Field of Science

Sex among siblings

A danish couple have been convicted in a court of law for having sex with each other. They are brother and sister, and incest is forbidden by law (article in Danish). Vagn Greve, professor of law, is of the opinion that it should not be forbidden, for as he says
The law is based on old [read: uninformed] ideas about morality, and so we have criminalized something that is allowed in countries like Holland, France, and Spain.
People will argue that incest should not take place because of the elevated risk of having children with genetic disorders. However, there is some serious disagreement about how much higher the risk is. For example, in the Danish article in Urban, Karen Brøndum-Nielsen, president of a research center in genetics, says that her best bet is that siblings has a risk of 10 to 30 percent for getting a "handicapped" child, while Vagn Greve contends that the risk increases from three percent among non-related parents to five percent among siblings (i.e. an irrelevant increase). Where do those numbers come from?

Alan Bittles, Australian geneticist, has collected data to show that the risk of birth defects increases from 2% in the general population to 4% when the parents are closely related (source). It would appear that Brøndum-Nielsen's numbers are pure fiction, while Greve actually based his assertion on real data.

Additionally, a comparison with the increased risk of birth defects in mother of high age reveals that the risk of birth defects has nothing at all to do with why people are universally against incest:
A woman over 40 has an elevated risk of giving birth to a child with defects. People who with autosomal dominant disorders have a 50% chance of having children with the same disorder. If we allow these people to give birth, why not allow the same for first-cousins (or, indeed, for them to marry).
This quote is from an article I mentioned earlier which argues that there is no sense in having laws against first-cousin marriage. Similarly, we also wouldn't dream of forbidding someone with cystic fibrosis, a terrible hereditary disorder, from choosing to have a child. Rather, we would all insist that it was their right to have one. And the baby's risk of getting the same disease is 50% (autosomal recessive disorder)

Our revulsion against incest can also be shown to be emotional and not based on hard facts by considering a hypothetical example: A brother and a sister are camping in a tent, and one day decide that they will try to have sex. They agree to use a condom, and after they're done, they agree that it was nice but that they won't do it again. Do you think what they did was wrong? Why? With no chance of becoming pregnant (you can even imagine that one of them are infertile), no other change in their personal lives, and no effect on any other person, because no one else will ever know, on what grounds is it that many of us think of the act as repugnant?

In a previous post I argued that science does indeed have something to say about morality. Not directly, but only inasmuch as our moral values are based on what we know about the real world. So, for example, if we think that incest leads to many children being born with defects, then we will think sex between siblings is wrong. And when we discover that what we thought we knew was based on anecdotal evidence, and has no statistical merit, then we should reconsider what we think about the matter. I am not saying that our emotions have nothing good to say about what we should do and think, but that when barring others from doing so, we should have better reasons than that.

And, please, for heaven's sake, stop saying things you know absolutely nothing about. Danish member of parliament Karen Hækkerup says "There will always be psychological damage when family members sleep with each other - even if they can't see it themselves right away." Such a statement is blatantly outrageous and paternalistic in the extreme. She knows nothing of the kind. Show me the numbers, or shut up!

Oh, and there is no evidence presented that the girl the couple had as a result of their affair has any defects whatsoever. Imagine a law saying that it was illegal if was born baby with any defects, and otherwise lawful. Imagine the same law applied to anyone else. Jail time for having a baby with a genetic disorder has got a certain ring to it, don't you think?

18 comments:

  1. I guess I want to make the following point here:

    Whatever science sais about it, it comes down to making a decision. Science can impossibly make this decision. And the example goes as follows: Suppose science shows that there is no risk to siblings offspring whatsoever. There is no psychological risk or harm to any one.

    I might still be against it. Not because it is justifyable imoral, or because I don't understand the facts. I don't want it for irrational reasons. Like I also don't want the music from the 80th come back ... ever! There is no harm to that music, I might just don't like it.
    Sometimes it is a matter of taste, personal opinion/prefferences. Does science tell you not to respect, or at least consider personal preferences? Imagine science proves the chinese cultural revolution to be the way to go. The best thing for the majority would be to ride a bike, kill political opposers, and all wear the same cloth? I wouldn't want that.
    Same with death penalty. See a little into the future, where scientist prove that consciousness is an illusion, and there is nothing "real" to life. And that we find human painless sufferless methods of capital punishment. I would still be against it, just becaue I am having the illusion of empathy.

    Facts do not necessarily change what we think is "right" and "wrong" (as moral terms).

    I am not against using science to make well informed decisions, not at all. In fact every not rationalized decision is most likely stupid. Still I want to keep the authority about what is right and wrong.

    And btw. science, however well done it is, and however good tested might still be wrong.
    To make an insane but probably analog example: You can not replace gods (not capital G, afterall this is a science Blog) authority and absolute morality, with another absolute moral and justify it's validity with something that in fact knows about it self that it is under no circumstance absolute. The key in science is the disprovability. If you have found something that is by itself undisprovable (and every absolute term is) you leave the scientific realm.

    You always end at: "To the best of our knowledge, we say..."

    Another argument against scientifically dictated moral:
    If I can seperate my moral from my own judgement, my own authority, my own responsibility, I can not held responsible for my actions. But in fact I want people to be responsible, I don't want people to justify their actions by saying:
    "The big thinking machine AI, considering the latest research, said I should do X!"
    If you want you can replace "thinking machine AI..." with "god"...

    Cheers Arend

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  2. Arend,

    That has to be the dumbest thing I've ever heard.

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  3. "Similarly, we also wouldn't dream of forbidding someone with cystic fibrosis, a terrible hereditary disorder, from choosing to have a child. Rather, we would all insist that it was their right to have one. And the baby's risk of getting the same disease is 50% (autosomal recessive disorder)"
    I would be fully supportive of banning procreation by such people. Removing those genes from the gene pool avoids having to treat future generations with equivalent issues and lower quality of life. They could adopt, assuming the risk of the parent dying while the child is relatively young is "ok" based on some measure of psychological health of children of such parents. Anyway. I always find it odd when it is assumed that everyone would be against having restrictions on who gets to have children.

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  4. Hi Bjorn, here's an interesting anthropological perspective.

    Personally, I would err on the side of caution. What I mean is that, if we have a near-universal moral reaction against incest (and I'm not convinced we do), then we should only override it if we understand why it evolved.

    In other words, it's not good enough to say that 'The genetic argument doesn't hold up, therefore we should ignore moral responses'. Because it's possible that there is a perfectly good rationale for the moral response, we just haven't figured it out yet.

    As a general principle, we should first try to understand the origin of a moral response (from an evolutionary perspective). And only then, if we determine that the rationale no longer holds in the modern world, should we be acceptant of transgressions.

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  5. As a general principle, we should first try to understand the origin of a moral response (from an evolutionary perspective). And only then, if we determine that the rationale no longer holds in the modern world, should we be acceptant of transgressions.I vehemently disagree. I think we should err on the side of caution in the sense that we should not impose our own moral values on others when the actions of others do not affect anyone else - unless we have a very good reason to. The argument I present is that we do not.

    Same goes for any sexual acts among consenting adults in private.

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  6. But if these moral responses are near universal, it suggests that they have evolved. That in turn suggests that there is some functional utility to them - some kind of fitness benefit.

    I think we need to understand what that is before dismissing them as irrelevant. They might well be irrelevant (especially to modern society), but we can't decide until we know why we have them!

    Most sexual taboos are not near-universal. Which suggests that they are largely cultural. So not comparable to this particular situation.

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  7. I think we need to understand what that is before dismissing them as irrelevant.Tom, I would very much more like to wait passing judgment on what others do in private until the science is totally sure, as opposed to let our evolved emotions decide prematurely. First establish that the evolved response to something is universal AND an adaptation AND why, then we can talk.

    Besides, the best science on this subject is that there is no risk we need to be concerned about.

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  8. I've found myself interested in the whole issue of incest between siblings, especially siblings of the same gender. Is there any research into such relationships (hetero or homo-sexual) as to their long term psychological effects? I speak here only of mutually consensual relationships - where there was no force or coercion and the participants are intellectually more or less peers, regardless of their chronological ages.

    I can find almost no actual research in the area, especially concerning siblings of the same gender. Anecdotal reports seem to suggest that (very)late adolescent to early adulthood couplings can and do occur without serious psychological harm far more often than most people think. Are you aware of any such reports or studies?

    Also from strictly a moral perspective, I can find no reason to condemn sibling incest where it is mutually consensual. Whether it is hetero or homo-sexual, acts between consenting partners do not usually affect society at large. I DO have a problem if one or the other of the participants has under age children who may be or are exposed to and knowing of the relationship and the sex - for some reason that is my 'tipping point'. But if adult siblings who are unmarried and childless want to cohabit, that's not my (or the government's) business IMHO. And if the cohabiting results in the birth of other children, well, then we may need to study the possible effects on the child/ren of societal disapproval of their parents' relationship. To me that's the point where it might become a valid societal issue - but the problem AFAI am concerned is with society.

    The ONLY reason I have ever been able to find for the reactions against sibling sexual relationships is the biblical proscription of them. Since the vast majority of people in Western culture are at least exposed to christian beliefs and taboos, even if they largely reject them later, I think that accounts for the nearly universal "ugh, that's repellant" reaction. Are there cultures where there is not this seemingly ingrained reaction to consensual incest? I'd be interested to know.

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  9. I just posted the long comment (June 11); my questions were serious an I hope you will consider them. I wanted to make it clear I am NOT the same person who posted an anonymous comment on April 17th though.

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  10. Great questions. I do not know of any research into any of those questions you pose. I also doubt there is any, since, I suspect, it's very hard to come by e.g. same gender siblings who have sex and are willing to tell anyone else about it. I'd be very interested in seeing it, too.

    But I agree with your view that if it's consenting, then it is no one else's business. That sort of what was I was trying to espouse in the main post.

    Sorry I couldn't be more helpful.

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  13. I really think it is ridiculous that such a thing is so criminalized. We all know it happens very often, we all know it is - one way or another - natural behavior of humans. Yet we refuse to morally accept it.

    I think sibling sex should be controlled so that there'd be no offspring involved. Mandating and making sure it is a bad idea. HOWEVER, I don't see the point of worrying to death because your 7 year old boy is touching your 5 year old girl. Who cares? Honestly, I read this in Human Sexuality 101, sibling sexual experiences can help children understand sex better.

    Again, I don't encourage an intercourse - simply because it is risky - but I seriously don't see the big deal in two kids satisfying each others needs.

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  14. We all know it happens very often

    I don't know that, and I don't think very many people do. How do you know?

    I think sibling sex should be controlled so that there'd be no offspring involved.

    Why no offspring? As I tried to make clear in the main post, the minor increased risk of having children with birth-defects is not enough to prevent it.

    HOWEVER, I don't see the point of worrying to death because your 7 year old boy is touching your 5 year old girl.

    Now you're talking about children, which is a whole other topic. What exactly does your textbook say that sibling sexual experiences constitute?

    I seriously don't see the big deal in two kids satisfying each others needs.

    What needs? (And at what age?)

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  15. Well... that's interessting but actually i have a hard time understanding it... wonder what others have to say..

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  16. Well... that's interessting but actually i have a hard time understanding it... wonder what others have to say..

    What is 'it'? Sibling sex? They're horny! That's normal. Doing it with each other? Because they're there, I suppose (I don't know why they would, but that's besides the point).

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  17. I think there are very few of us that experience true love and warmth from our partners. The only conditions here should be consenting adults. Adults being the key word as then you are in tune with your emotions. Other then that why should people decide to let go of something beautiful just because the rest of the jealous world can not let them live in peace. I truely belieave that love between a brother and sister is a special bond. Often men go around looking for girl friends that look or have the same characteristics as their sister. But what they realy want is her warmth. What they realy want is to be able to hold her and love her. Their is a sense of safety and peace. A sense of belonging. And a feeling that this is truely unselfish. When growing up your sister stands by you when you are being bullied or when you hold her hand when her date lets her down. Other girls will run away with the famous guys. Guys will reward the guys for "scoring". But a brother and sister will stand by. What they realy want to say is that if only...if only I could you would not have to go through all this..I am here..no one will love you like me..but I can not love you because..Ah, no one has the answer to this because.....!!!

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  18. Your views sound very personal. Are they? Or are you really trying to speak generally? If so, where do you know this from? Are there studies that show this? What?

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