Two of the related arguments made by people who oppose same-sex marriage (especially those who do it on the basis of "marriage is for raising kids"):

  1. The slippery slope argument. Usually goes "if you let two guys marry, why not 2 siblings?"

  2. A similar but opposite direction policy argument: "You claim to support same sex marriage because the government can not treat people differently by saying who they can't marry. But you yourself support government telling people that they can't marry, e.g. between siblings".

The problem is that such arguments are based on unsupported assumption that there are indeed people who support one and not the other.

Thus, I have a couple of sub-questions:

  1. How valid is that assumption?

    What I seek as an answer is a comparison of polls/surveys which show support for same-sex marriage vs polls that show opinion about sibling marriage. It's probably too much to ask for a poll asking the 2 from the same people, but that would be ideal.

  2. If there are indeed statistically significant amount of people who support same sex marriage but oppose letting siblings marry - based on the first question - what are their main arguments for why there are differences between the two situations (legally, ethically or practically)?

    I would prefer this to be ideally somehow referenced (e.g. a policy paper, a speech, or at least a blog post articulating the position).

Please note that I am not asking for the overall reasons why people oppose incestual marriage - those are obvious and well documented (Rooted in Old Testament and then Christian marriage laws, and more likely than not arising out of genetics issues arising from children of incest).

Also, I am explicitly excluding inter-generational incest, since a rational argument can be made against it based on unequal power distribution.

  • 5
    In the interest of full disclosure - as noted in another Q, I am personally in the camp of "the government shouldn't define marriage in the first place, simply endorce standardized private contracts".
    – user4012
    Commented Dec 21, 2012 at 16:36
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    "Valid" has a very specific definition in the world of logic and reason and as the "slippery slope" argument is an informal logical fallacy, it cannot be valid regardless of what an analysis of the data purports to say. Commented Dec 21, 2012 at 21:28
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    @RobertHarvey - see my point #2. Marriage and children aren't coupled in USA in 2012 anymore, same sex couples or not.
    – user4012
    Commented Jan 11, 2013 at 1:14
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    Do any surveys on societal attitudes to incestual marriages even exist?
    – TRiG
    Commented Jan 12, 2013 at 18:47
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    I'm not seeing a question with two sub-questions here, I'm seeing two questions and a statement of an assumption you assume is unsupported. Why not first just ask for any data supporting or refuting the assumption, and then (if you get the data) ask separate follow-up questions?
    – Shog9
    Commented Jan 12, 2013 at 19:08

3 Answers 3


Summary of answer:

  1. A poll that would satisfy the conditions for this question is unlikely to exist, given the common negative (and sometimes inaccurate) associations of the term "incest" or "incestuous."
  2. The standard arguments made to differentiate incestuous unions from gay unions are discussed in many media sources that have covered the gay marriage debate, including, for example, here and here. (For a summary of points from these links, see the bulleted list below.)

For a more detailed consideration of the questions, read on.

For the first question, I seriously doubt you're going to find any polls asking people whether they are in favor of incest. From a brief search, I couldn't (though there are some polls asking people if they have been the victims of incest or know of incestuous abuse). For most people, the term "incest" generally conjures up images of abuse, often father-daughter relationships (or father with minor child in general), or perhaps brother-sister abusive interactions. I don't think we need to do a poll to see that this is the common association of the term (at least in the U.S.); just look at its usage in any news story or really most of the time it comes up.

Obviously, consensual incestuous relationships are theoretically possible and probably do happen in a minority of cases. But it's unlikely that the vast majority of people would ever say they are "in favor of incest" in a poll, because it is generally taken to be synonymous with abusive incest. In a way, it's not unlike the associations that homosexuality had decades ago: it was assumed to be deviant, including mostly weird people and child molesters. (To this day, there are many people who maintain that association with homosexuality. They do not see a difference between pedophilia and homosexuality, assuming that someone who practices one is just as likely to practice the other.) Similarly, to many people "polygamy" is equivalent to the idea of a guy abusing young girls and forcing them into marriage. That may be changing slightly with media stories, the series Big Love, etc. Still, the idea that "polygamy" could refer to an adult woman with multiple (consensual) male partners or even three women in a consensual relationship is just not what most people think of when the term "polygamy" comes up. (Again, just look at most news coverage to see these assumptions and associations.)

Because of these associations with incest, you're probably not going to find a poll. And if you did, it would be meaningless anyway to answer your question unless the pollsters took time to explain to those polled what consensual incest might actually mean and convince people that such things might exist.

I also think, in the absence of polls, that you can look at the way incestuous relationships tend to be portrayed in the media and in the courts, even among liberals. The case of Patrick Stübing, who has had a consensual incestuous relationship with his sister for many years, shows that a country with liberal marriage laws (gay partnerships have been legal in Germany since 2001) still will put someone in jail for consensual incest. The European Court of Human Rights also upheld that conviction, again despite having many member nations that approve of gay marriage and/or gay partnerships. A similar prosecution happened to Columbia professor David Epstein, who had an apparently consensual sexual relationship with his adult daughter. In that case, most gay marriage advocates in the press found themselves coming up with all sorts of reasons why incest is different.

And that leads us to the second question: clearly most people who write for the media in favor of gay marriage tend NOT to be in favor of incestuous marriage. All one needs to do is scan the news for any point in the past decade when any major public figure has associated the two, and you'll see days of coverage afterward while that person is denounced by those in favor of gay marriage. I have no idea what the general public might think, but clearly the public voices of the gay marriage movement react in strong negative association to ANY comparison to incest.

Since you ask for specific examples of arguments public figures tend to use to differentiate the two, here's a detailed article (which covers polygamy, incest, and bestiality). The David Epstein article linked above also contains a lot of the standard arguments.

In sum, here are the standard arguments, which you can read about in more detail in the links:

  • Genetics (obviously not applicable to homosexual incest)
  • Exploitation/abuse (discussed above)
  • Disruption of family dynamics: as the German court ruled: "Incestuous connections lead to an overlap of family relationships and social roles and thus to a disturbance of a family bereft of [clear] assignments. … Children of an incestuous relationship have great difficulty finding their place in the family structure and building relationships of trust with their next caregivers. The vital function of the family for the human community … is crucially disturbed if its ordered structure is shaken by incestuous relations."
  • The "Many Other Possible Partners Theory": essentially, the idea here is that people have other choices to form relationships outside of their family. A homosexual who is banned from homosexual relationships has no possibility for a relationship, while people in incestuous relationships could still have many other opportunities for relationships outside their family unit.

The last two of these are the ones that typically come up when incest is discussed beyond the possibility of abusive incest. Here's an extended explanation from the Epstein article linked above:

When a young man falls in love with another man, no family is destroyed. Homosexuality is largely immutable, as the chronic failure of "ex-gay" ministries attests. So if you forbid sex between these two men, neither of them is likely to form a happy, faithful heterosexual family. The best way to help them form a stable family is to encourage them to marry each other.

Incest spectacularly flunks this test. By definition, it occurs within an already existing family. So it offers no benefit in terms of family formation. On the contrary, it injects a notoriously incendiary dynamic—sexual tension—into the mix. Think of all the opposite-sex friendships you and your friends have cumulatively destroyed by "crossing the line." Now imagine doing that to your family. That's what incest does. Don't take my word for it. Read The Kiss. Or the sad threads on pro-incest message boards. Or what Woody Allen's son says about his dad: "He's my father married to my sister. That makes me his son and his brother-in-law. That is such a moral transgression. I cannot see him. I cannot have a relationship with my father …"

Homosexuality is an orientation. Incest isn't. If the law bans gay sex, a lesbian can't have a sex life. But if you're hot for your sister, and the law says you can't sleep with her, you have billions of other options. Get out of your house, for God's sake. You'll find somebody to love without incinerating your family. And don't tell me you're just adding a second kind of love to your relationship. That's like adding a second kind of life to your body. When a second kind of life grows in your body, we call it cancer. That's what incest is: cancer of the family.

These are the general sentiments that usually comes up in the media when your question is asked. One can obviously question the logic of these arguments. For example: Don't all marriages have power dynamics, many of them unhealthy? These unhealthy power dynamics might in fact have something to do with the fact that until recent decades, women were generally an oppressed class. If we're going to outlaw marriages that result in screwed up relationships with messed up power dynamics, we might have to cast the net pretty wide.

And as for the "just find another partner outside your family" argument, is that argument applicable when we're talking about a fundamental right (as marriage is generally characterized in the gay marriage debate)? Should you lose your right to marry someone just because there's a social stigma, and "there's lots of other fish in the sea"?

(From a purely legal standpoint, actual marriage brings up other practical problems about inheritance and other marriage benefits. Particularly if you allow intergenerational incest, you would end up messing up a lot of the legal apparatus that deals with marriage vs. heirs/relatives.)

There are lots of arguments out there. But it's pretty clear from even a few cursory searches that the common view among gay marriage advocates is that incest is different -- among the media anyway, and probably on the internet in general, if comments on forums are any indication. Whether or not you find arguments on either side convincing usually seems to be based on how you "stack the deck" from the outset.

For example, in the question, it is already presumed that there must be an obvious "rational argument" against intergenerational incest. To many other people, there are obvious "rational arguments" against all incest, against gay marriage, against interracial marriage, against just about anything. What is "rational" depends fundamentally on what assumptions you begin with. I think it's undeniable that it is possible for an intergenerational incestuous relationship to be consensual (and perhaps even have a power dynamic that is very different from what you would assume). I also think it's undeniable that the vast majority of incestuous relationships among close relatives (not talking cousins here: fathers/mothers with sons/daughters, siblings, aunts/uncles, etc.) tend to be psychologically damaging, if not outright abusive.

In the end, the question comes down to how much freedom you're willing to grant vs. how bad you think the chances of abuse are (for whatever sort of relationship you want to allow). Obviously abuse is illegal in all relationships, but the main concern among most people is probably that legal incest would encourage more abusive relationships. Since there haven't been a lot of countries experimenting with legalization of incestuous marriages, we don't have any data on what effect legalization might have. So, "rational arguments" usually tend to just be about what you're personally comfortable with.

  • "Incest spectacularly flunks this test. By definition, it occurs within an already existing family. So it offers no benefit in terms of family formation. On the contrary, it injects a notoriously incendiary dynamic—sexual tension—into the mix." I guess that is why married people stop having sex. You seem to conflagrate sex with marriage in many of your positions. For example, you say that homosexuality is a orientation, and incest isn't. But, what does sex have to do with marriage? Are we supposed to outlaw open marriages or adultery(encourage it instead to help form new family relations?)
    – user1873
    Commented Apr 18, 2013 at 4:00
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    @user1873 the very definition of 'incest' is sexual intercourse. So, perhaps the issue is that the term 'incest' is being used in the argument.
    – user1530
    Commented Apr 18, 2013 at 4:05
  • @DA., "incestual marriage" is used, not incest to describe the relationship (in shorthand) within family relations.Athanasius what about the Hebrew/Islamic customs, since they don't offer any benefit in terms of family formation. "The Hebrew Bible forbids a man from marrying his brother's widow with the exception that, if his brother died childless, the man is instead required to marry his brother's widow so as to "raise up seed to him" (taken from Deuteronomy 25:5–6). According to Islamic ideology ... Marrying the widow of a brother, or the sister of deceased or divorced wife is allowed"
    – user1873
    Commented Apr 18, 2013 at 4:30
  • @user1873 the term 'incest' or 'incestual' is a sexual term, though. So if we're going to use that term, by definition, it's going to refer to sex.
    – user1530
    Commented Apr 18, 2013 at 4:39
  • @DA., not necessairiy. same-sex is also a sexual term, "of or involving a sexual relationship between two men or between two women: same-sex marriage." In the context it is used, "incestual" and "same-sex" both refer to the relationship between (in this argument), two people who love each other. It seems kind of weird to argue that incestual relations should be denied marrying the person they love.
    – user1873
    Commented Apr 18, 2013 at 4:54

I think the best way address your question(s) is by using a legal analysis. This makes sense because the pro-gay marriage position is that in denying gays the right to marry, state governments are denying them equal protection of law. This is why gays argue that laws limiting marriage to heterosexual couples are unconstitutional. In some states, the state supreme courts, interpreting the states' constitutions have accepted this argument. In these situations, the state legislatures cannot ban gay marriage without amending the state constitution.

The question of whether state laws that do not allow gay marriage are unconstitutional under the US Constitution has not been decided, but the US Supreme Court will probably deal with this question during the current term in the case involving the Federal Defense of Marriage Act.

As a matter of constitutional law, when a law disadvantages a particular group of people, the courts must determine whether there is a legitimate state purpose for the law. This balancing of the state interest versus individual interests is done somewhat differently depending on the group of people involved. If the group is a "suspect class" such as a racial group, then the state must meet a higher standard than in other cases.

At this point, neither gays nor siblings are a suspect class, so the standard to show a legitimate state purpose is not as high as it would be in, say, a case involving a law banning inter-racial marriage. There is the possibility that the Supreme Court could decide that Gays are a suspect class or close to it (as the court has with women), but I think that's unlikely given the conservatism of the current court.

There is little question that state governments do have legitimate interests in preventing incestuous marriages, which is why these laws have stood. In addition to the likely health problems of any children born of the union, there is also the very real possibility of coercion, particularly in father-daughter and older brother-younger sister unions. Clearly, these reasons do not apply in the case of gay marriages. So any argument equating the two classes of marriages is not convincing.

As for slippery slope arguments, one of the first things one learns in law school is that while these might sound good in a debate (or on a TV talk show), they are meaningless. In reality, the law stops slides down slippery slopes all the time, because a legal system won't work if people do not draw lines. For example, it is illegal to steal things from people, but that does not mean that once we pass a law against robbery, we give every thief the maximum punishment. Stealing an apple is not the same as stealing an iPod or stealing a car or carrying out an armed robbery of a bank. These are all on the same slope, but we have been able to decide how far down the hill they are.

  • Sorry, but this long and seemingly well reasoned argument seems to base its main conclusion (" any argument equating the two classes of marraiges is not convincing") based on 2 assumptions contrary to the question: "In addition to the likely health problems of any children born of the union" (the whole point of pro-gay-marriage is to disassociate marriage from procreation) and "coersion, particularly in father-daughter and older brother-younger sister unions" (Q is only discussing siblings who are both of age. You might as well prohibit ANY marriage if a man is much older on same grounds).
    – user4012
    Commented Feb 3, 2013 at 13:03

What troubles most people about same-sex and sibling marriages is their likely impact on childbearing.

Homosexual marriages can't produce children (within the marriage) BY DEFINITION. That probably offends religious, more than secular authorities.

Sibling marriages CAN produce children, but they are likely to be genetically deformed. That explains most of the civil legislation against it.

I doubt that you will be able to find the bases for opposition in polls. More like laws, and religious encyclopedicals, plus marriage policy papers, and maybe some sociological studies.

Probably fewer people will object to same-sex marriage because it only causes "negative harm," (no children), rather than "positive harm" (deformed children) in the case of sibling marriage.

The difference between the two types of harm can be found in my answer to this question:

Are clergy required to perform interracial and/or same-sex marriages in the United States?

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