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!

References

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)