Why shouldn’t you marry your sister?

There is an intrinsic “eugh” factor in contemplating an incestuous relationship. But how much of this is brought about by social pressures, and how much by biological issues?

In the animal world reproduction between close relatives is very common. In fact it was fairly common in humans in historical times to preserve bloodlines or inheritance. Even Cain (in the Old Testament) may have married his sister.

Marriage of related individuals is still common in numerous societies including people from South-East Asia, Israel and Saudi Arabia. It is estimated that in the World, 8.5% of children have consanguineous (more closely related than 2nd cousin) parents. Some research even suggests that marriages within the family are more stable than those between non-relatives. In some cultures it is thought to be a method of providing support and protection for women.

Indeed the notion of negative effects of “inbreeding” don’t seem to have been present even in English communities until around 1888 (though marriage to a sibling is banned in Christian dogma). Thus current Western revulsion at relatives marrying may correspond more to echoes from the Eugenics programs of the early 20th century than to any long held community beliefs.

The cost of such partnerings can theoretically be high, however.

Take for example a hypothetical disease characterised by a genetic fault on gene d. Humans have two copies of each (non-X/Y sex) chromosome, and thus each gene. Most people in a population (say 99%) will have two copies of the healthy D gene (DD). One percent, however, will carry one copy of the diseased gene (Dd).

The disease is recessive. That is, people are healthy unless they have two copies of the diseased gene (dd).

If your mother carries one copy of the gene (Dd), you have a 50% chance of inheriting it. So does your sibling. If you marry a random person in the population (who has a 1% chance of carrying the disease), the probability of you both carrying the defective gene is 50/100 * 1/100 or 0.5%.

If you were to marry your sibling, the probability would jump to 50/100 * 50/100 or 25%.

Even if you both carry the gene, there is only a 25% chance that any child you had would be dd, that is will have not just the gene, but the disease. (This is an over-simplified explanation – often diseases have more than one gene, for example, but it gives an idea of the concepts).

So the likelihood of children being affected is still very small even if you do marry a relative, yet if you multiply the probabilities by the number of potential genetic diseases, they start to look a little more worrying. Research has shown that for consanguineous partnerings, the prevalence of genetic disorders may be up to twice as high as those from unrelated marriages.

The probabilities of having the same gene as a potential mate can be looked at in terms of relatedness (what proportion of your genes you would expect to share with them). For a sibling it’s 50%. For a cousin, 12.5%.

But say you marry your cousin (which is legal), and your child marries their cousin (on the same side). This gives your child and their partner a relatedness factor of 25% – the same as a half sister or brother. Continue marrying cousin to cousin for a couple more generations and, genetically, it can be equivalent to marrying a direct sibling.

This doesn’t mean that there will be genetic problems, just that the chance of them is higher. In fact if you look at the history of European royalty (with a history of consanguineous marriages) you can see some of the genetic problems surfacing – think Alexis – son of Tsar Nicholas II – who suffered from haemophilia. In a visit to Indeed if you go to the Kremlin and look at clothing from the royal lineage, you will see that many of the queens had extremely small feet – another genetic abnormality.

Similarly in other closed populations where people generally marry relatives through religious or geographical isolation, genetic problems will increase in prevalence (amongst the Amish in Pennsylvania or in Hasidic Jewish populations in New York for example).

So there are some good reasons for banning incest. But many of the laws against it were laid down long before genetics was understood. So some other forces must be at play.

In Israel, children in Kibbutz’s are raised cooperatively – that is they are raised with each other, external to any family group. In the 70’s it was noted that people brought up together in a single kibbutz rarely engaged in sexual affairs, nor married within that group – rather they looked to outsiders. This suggests that at some point in childhood the notion of kinship is developed and people become biologically programmed to avoid as potential mates those people they consider closely related. This view has been strengthened by more recent research.

There appears to be an intrinsic biological avoidance of mating with perceived siblings. While some argue that it could be a result of subconscious recognition of related individuals (perhaps by smell), the Kibbutz studies make it seem more likely due to an imprinting period early in life.

Our sense of morality has, at least recently, come from concern over potential birth abnormalities. In current European society, nearly 20% of women are choosing not to have children. Thus for numerous couples, genetic compatibility is not an issue.

With the availability of extensive genetic testing, any potential problems could be easily screened out – in techniques such as those employed in Israel – where couples can be screened for common genetic incompatibilities either prior to marriage (for those opposed to abortion) or during pregnancy.

Without the concerns for health, as a society, should we still choose to prohibit inter-family marriages?

I think I’ll leave that debate to the ethicists!


Bach G, Zeigler M, Zlotogora J (2007) Prevention of lysosomal storage disorders in Israel Mol Genet Metab. 90(4):353-7.

Hakim C (2004) Childlessness in Europe http://www.esrcsocietytoday.ac.uk/ESRCInfoCentre/PO/releases/2004/january/family.aspx?ComponentId=2010&SourcePageId=1405

Kaback MM. (2001) Screening and prevention in Tay-Sachs disease: origins, update, and impact. Adv Genet. 44:253-65.

Modell B, Darr A (2002) Science and society: genetic counselling and customary consanguineous marriage Nat Rev Genet. 3(3):225-9.

Patton MA (2005) Genetic studies in the Amish community Ann Hum Biol. 32(2):163-7.

Shepher J (1971) Mate selection among second generation kibbutz adolescents and adults: Incest avoidance and negative imprinting Journal Archives of Sexual Behavior 1(4)



Filed under Curious Science, Science

50 responses to “Why shouldn’t you marry your sister?

  1. There have been a number of cases of siblings who were separated as children, met as adults, fell in love & ended up in trouble with the law as they were committing incest.

    I remember reading about one such case here in New Zealand, where the couple involved were harassed by the Police on a daily basis until they ended up fleeing their home town and changing their names.

    These cases support the imprinting theory (or at least doesn’t contradict it), as those involved wouldn’t have had the opportunity to know each other as siblings as children.

    On an unrelated note, the issue of marrying a sibling is one of those which highlights gross inconsistency in the Old Testament; see here for details.

  2. It seems unfair to punish peeople who didn’t even know before they became involved.

    It becomes even more ethically “interesting” when you consider assisted reproductive techniques.

    In most countries sperm donation is still anonymous – and while there is generally legislation limiting the number of offspring a donor can father, there is a chance that half siblings may marry unknowingly.

    Then you have cases like Dr Jacobson (http://query.nytimes.com/gst/fullpage.html?sec=health&res=9E0CE0DE1F3BF93AA35756C0A964958260)
    who fathered over 80 children, most through using his own semen in sperm donor cases.

  3. Evie

    Marrying you r sister is just wrong

  4. Mike

    I truly believe that if they can’t have children then there is nothing wrong b/c if two people truly love each other then what is the harm of marrying each other. I believe that the laws need to be looked at closer in cases such as I just pointed out.

  5. katelynn:)

    umm yeahh. if you didnt kno prior too your relationship,it shouldnt count in anyway againsts you . its not your fault that you like someone that your related too.like if you grew uo with that person you wouldnt like them that way . but since you didnt grow up with that sibling, your appeal isnt down fer that person,

  6. m

    I wonder how often it happens that people unknowingly marry a half sibling?

    • DWornock

      The odds for any individual is very low but doesn’t imply it is not common. Think of sperm banks. Certain gifted (high intelligence, athletic ability, etc.) sperm donors are many times more likely to be selected than average sperm donors. Therefore, they may have hundreds of biological children within a city or within a few tens of miles of each other. The odds that such half siblings meet and marry is not great for any specific half sibling, but combined it becomes substantial and many times greater when you factor in all the sperm banks within the USA.

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  8. stephen

    September 2, 2007 at 4:02 pm
    Marrying you r sister is just wrong”


    • Scepter

      People like “Evie” say its wrong, but like “stephen” said because?. People don’t seem to have an explanation to this question. They are just as ignorant as people that say gay people shouldn’t be together. Why not? If thats what makes them happy and if they both want to be together then who are we to get in between them?

      • Awesomegal

        Just because it makes you happy doesn’t means it’s right. What you feel is right isn’t always right. Like, having an affair and your SO never knew about it. It’s not hurting anyone but it’s still wrong

    • nikhil

      Marring your half siblings, for me its not a problem. loving some one is not just for sexual relationship. we are connected to a person mentally and then its come to a sexual desire. where we talk on a sexual desire, we can use and have protection from the so called disease. likely for the females they are fond of giving birth, the can be done by test tube treatment. hence a couple can have every thing in the life. healthy sex,mental and child pleasure.And a happy life. its not wrong to love.

      • Amb

        Does anyone dare to bring up the prospect of gay siblings marrying each other? I think someone’s head may explode with the thought.

  9. James

    I personally have the eww factor when thinking about marrying a sister. But, in my pursuit of analytical thinking, I cannot find a reason why it would need to be banned.

    Certainly the biologically implications of reproduction are a concerned; but reproduction is not a requirement for marriage. If it is the reason, then it would be prudent to apply the same restrictions on everybody by requiring genetic tests for marriage.

    Please, if you have a good argument against it, I am willing to hear it.

    • Scepter

      Good one! I am also willing to hear it. The thing is, I doubt anyone does have a good argument against it. People are just ignorant and don’t think for themselves.

      • Yeti

        Why is it necessary for someone to draw the same conclusion in order to avoid being “ignorant”?

        It’s the biological concern for successful procreation by the standard of “survival of the fittest” that suggests gay marriage, or “high risk” marriage (read: high risk for birth defect through probability by sibling marriage) is counter-intuitive, and counter-productive.

        There are many many people on the planet, so procreation isn’t a necessity. But the instinct to have sex is a chemical driven element for the purposes of continuing the human race. Were there a threat, and fewer humans, the obvious would simply be more obvious. It doesn’t mean it’s “ok” just because “you can”. Or, because it’s less obvious that it serves this purpose, and need is low, doesn’t make it “different” than original intended purpose.

        And, to “Duncan Bayne[‘s]” post; The contradiction in that link simply isn’t contextually accurate objection. I can’t find a mind changing. I see a component of time where something wasn’t specifically noted, but not forbidden, then I see a time where it is forbidden. A decision to be made at a later time is not a mind changing. But, a time where a decision was made. Perhaps genetic purity existed for a time; later, imperfection in the blood line through defect prevalence caused it to be necessary to avoid “from hence forth”. Implicit must be understood in light of explicit. Not the other way around. That said, define “cursed”. Cursed (later) genetically by proxy of birth defect could as easily be inferred as the claim made in that so-called contradiction. Science shows that indeed, as this article states, probability of one marrying a sibling as a high probability of producing a “curse”, or, damaged offspring, genetically speaking.

    • Troy

      “…then it would be prudent to apply the same restrictions on everybody by requiring genetic tests for marriage.”

      It was shown by genetics above that by marrying closer relatives, the percentage of inheriting recessive diseased genes increases. And one thing to know is that recessive diseased genes are usually rather rare; if one of your parents(or grandparents) has one, then the chance of you and your sibling(or relative) having is extremely higher relative to marrying non-relatives.

      Then the either going for the incest or not will completely depend on the person. When people marry non-relatives, they have a low statistical risk, so one usually wouldn’t get busy to do a genetic check-up. Because, the risk is very, very, Very, low. But when marrying relatives, since the statistical risk skyrockets, one will have to decide if marrying the relative is a greater priority than Avoiding diseased/disabled children. IF one decides that Avoding diseased/disabled children is a greater priority, or at least decides that it is of some importance, they will go and get a genetic check-up.

      My point is, it should really depend on the person’s world views and philosophies. And the world I suggest should deal with this issue with how some other countries deal with abortion; by supporting the people, but showing the facts alongside.

      But even after all the long text, I wouldn’t mind seeing anyone who decided that this was their way of things. That’s what they wanted, and I see no moral/ethical issues.

  10. Edward

    i totally wouldn’t mind marrying my sister, as long as she’s hawt. lol
    except i dont have a sister. 😦

  11. janaka

    i like to marry may sister. i like her lips. bobs. & ass

  12. Kareena

    I totally agree with Stephen…what’s so wrong about marrying your sibling? I make my characters marry and they are bro and sis.

  13. Abraham

    i love my beautiful sister , and i wish i marry her ,,,

  14. Randy

    Incest has always been an interesting subject for me. Having grown up attracted to my sisters, cousins and a couple of aunts back in my youth.

    So its always interesting to read other peoples take on this. From what I have seen in my life, there are a certain amount of people who would never get into a relationship with any kind of relative, and then there are some people who do, and then there are the ones that just fantasize about it. Fantasy and reality are two very different things and frankly sometimes its better to keep some things on the fantasy level.

    • Scepter

      Well if fantasy can become a reality then why not? If someone fantasizes about being gay, then why not go for it if thats what you feel will make you happy and if that is what you truly want. Honestly I get along better with my half sister than I did with any other female in the past. We have everything in common and like all the same things and we make each other laugh all the time, and we sometimes finish each other sentences, I have the best time with her. If we get along so well then why not peruse something? I will probably treat her better than any other man will. And I think we could have a very stable relationship considering we have such a great friendship already.

  15. pippa

    Just because you fantasize about something does NOT make it okay. Just because you want something does not make it okay. Humans can control themselves. It is what makes us human. Marrying a sibling is gross.

  16. pippa

    And besides–who says there has to be a scientific/medical reason? Wrong can simply be wrong. Yes–some things ARE black and white.

  17. Ezeunachukwu samuel.c

    From what marriage means,incest ar supposed to be legal

  18. Logical Mind

    All of you morons saying its okay just because there is no reason why its wrong need to shut up. Just because there is no reason why nudity is wrong, doesn’t mean I should be allowed to walk around outside with my dick hanging out. Some things aren’t necessarily wrong, but are inappropriate. I highly doubt over 90% of the world would say fucking your sibling is wrong if there really was no problem. Its just disgusting. Whether you make a baby or not. Humans have evolved for a reason. To know one another and be social. Fucking your own blood is not being social. And for those of you who think I’m ignorant, answer this. Is it wrong for me to walk around in public naked in front of people? Even children? If it is wrong, then tell me why. There is no reason, beyond it CAN corrupt the minds of people and it is what? Disgusting. Some things are gross just because they’re gross. Why is a bad smell bad to smell? Just because you don’t like it? Don’t be stupid. Its very simple. Fuck people outside your family, its better for you and its better for your family.

    • Bob

      Are not seeing this u will have fucked up children

    • Scott

      If the likelihood of passing on repressed genes increases by marrying and reproducing within your family, does not the likelihood of passing on desireable genes within the family increase as well?

    • Troy

      Lol nudity would bother some people sexually (and most people just ethically), that is why it is unethical. And nobody, wants, to be, bothered. It just reduces productivity or rest. And as far as I know, it is illegal too; well maybe at least in some countries. Same logic applies for banning loud noise at night(in my country it is illegal(and don’t you dare say the lawmakers are just stupid)), it Bothers people. Bad smells are bad because the brain either does not sense any apparent chemical/biological advantages from ingesting that, or does see a chemical/biological disadvantage in ingesting/inhaling it. These cases, have reasons to be avoided.

      Statistics do show something of a ‘reason’ to avoid incest. But it really should depend on the person who is to decide. It’s all about priorities, and happiness. If you can find my other comment, please try reading it. Should be noticeable, it’s a rather long one.

      And woah, not being social? You have to lay out your assumptions first. You’ve got plenty that are illogical, Mr. logical mind.

      Well, it is an old comment and things might’ve changed, so I’ll apologize for any unnecessary offending words.

  19. Sharif ansar

    I would luv to fuck my sis if i had one!!!! But i would like to fuck kareena!!!!

  20. rosatie

    why dont u fuck urs mom instead of urs sis doesnt she has boobs.ass,etc

  21. rosatie

    curse on sister and brother mariages

  22. santhosh

    I have a cousin sister I really love her very much I really want to marry her is it legal does she marry me

  23. Samuel

    Marry whoever you want and fuck whoever. Who cares about society? Whatever you do, society will always try to screw you, so you can ask them suck up.

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  27. Henrique

    So much hate speech, well it’s kinda understandable that people would be against. It’s hard to someone accept that their view of the black and white world can be…well…wrong and that they should start minding their own business and that their life and their issues should be more important than some guy marrying his sister or something like this. You think it’s gross, well nice for you, guess what you don’t have to say…be polite and don’t threat badly someone that’s minding their own business, doing no harm to you, keep those hatred feelings and words to yourself you know BE POLITE, you don’t have to agree but you also don’t have to disagree…They are happy? well be happy that they are happy, or don’t…whatever keep to yourself. As the medical/scientific implications…well a higher chance isn’t the same as YOU ARE GOING TO HAVE…I’m left handed, which means that I have a higher chance THAN A RIGHT HANDED to develop schizophrenia, but guess what I don’t have it…I’m completely fine. So siblings should be allowed to (which is different from encouraged note that) do whatever the hell they want with their lives, and they should be allowed to have children, after all we DO have technology to practically rule out the chances of genetic problems. Hell even couples that are not siblings have their chances of having a child with some kind of genetic problem, so with the adequate tests and exams it’s possible to get a 100% perfect offspring just like any other couple would, and it’s perfectly safe even without those technologies. Well of course there are many people in the world, but sometimes you don’t have to star a journey and travel the whole world to find a precious thing, sometimes it’s right at the grasp of your hands…you know everyone has their perfect match, some just happen to be a relative of a sibling, just that…simply as that…judge less and live more…

    And for any of you that may be thinking that I’m in such a situation, guess what…I’m not in fact I don’t even have a sister…I’ve just enlightened myself enough to know that I couldn’t care less if some guy is doing X or Y with his sister, or if some girl likes other girls and anything like that. I’ll just let them be, and hope the have a happy life and all that stuff…now if you excuse me I have things to do and places to be…

  28. Kris

    If you think that it is wrong, then don’t do it. But don’t tell me that I should not be doing it!! It is my life and I will enjoy the way I like it. Life is short baby!!! Enjoy or perish.

  29. Abdul

    I Totally agree with logical mind, there is must be difference between animal & human.
    Respect you family

    • Wayne

      For all of you who think it’s ok to marry your siblings because it’s all about “love”, then I guess it’s ok to marry my mother, grandmother, father, grandfather, etc. It will all be the same wouldn’t it?

  30. Pingback: Mother, daughter arrested after alleged incestuous marriage – Southern Nation News

  31. DWornock

    The genetic risk of marrying and having children with a relative depends on how closely related they are. On average, the genetic risk for first cousins (12.5% related) is 2% higher (5% vs 3%) than for unrelated individuals. The genetic risk for second cousins (6.25%) is considered to be no greater than unrelated individuals. I couldn’t find any definite genetic risk for uncle/niece, double first cousins, or half siblings (25% related). However, one web site indicated the genetic risk is 4% to 8% greater than for unrelated individuals. If so, then the genetic risk for full siblings or parent child (50% related) is substantial.

    Therefore, unless there is a common occurrence of a genetic defect within the family, there is no reason to be concerned about cousin marriages. Nevertheless, first cousins may wish to get genetic counseling, especially if they are aware of some genetic defect within their family and close relatives.

  32. Tourist

    Actually I don’t have a reason why it’s wrong. But frm my concept, I think it’s fun to check out behaviors of other people frm different family, if u get married eventually from another family, d families become larger, and you get to meet new people. It’s like going on tour to other countries, but it’s not as if your country doesn’t av a tourist destination.
    Another reason is this, that means you don’t need to pay her dowry, she’s your sister, you wud b paying her dowry to your mum n dad. What am saying is just get married outside the box, it’s fun to experience new people n meet new people.

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