Why inbreeding is a bad idea

It seems like humanity is particularly sensitive to inbreeding.  If so it is because evolution has changed our ancestors too quickly.  The locomotor system has changed considerably in 3.5 million years.  Our ancestors went from mostly swinging trees to walking bipedally on the ground.  Later but overlapping the size of the brain has grown enormously.  It has become 4.5 times as big in 3 million years.  Such a quick transformation has a great deal of by-effects.  This could also explain our relatively weak immune system.  We are more sensitive to contagious diseases than many other mammals.

Hereditary diseases are not something only some are carrying.  All humans carry hereditary characters for several different such.  However, one usually needs to have two of the relevant gene to have the disease.  Few people have more than one of such a harmful gene.  Moreover there are hundreds of different ones.  People not being closely related for this reason rarely carry the same.  This means a low risk of getting one from each parent.  So the risk of having a child with a hereditary disease is normally not particularly high.

Consanguinity is something gradual.  Parents and children share half of humanity’s variable genes.  On average this applies to full siblings too.  Halfsiblings on average share 1/3 of their genes.  As a rule of thumb one can say that genetic similarity decrease by half for each degree of relationship.  If they are not halfsiblings for then it is 2/3.  The person’s sex is unimportant since the difference is minimal.  All this only applies to biological relatives.  In-laws and adopted are irrelevant in context.

This means people share 1/4 of their genes with their parents’ siblings.  As such we share this with aunts and uncles.  With first cousins we share 1/8.  If they are not double first cousins which share 1/4.  Such individuals are first cousins on both their father’s and their mother’s side.  On the other hand half-cousins only share 1/12 of their genes.  With our first cousin’s children we share 1/16 of humanity’s variable genes.  Second cousins share as little as 1/32.  This presupposes a society were reproduction between known relatives are not particularly common.  If cousin marriages are common similarity increases over time.  This makes it more and more risky to continue such a tradition.  But people don’t always realise this due to myths about hereditability.

The Spanish branch of the Habsburgs did not only practice cousin marriage.  They also had several marriages between sibling’s children and parent’s siblings.  The degree of inbreeding increased over time with disastrous consequences.  Felipe IV and Maria Anna were uncle and nice.  However, they were at least as genetically similar as full siblings.  (For one of their ancestors the fatherhood is uncertain.)  Out of their five children two survived.  Their son Carlos was malformed and was born with syphilis.  Their daughter Margarita was older and did not have this disease.  Out of her four children only Maria Antonia survived.  All other children died as did all her daughter’s children.  Carlos never managed to have any children so the branch went extinct with him.  Both the royal house of France and a different branch of the Habsburgs then claimed the throne.  The resulting war was eventually won by France.  Since then all of Spain’s kings except one have had de Capet as their surname.

There are people maintaining those things are not such a large problem anymore because groups are less inbred.  I dare to say such risks still exist today.  Reproducing with siblings or siblings’ children is still a bad idea.  Not to talk about the exploitive in incest between parents and children.  Also in the other cases it may be exploitive.  But it does not have to be so.  I know about a modern case of sibling incest in Germany.  They did not know they were siblings since they were raised separately.  Unknowing about their close consanguinity they married each other.  If I remember it correctly more than half of their children were handicapped.  Please note present-day Germans are not particularly inbred.  This applies to most of present-day Europeans.

Such documented cases can teach us on the limits of inbreeding.  Reproducing with full or halfsiblings generation after generation simply does not work.  Too many children would have died or themselves not have been able to have children.  Male sterility is a common effect of inbreeding in humans.  Female is considerably less common.  Many believe the pharaohs used to marry their sisters.  However, I think this was a metaphor which has been taken literarily.  Anyway, the pharaohs were married to several women at the same time.  As such they had children within marriage with at least two unrelated women.  Or at least with women being more distantly related.  This provided enough genetic variation for the survival of the dynasty.  Otherwise each of them would have gone extinct in 3 – 4 generations.

In modern times there are clans, casts and metaphorical brotherhoods.  People oftentimes feel obligated to marry within their group.  For the casts of India this is usually not a problem.  They are descended from occupational groups and were formalised during colonialism.  Moreover Hinduism prohibits marriages between first cousins and half-cousins.  In contrast this is a problem in the neighbouring Pakistan.  Members of metaphorical brotherhoods promote marriage within them.  The result is Pakistan having the world’s highest rate of cousin marriage.  But Afghanistan, West Asia and North Africa is not far behind.  There it is a matter of clans believed to have collective character.  It is a common type of prejudice in those parts of the world.  To uphold the clan’s reputation, social status and economy many marry within it.  Which in the long run can cause problems.

Finally there are ethnic groups being inbred as a whole.  There are ethnic minorities descended almost entirely from a too small number of founders.  Such are much affected by hereditary diseases.  Some countries’ majorities can suffer to less extent.  Finns have an overrepresentation of hereditary diseases.  This despite cousin marriage being taboo among the Finns.  There it instead is about a small founding population.  Same applies to Iceland.  Moreover the Icelandic have suffered repeated bottlenecks.  Large volcanic eruptions have led to a considerable part of the population dying.  This has been compensated for by a higher immigration than what Finland has had.

 

Uploaded on the 25th of June 2025.