrelated people who have a similar chance of producing defective children.

In light of the above, the incest gambit against same-sex marriage would seem to fail. However, it also seems to follow that incestuous marriage would be acceptable in some cases.

Obviously enough, I have an emotional opposition to incest and believe that it should not be allowed. Of course, how I feel about it is no indication of its correctness or incorrectness. I do, of course, have argments against incest.

Many cases of incest involve a lack of consent, coercion or actual rape. Such cases often involve an older relative having sexual relations with a child. This sort of incest is clearly wrong and arguments for this are easy enough to provide-after all, one can make use of the usual arguments against coercion, child molestation and rape.

Where matters get rather more difficult is incest involving two consenting adults-be they of the same or different sexes. After all, the moral arguments that are based on a lack of consent no longer apply. Appealing to tradition will not work here-after all, that is a fallacy. The claim that it makes me uncomfortable or even sick would also not have any logical weight. As J.S. Mill argued, I have no right to prevent people from engaging in consenual activity just because I think it is offensive. What would be needed would be evidence of harm being done to others without their consent.

I have considered the idea that allowing incestuous marriage would be damaging to family relations. That is, the proper moral relations between relatives is such that incest would be harmful to the family as a whole. This is, obviously enough, analogous to the arguments made by those who oppose same-sex marriage. They argue that allowing same-sex marriage would be damaging to family relations because the proper moral relation between a married couple is such that same-sex marriage would damage to the family as a whole. As it stands, the evidence is that same-sex couples do not create such harm. Naturally, there is not much evidence involving incestuous marriages or relationships. However, if it could be shown that incestuous relationships between consenting adults were harmful, then they could thus be justly forbidden on utilitarian grounds. Naturally, the same would hold true of same-sex relationships.

Reflecting on incestuous marriage has, interestingly enough, given me some sympathy for people who have reflected on same-sex marriage and believe that there is something wrong about it. After all, I am against incestuous marriage and thinking of it makes me feel ill. However, I am at a loss for a truly compelling moral argument against it that would not also apply to non-related couples. My best argument, as I see it, is the harm argument. This is, as noted above, analogous to the harm argument used by opponents of same-sex marriage. The main difference is, of course, that the harm arguments presented by opponents of same sex-marriage have been shown to have premises that are not true. For example, claims about the alleged harms to children from having same-sex parents have been shown to be untrue. As such, I am not against same-sex marriage, but I am opposed to incestuous marriage-be it same or different sexes.

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  1. In my opinion your conclusion are pointing in the right direction but you’re doing the politicians dance.

    You being offended by the idea of incestious marriages has no bearing at all whether it’s a fundamental human right, ex. whites used to be offended by the idea of black people riding the same bus. It simply means you were brought up with indoctrinated ideas and “values”, that my or may not be wrong.

    In my opinion, the harm argument is the only valid argument as such only coupling that will result in high potential for genetic defects should be prevented.

    Society and government has it the wrong way around. ALL freedoms should be provided and only those taken away that are either proven to cause harm or are found to cause harm.

  2. Anthony Juan Bautista


    Very nicely thought-out! Your analysis presents a rationale for how “society” might analyze these distinctions. However, (as is oftentimes sadly the case) society will NOT address this matter—legal precedent will address this matter.

    You should read Walker’s ruling on Prop 8 (start around page 124):

    Now, wherever is written the word “gender”, as in “gender restrictions” or “restricting an individual’s choice of spouse based on gender”, I want you to substitute the word “genotype”. So the phrasing becomes “restricting an individual’s choice of spouse based on genotype.” (And remember with rare exception, gender is simply a phenotype derived from genotype—meaning gender is subordinate, shall we say.)

    You’ll be very surprised that the legal rationale is entirely sustained and remains extremely coherent, even with the word substitution!

    Also, your notion that the slippery slope is a fallacy is rejected outright. I previously mentioned a child’s modern-day “right” to an abortion, “slipping” from a 1973 SCOTUS opinion relating to mature adult women.

  3. Anthony Juan Bautista

    To finish the thought:

    Over 100-plus yrs we have outlawed previously codified restrictions and various unequal treatment based solely on phenotype—think gender and race. Hence, if we cannot discriminate based on phenotype, we most certainly (logically) cannot discriminate based on genotype. The former is subsumed by the latter.

  4. “I have considered the idea that allowing incestuous marriage would be damaging to family relations. That is, the proper moral relations between relatives is such that incest would be harmful to the family as a whole.”

    Interestingly, this is the main argument of the German Constitutional Court (BVerfG) in its decision (28 February 2008) to uphold § 173 of the German Penal Code (StGB) which prohibits sexual intercourse between close relatives.

    As any kind of eugenic argument is implossible to make under German law (and, I would argue, human rights provisions), the court had to appeal to the ‘integrity of the family’ to rule a prohibition of incest constitutional.
    The one dissenting opinion (by judge Winfried Hassemer) disagreed with this assessment on several grounds: He argued that the prohibition violated the proportionality principle (as the penalty is rather severe: a maximum of three years imprisonment) and failed to address any victim – he called incest between consenting adults a “victimless crime”. Furthermore, he questioned the intent of the law, which (historically) DID involve a eugenic angle. However, the law does not prohibit other sexual relations which might be even more likely to produce children with genetic disorders. And to make things even more absurd, § 173 StGB prohibits only intercourse (“Beischlaf” in German), which by legal definition is limited to vaginal penetration (again, betraying the law’s historical intent of eugenics). For example, consenting siblings can legally engage in all kinds of sexual acts – except intercourse!
    Following the court’s ruling – which was controversial not least because of Hassemer’s arguments – the German Society for Human Genetics criticised the decision on the same grounds as Hassemer (that is, nobody would accept a law prohibiting parents with genetic disorders or a predisposition for genetic disorders from reproducing).

  5. Ah, the old incestuous marriage canard.

    Sanctimonious bible bashers are not on such firm ground when it comes to this argument. In fact, it’s where you can pick up that good ole bible and give them a good ole bash back with it – with a simple question; “What about Lot?”

    What about Lot?

    Well, when God was through nuking two cities, and turning Lot’s wife into a pillar of salt for the capital crime of taking a peek at him mashing up her house, in one of his crazy heavy drinking meth smoking rages he used to get into in his old testament days (the world was young, and so was God – he was out of control, but who isn’t at that age). Lot retreats to a cave with his two daughters. His daughters get him drunk every night, so he will have sex with them, and get them pregnant. And that is the literal word of the Lord.

    Any interpretation is problematic. Does God really think, that if you are pious individual it’s all wholesome to get drunk and impregnate your daughters.

  6. Despite its characterization of the incest argument as a slippery slope, I find it ironic that this article tried its best to prove in favor of incest marriages on account of same-sex and different-sex marriages. A same-sex marriage proponent arguing on account of all the reasons listed in the article should consider, for the sake of consistency, also supporting incest marriages, otherwise it might be hypocritical to argue against opponents of same-sex because they find it disgusting when you’re against incest because you find it disgusting or it makes you ill.

  7. Jonathan,

    I actually address the points you make in the post.

  8. I am a very pragmatic person. I think that if marriage fulfills a purpose, and that if said purpose applies to same-sex (or incestuos) couples as well, then they should be allowed to get married.

    I agree with Anthony that “we most certainly (logically) cannot discriminate based on genotype”. As you pointed out, there is not much evidence involving incestuous marriages or relationships.

    Maybe the greatest harm about incestuos realtionships is society’s reject on moral grounds. The reproduction issue is a weak one, as your article pointed out. The only way to assess the harm will be stopping society from condemning incestuos relations, so we can study the phenomenom from a free of intolerance point of view.

    (please disregard my poor english)

  9. The thing about this debate as pertains specifically to gay marriage is that we don’t even need to identify or defend any sort of argument against incest to show the fallacy. The argument is saying that if incest is to be banned on the sole grounds of being gross, then so must anything likewise tabooed. It effectively holds gay civil rights hostage to a completely unrelated area of social mores.

    Perhaps the core problem of almost any slippery slope of thisform is: it simultaneously assumes a consequence is bad, and that the people will make no chioce other than to enact the consequence once a given point is reached. In other words, the fallacy depends on the existence of “inexplicably bad” things. It further assumes that the badness must overwhelm everything else in our considerations. Yet the potential set of “inexplicably bad” things is infinite. At one time, there was a strong taboo against interracial marriage; the “incest argument” necessarily leads to an opposition to interracial marriage, on the basis that “If anyone has the right to marry whomever they love, then…”

    You could even invent new taboos just to add to the “hostages”. No one should marry anyone with a different hair color unless their names start with different letters. It’s just disgusting. If you disagree, then we’ll have to bring in your unexamined opposition to incest, and I assume you value that opposition more than you value some made-up right to marry just anyone

  10. Lenoxus,
    If incest isn’t “inexplicably bad”, then why do gay marriage proponents have a hard time just accepting the incest argument?

  11. @Jon

    I’m a gay marriage proponent and I accept the incest argument. Let ‘em marry.

    I am, however, very creeped by the idea of parent-child marriage. I at least recognize, though, that this is because of their typical social rather than biological relation.

    For example, Oedipus seems ironic rather than creepy to me, but I find a parent marrying their adopted child really disturbing. It’s about social relation, not biology.

    Sibling and close-cousin marriage seems fine, apart from the recessive gene concern.

  12. Jon: What do you mean by “accepting the incest argument”? Do you mean philosophically biting the bullet? In other words, you ask: if no concrete anti-incest argument can be made, then why don’t gay marriage proponents accept the possibility of incestuous marriage?

    Well, maybe the “bad” part outweighs the “inexplicable” part here; it can do that without encroaching on other forms of marriage. Also, we aren’t operating in a political vacuum. If gay marriage proponents (or even anyone else) all started actively campaigning for the legalization of incestuous marriage on civil-rights grounds, that would seriously damage, and perhaps,destroy, the same-sex-marriage cause. Most likely, though, gay-marriage proponents are against incest for the same reasons listed on this page.

    Regardless, I hope I made it clear why the existence or absence of explicit anti-incest arguments is unnecessary with respect to the support of gay marriage. The slippery slope argument is almost a non-sequiter, unless we consider the prevention of incest to trump every other consideration. (In which case it would be best to abolish marriage altogether so as to not even climb the mountain, much less approach the slope. We don’t even need out-of-wedlock sex, another tabooed activity, to occur, since sex is no longer a prerequisite for human reproduction.)

    Meanwhile, to the extent that incest is “explicably” bad, then the argument fares even worse. All we have to do is consider the points against incest (which do not also exist as points against homosexuality) to be sufficient grounds for its continued illegality; problem solved.

  13. I think a big distinction worth considering is that in countries who condemn homosexuals, there are or at least were cases of polygamy and incest being accepted. In contrast, homosexuality is becoming increasingly accepted in western countries while polygamy and incest are generally frowned upon. There is a shift against the two but towards homosexuality so the argument is invalid.

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