Incest, morality and genetics

By Razib Khan | April 13, 2009 12:04 pm

Over at Secular Right I point out that Romania is set to decriminalize consensual sexual relations between adult first-order kin. That is, incest. There are a few angles that this story offers. First is the moral-ethical one. From a rational individualist perspective how reasonable are laws which ban consensual sex between adults who just happen to share distinctive genetic information? The love that dare not speak its name because of alleles which are identical by descent?
Of course all this rational talk is irrelevant for most people. Incest is gross, repugnant and immoral. Not because incest is irrational or unreasonable, human instinct just tells us that it’s wrong. For most humans the repulsiveness of incest requires as much justification as the repulsiveness of consuming feces. My earlier post, The biological roots of moral sentiments?, and Jonathan Haidt’s distinctions between liberal and conservative morality, are relevant in explaining some of the variation in condemnation you see across the population toward “victimless crimes.” Since I’ve addressed these issues before I won’t touch upon the rightness or wrongness of incest any further.
Rather, a commenter at Secular Right points out a very utilitarian reason for criminalizing sex between near relations:

Were I a Romanian, paying taxes that went to a public health system, I’d probably oppose legalizing incest because of the possibility of the unnecessary burden on the public. We actually have this problem here in Arizona from Warren Jeffs’ FLDS. (Fundie Mormons, if you hadn’t heard of them.) Incest is fairly common among insular religious communities.
Here in Az, the FLDS up in Colorado City practice polygamy and have multiple marriages amongst the same few families. (We know of granddaughters being married to their grandfathers, among other convoluted and forced relationships.) The state’s public health system covers the results. Here’s a piece of local investigative reporting on the matter: http://www.phoenixnewtimes.com/2005-12-29/news/forbidden-fruit
I went to high school at a Mennonite Church-run high school and knew people who were only a generation removed from Amish communities. We had one girl who was a dwarf as a result of Amish inbreeding.

This is one that comes up frequently. In a system of socialized medicine where you are your brother’s keeper, you are also your brother’s minder, and their business is your business. Recessive diseases are a major issue when it comes to incest between relatives whose coefficient of relatedness is 1/2 (parent-child, full sibling). Consider:

The medical risks of in-breeding are illustrated in the few well-documented cases of consensual incestuous couples. In Germany, a brother and sister couple challenged the courts so they could continue their relationship. They had four children, three of whom are in foster care and two have unspecified disabilities. In Australia, John Deaves and his daughter Jenny have been in a relationship for the past seven years. The pair had two daughters but one died shortly after birth because of a congenital heart disease.

One of Elisabeth Fritzl’s sons by her father died soon after being born because of a congenital heart problem. I’ll be posting on a paper tomorrow which suggests that mortality due to first-order incest is far lower than it was in the past because of modern medicine, but modern medicine might be better at transforming mortality into morbidity than abolishing the physiological downsides.
pedincest.pngWhy are the offspring of two siblings who mate at so much greater risk than those born from two cousins? To the left you see two simple pedigrees with negative outcomes: the offspring express deleterious phenotypes because they get two copies of a defective gene from their ancestors. As you can see there are many more steps in first cousin matings than in sibling matings. Siblings share ~1/2 of their genes identical by descent from their parents. First cousins only share ~1/8 of their genes. Consider an individual who has a very rare deleterious mutant that is masked in a heterozygote because of recessive expression. This is all of us, as we all carry many deleterious mutants which are masked, and likely a handful of lethal ones too. There is a 1 out of 2 chance that any given one of your children will carry the mutant. There is a 1 out of 4 chance that any given one of your grandchildren will carry this mutant. So as you can see the risks diminish down the generations as the likelihood of any two individuals across the family tree of carrying a mutant is diluted. But in brother-sister matings mutants only need to jump a few steps of independent probabilities. Of course when you have an inbred clan, like the radical Mormons alluded to above, the inbreeding coefficients start to go up very high and can approach that of siblings between relatives who are not technically siblings.

CATEGORIZED UNDER: Culture, Genetics
  • http://www.silphium.net/blog Larry Ayers

    Facinating post, Razib! My ex’s folks are Mormon converts and I heard more than I really wanted to know about the religion over the years; the sacred underwear and such.

  • http://ranaban.blogspot.com RNB

    You say in bold I’ll be posting on a paper tomorrow which suggests that morality due to first-order incest is far lower than it was in the past but did you mean mortality as that’s a provisional conclusion I’ve seen recently?

  • –bill

    “morality due to first-order incest”!?
    perhaps you mean
    “mortality due to first-order incest”

  • Pierce R. Butler

    Incest is gross, repugnant and immoral.
    You state that as an absolute, but in practice doesn’t it vary in inverse proportion to the foxiness of your sister?
    But aren’t laws against incest a form of eugenics??!?1?
    All Right-Thinking People™ are against eugenics!

  • Jancis M. Andrews

    Did the politicians who are legalising incest give even one thought to how much more difficult it will be now for an abused girl to lay charges against the relative sexually exploiting her? Rape requires proof, and the abuser can now easily say, “Oh, she consented to sex, but she’s mad at me for something I’ve done, and this is the way she’s getting her revenge.” And how can the girl prove that’s not true? My father, who liked to expose himself to my sister and me, would have LOVED this law! Legalising incest puts girls and women in danger, and greatly reduces their chances of obtaining justice. What a bunch of stupid, blind, misogynists these politicians are!

  • Martin

    I’ll be posting on a paper tomorrow which suggests that morality due to first-order incest is far lower than it was in the past because of modern medicine

    Did you mean “mortality?”

  • http://changelog.ca/ Charles Iliya Krempeaux

    From a rational individualist perspective how reasonable are laws which ban consensual sex between adults who just happen to share distinctive genetic information?


    Were I a Romanian, paying taxes that went to a public health system, I’d probably oppose legalizing incest because of the possibility of the unnecessary burden on the public.

    A rational individualist might suggest that the public health system be eliminated as a solution to the problem (in favor of a privatized and voluntary system). (Then “the public” wouldn’t so necessarily be burdened.)

  • http://scienceblogs.com/gnxp razib

    mortality.
    when i say incest, i mean first-order incest of near kin.

  • http://changelog.ca/ Charles Iliya Krempeaux

    @Jancis M. Andrews, you said…

    Legalising incest puts girls and women in danger, and greatly reduces [a raped women's] chances of obtaining justice.

    Even if one assumes that is true, one could then consider the (perhaps harsh) question: does it matter?
    A rational individualist would likely argue that the ends don’t justify the means. And while acknowledging that rape is horrible and wrong, that laws against incest (that would put someone who engaged in it in prison) are also horrible and wrong. (For a rational individualist the horribleness and wrongness, of each, would be derived from the concept of self-ownership.)
    For those without such beliefs you’d likely get different results.
    @Jancis M. Andrews, you also said…

    What a bunch of stupid, blind, misogynists these politicians are!

    That sounds like Ad Hominem.

  • http://scienceblogs.com/gnxp razib

    You state that as an absolute, but in practice doesn’t it vary in inverse proportion to the foxiness of your sister?

    dude, this is too much information about you! :-)
    Did the politicians who are legalising incest give even one thought to how much more difficult it will be now for an abused girl to lay charges against the relative sexually exploiting her
    the law covers only adults. so substitute “woman” for “girl.”
    A rational individualist might suggest that the public health system be eliminated as a solution to the problem (in favor of a privatized and voluntary system). (Then “the public” wouldn’t so necessarily be burdened.)
    the perfect the enemy of the good? :-)

  • Joey

    It’s not illegal to eat feces. Incest itself should not be illegal, no matter how gross it is. If you are concerned about the offspring, suggest making a law requiring genetic testing of embryos of any incestuous relationship for known genetic diseases, which is after all the real issue isn’t it?
    Nah. The real issue is political with you, I think.

  • me

    Here in Az, the FLDS up in Colorado City practice polygamy and have multiple marriages amongst the same few families. (We know of granddaughters being married to their grandfathers, among other convoluted and forced relationships.) The state’s public health system covers the results. Here’s a piece of local investigative reporting on the matter: http://www.phoenixnewtimes.com/2005-12-29/news/forbidden-fruit
    I have lived there all my life. And the phoenix times is full of Crap!! And you are for listening to them. there is a web site with the truth http://www.fldstruth.org You should do your research before you make such an immoral accusation against people you have never met. we have been very clear about the way we live and believe and you people have no right to state these sick actions as fact.

  • http://changelog.ca/ Charles Iliya Krempeaux

    @Razib, you said…

    A rational individualist might suggest that the public health system be eliminated as a solution to the problem (in favor of a privatized and voluntary system). (Then “the public” wouldn’t so necessarily be burdened.)

    the perfect the enemy of the good? :-)

    Arguing again from the point-of-view of the rational individualist
    You could argue (against eliminating the public health system, in favor of a privatized and voluntary system) that, in the world we live in, that it’s unlikely that the public health system will eliminated. (I.e., “perfect the enemy of the good”… also known as the Nirvana fallacy.) But given that we’re talking about incest being legalized (something that I’d also have thought unlikely) I’d imagine that that opened the door for other things that are unlikely too. (Like the elimination of the public health care system.)

  • http://scienceblogs.com/gnxp razib

    .But given that we’re talking about incest being legalized (something that I’d also have thought unlikely) I’d imagine that that opened the door for other things that are unlikely too. (Like the elimination of the public health care system.)
    you might be rational, but you sure aren’t realistic.

  • Rogue Epidemiologist

    @FDLSme
    How about that. You’ve got the privilieges to use a computer with Internet access, but you think it’s prudent to not educate your children beyond the 8th grade, and you think it’s okay for 13-year-old girls to marry.
    Here’s an idea: get your clan into the 21st Century. Let your women do things like cut their hair, graduate high school and consent to when they conceive and give birth.
    Then maybe we’ll stop marginalizing you as a bunch of baby-fuckers.

  • Momo

    It seems unlikely that repealing incest laws will result in a rash of sibling lovin’ so large that it places a significant burden of the public.

  • me

    Again i dont know where your getting your info ,but its crap i dont know were this lack of technology crap came from we very much beyond the 21 st century we have nurses doctors teachers and a choice in all things. All my sisters and there friends have a 12th grade diploma. and who are you talking about there was no one married at 13 or 14 or 15 its always been the legal age so dont tell me what is going on in my own house use your head for a change. my wife and i were married when i was 25 and she was 19 and we BOTH chose to do it we have been married for five years and we have one child i could never imagine making her have a baby when she was not ready to and as far as them cutting their hair is that not a personal choice for everyone if your wife wanted to keep her hair long for any reason other than what society says would u make her cut it we chose to do the things we do because we have a purpose for all things really open your mind and ask yourself what sounds right to u take all judgment out and u will see the truth all we want is to be happy and bless others

  • http://scienceblogs.com/gnxp razib

    ok people, let’s stop the FDLS talk now. not getting anywhere….

  • http://rrresearch.blogspot.com Rosie Redfield

    Dwarfism, at least the common achondroplastic type, is caused by a dominant mutation. Inbreeding wouldn’t be responsible.

  • http://www.open.ac.uk/blogs/ideasblog/ Ben

    What if incest were beneficial to children? I’d be suspicious of judgements that wouldn’t affected by this. (Preventing dilution of resources is the likely benefit though screening deleterious recessives through spontaneous abortions is another). Anyway: can I take a stab at being a rational statist?
    The opportunity costs of incest are likely higher for women than for men (who can just get an extra, if defective, offspring) so incest will be of benefit to males over a broader range of parameter values than for females (see this paper for the general model). The law ought to protect the vulnerable in that zone of asymmetry between the sexes and a simple way to do this is by banning incest. But I can imagine unwitting incest between long-lost siblings. Perhaps the law should require vasectomy in these cases but not try to split apart a loving couple.
    And I like the fact that socialised medicine incentivises the government to diminish morbidity – especially if they have been delegated that authority by the people. Morality is (or is about) social coordination and this is one way to achieve it.

  • http://changelog.ca/ Charles Iliya Krempeaux

    What if incest were beneficial to children?

    Isn’t incest used in breeding dogs and other domesticated animals to fix certain characteristics. I can imagine that there are some measurement system that can find incest “beneficial” (for even humans). (Regardless of whether you or I find it “gross”.)

    especially if they have been delegated that authority by the people

    I’ll silence the rational individualist in me for now, and not comment :-)

  • Julia

    It’s difficult enough going through puberty and maturing, if incest was legal and common enough that sexual relations with family members was no longer taboo, the safety and trust a teenager felt among family relationships would disappear. Part of becoming an adult is understanding what shields you put up between yourself and others before you engage or become interested in a sexual relationship with someone. Without legal ramifications the taboo would dissolve and as soon as a child started looking like an adult, sexual interest could be more likely to develop. If the adult or teen sibling was moral or unscrupulous, this switch from affection to attraction would damage the intimacy of the relationship and hurt emotional development of the teenager/young adult. And even as adults it can be more comfortable, healthy and freeing to have relationships with a group of people that you know aren’t going to hit on you. The ramifications of a society of people who have lost the safety from sexual dynamics bubble of familial relationship goes far beyond the chances of having a genetically effected child.

  • genemachine

    I’m surprised that nobody has mentioned longer term benefits.
    When the inbreeding groups return to outbreeding their genetics will have been cleansed of many of the harmful mutations. These people do a service to genetics, letting us identify the function of genes and causes of many diseases.
    (It’s still amoral to cause disability in the next generation of course)

  • Pierce R. Butler

    dude, this is too much information about you! :-)
    Well, I am from Mississippi – but I’m an only child!

  • Challen

    “”"I went to high school at a Mennonite Church-run high school and knew people who were only a generation removed from Amish communities. We had one girl who was a dwarf as a result of Amish inbreeding”"”
    This is a first time I’ve heard of this. Dwarfism is a result of inbreeding? LOL, such a comment is so offensive to those that are dwarfs, it’s just crazy.
    Pull off the highway onto a dirty, just about anywhere in Kentucky, or neighboring states and you will see the result of inbreeding. Eyes not straight on the face, ears not level, that sort of thing. Dwarfism isn’t a result of inbreeding.

  • Levi

    Without legal ramifications the taboo would dissolve and as soon as a child started looking like an adult, sexual interest could be more likely to develop.

    So without artificial legal ramifications, a preferance that is the result of our evolutionary history (at minimum several hundred thouand year old) is going to dissolve? Pardon me if I am skeptical.

  • http://changelog.ca/ Charles Iliya Krempeaux

    Without legal ramifications the taboo would dissolve and as soon as a child started looking like an adult, sexual interest could be more likely to develop.

    Can you offer some proof for that?
    I find it difficult to believe that taboos exist only because of laws?
    Also, I’m skeptical that the only thing that’s stopping brother’s an sisters from doing it is the law. I think I remember reading that (across cultures and races) kids raised together before a certain age will not find each other sexual attractive (regardless if they are brother or sister or not). (I.e. that would seem to suggest to me that there’s likely some kind of genetic basis to it.)
    Also, I suspect “culture” can play a strong role in shaping behavior.
    One litmus test for your assertion may be to go to various “lawless areas” and see if brothers and sisters are going at it.

  • http://twitter.com/marcellojun Marcello Jun de Oliveira

    Your comments insisting that incest is “gross, repugnant and immoral” is a values judgement, which is based on your cultural and philosophical context and personality make-up. In hypothetical (or historically actual) different cultural contexts, you would not be so adamantly manicheistic about it.
    In a biological context, there is nothing “gross, repugnant and immoral” about it. One may argue, as you did, that there is an evolutionary disadvantage to such sexual procreation. Naturally, this disadvantage depends heavily on the environment at any given time, and cannot ever be a clear-cut once-and-for-all-time bi-polar argument.
    That said, there is absolutely no reason why unorthodox sexual relationships should be mandated or negated by the State. Christian homophobes use the biological argument against Homosexual Marriage (sexual relationships that bear no children; they also make the judgement claim that homosexual relations are “unnatural” and “gross, repugnant and immoral”) to institute legalized marginalization based on sexual preferences.
    Your use of Mormon polygamy as an example is a strawman. The problem with Mormon polygamy is not incest, and it´s not even polyandry, but rather child abuse — sexual relations in assymetric positions of power, i.e. an adult male versus an infant female. Romanian legalization of incest, from what I gather from your post, relates to situations of symmetric sexual relations between consenting adults.
    If your standard for criminalization of sexuality is based on the potentiality of inherited genetic disadvantages, then open the precedece for mandatory genetic screening and State-power to dissolve relations and/or prevent procreation in certain couples.
    If your concern is genetic-medical implications, which is a fair point, then I propose investing in education and genetic-medical counselling is a far better route than criminalization would ever be. Besides, there is sufficient evidence that, sexually speaking, people will do what people will do — and historically incest is FAR more common than you would care to admit. Far better, then, to empower people (drawing an age line for when it is permissible) and educate them (taking genetic responsability conscientiously).
    I am personally not arguing FOR incest. I do, however, flinch when people want to endow the State with power to curtail rights of OTHER people can or cannot do in their bedrooms.

  • http://falterer.blogspot.com falterer

    Marcello, unlike incest, there are no biological consequences peculiar to MSM or WSW sexuality. Anal sex is commonly associated with MSM sexuality and is a riskier sex act, but it is neither restricted to nor required for MSM relationships. The ability or inability of a couple to produce offspring is only a financial burden to the state if a couple pursues a medical solution for their infertility problem through socialized medicine. (To the best of my knowledge, no same-gender couple has.)
    On the subject of homosexuality, the utilitarian argument put forward in this post–that the biological consequences of incest are financially burdensome for socialized medicine to support–cannot be used to condemn same-gender incestuous sexual relationships. Is this a shortcoming of the argument, or are we less inclined to condemn such relationships?

  • Troy

    Who taught you to do punnett squares? there is just as good a probability that a pair with a single Deleterious gene could have an offspring without one at all.

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This blog is about evolution, genetics, genomics and their interstices. Please beware that comments are aggressively moderated. Uncivil or churlish comments will likely get you banned immediately, so make any contribution count!

About Razib Khan

I have degrees in biology and biochemistry, a passion for genetics, history, and philosophy, and shrimp is my favorite food. In relation to nationality I'm a American Northwesterner, in politics I'm a reactionary, and as for religion I have none (I'm an atheist). If you want to know more, see the links at http://www.razib.com

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