If my understanding of history is correct, it was common for much of European history for nobles and royalty to need to keep the blood 'pure' by marrying only people of similar standing. This resulted in all the potential marriage options often being first or second cousins in some manner due to a low 'breeding stock' existing, too few people deemed high enough in rank to be in consideration of marriage.

I'm wondering how aware the nobility was of consanguinity and its downsides. I'm pretty sure there was still a taboo against direct incest, but how aware were they of the harm that came from constantly intermingling the same limited genetics over and over again? Put another way, were they aware that a marriage between 2 cousins amongst the ruling elite was more dangerous then it would be amongst the peasants due to the significant amount of genetics everyone shared due to the repeated inbreeding over generations?

Did any of them realize they were rendering themselves more prone to all kinds of genetic disorders then the 'lower' class through this policy? Did anyone suggest that occasional marriages to lower nobility, or even merchant class, individuals in order to bring in fresh blood may be a wise idea?

In short, I'm wondering how much the nobility were aware of the risk that came with only marrying other nobility and whether they simply chose to ignore it due to political gains or simple tradition, and how much were they just not aware of the danger they were putting themselves and their children in by the practice?

If they were aware of the risk, what was the primary motivation for taking that risk? Was it purely political, the need to ensure alliances; or was it about tradition and the claim that they were somehow superior to others and thus too good to marry them?

I'm tagging this as middle ages but I'm honestly interested in the views as they adjusted over time as well.

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    It should be noted that consanguinity was a problem just for a few families, like the Habsburgs. The pool of nobles was large enough so that marrying other nobles wasn't going to cause genetic problems. Marrying repeatedly just between two branches of the same family - as the Habsburgs did - is a bit different.
    – Pere
    Oct 15, 2021 at 20:06
  • 5
    Medievals knew about animal breeding and the risks -- and benefits -- of inbreeding. I suggest that you look at that and to what degree they applied that knowledge to themselves.
    – Mark Olson
    Oct 15, 2021 at 20:54
  • 2
    The question suggests the assumption that it happened only in Europe. For long, similar practices were/are common (e.g. pharaohs, Japanese emperors). Since to genetic diversity of the human race is relatively low, inbreeding of specific bloodlines is a smaller issue than most think. Only two things might be unique to Europe: relatively long-living royal families and monogamy.
    – Greg
    Oct 18, 2021 at 5:23
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    According to scholarworks.sjsu.edu/cgi/…, from 990 to 1150, the Church take consanguinity as a very serious matter, relaxed the prohibitions after 1150 and after 1215 allowed marriages with high consanguinity "into closer conformity with prevailing marriage practices." Oct 19, 2021 at 9:18
  • 1
    As late as the 17th century, Philip IV of Spain married his own niece.
    – Spencer
    Jun 15, 2022 at 21:52

3 Answers 3


Well, any sound answer should specify WHEN.

According to ThoughtCo.com, marriage between persons related within four degrees of consanguinity violated the civil code. Until the 13th century (same source), such marriage was banned within 7 degrees.

The degrees are (same source)

  • The first degree of kinship includes: parents and children (direct line)
  • The second degree of kinship includes: brothers and sisters; grandparents and grandchildren (direct line)
  • The third degree of kinship includes: uncles/aunts and nieces/nephews; great-grandchildren and great-grandparents (direct line)
  • The fourth degree of kinship includes: first cousins (children sharing a pair of common grandparents); great uncles/great aunts and grand nephews/grand nieces; great grandchildren and great grandparents
  • The fifth degree of kinship includes: first cousins once removed; great grand nephews/great grand nieces and great grand uncles/great grand aunts
  • The sixth degree of kinship includes: second cousins; first cousins twice removed
  • The seventh degree of kinship includes: second cousins once removed; first cousins three times removed
  • The eighth degree of kinship includes: third cousins; second cousins twice removed; first cousins four times removed

The Pope could grant exception to the rule. In fact, that was what Henry VIII wanted Pope Clement to grant an annulment from his marriage to Catherine of Aragon so that he could marry Anne Boleyn (in that case, it was 4 degrees of consanguinity, but Clement wasn't about to do that and sent Henry a letter forbidding him to marry anyone while Catherine lived. Next stop: The Church of England.see https://www.history.com/this-day-in-history/pope-clement-vii-forbids-king-henry-viii-from-remarrying

Now, to what extent laypersons - or even those learned in "science" - understood precisely WHY such marriages were a bad idea, I suppose depends upon whom we're talking about. Biology was largely a matter of superstition until... the 17th century or so.

  • 2
    "marriage was banned within 7 degrees." -banned by whom and where? Jun 15, 2022 at 22:06

It might be worth noting that marriage was, for most of history, a matter of property management in many areas of the planet.

  • 3
    Hi. Once you have a little rep (One good answer should do it), you can comment anywhere on History SE.
    – Spencer
    Jun 15, 2022 at 21:56
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    I'm not sure this answers the question; I think you could expand it to answer the question and to provide sources to support the argument.
    – MCW
    Jun 16, 2022 at 0:26

Genetics was discovered in the 19th and 20th centuries.

Half of an organism's genes come from its male parent and half from its female parent.

Various genes can be good or bad for someone's health. If a gene is dominant, it will be expressed and effective if someone gets it only one parent. If a gene is recessive, it will be expressed and effective if someone gets it from both parents but it will not be expressed or effective if someone gets it from only one parent.

If someone gets one or two copies of a bad dominant gene, they will suffer. If someone gets one copy of a bad recessive gene, it will not affect them. If they get two copies of a bade recessive gene, one from each parent, they will suffer.

The more closely people are related, the more genes they have in common. That includes bad recessive genes. So if someone's parents are closely related, they are more likely to get two copies each of bad recessive genes and suffer from them.

Of course human genetics was not understood until the 19th and 20th centuries.

In ancient and medieval times, there was a medical theory that all of a person's genes came from the father, despite the fact that many children did not outwardly resemble their fathers but did resemble their mother's. It was also believed that it was possible for two or more men to contribute genes to a child if they had sex with the mother at about the same time.

According to legend Merovech or Meroveus, a 5th century Frankish king, was the son of the wife of King Clodio and a river giant in the form of a bull who raped her. And yet some royal families traced their ancestry back to the royal family of Troy though Meroveus, despite the fact that Meroveus's father would have been 100 percent river giant. According to medieval ideas, Meroveus would have been half human because half his genes came from the river giant and half from king Clodio. While modern people (if they believed the story) would say that Meroveus was half human because half his genes came from Clodio's human wife and half from the river giant.

So there wasn't a very strong knowledge of genetics in the Middle Ages in Europe.

There is a natural tendency in humans to not be sexually attracted to people that they have been raised from childhood with. And that could have arisen through natural selection. People who didn't to have sex with their siblings would have children a little less likely to inherit bad recessive genes from both parents and so those children would be a little more likely to survive long enough to have children of there own.

So over thousands of generations humans evolved a repugnance and reluctance to have sex with their siblings.

And that might possibly be all the biological basis for incest taboos. It is possible that all other ideas of what relationships would be incestuous are purely cultural and based on various cultural trends.

I think that according to biology, relationships closer than 2nd cousins have an increased probability of inheriting bad genes, while relationships between 2nd cousins do not increase the probability of inheriting bad genes.

And many societies have been okay with marriages between first cousins, and other societies have prohibited much more distant relationships.

Christian clergymen gained a lot of control over marriage in Christian society. They gradually made it more and more against the rules for illegitimate children to inherit property, and gained more and more control about who would allowed to marry who.

And of course different medieval Christian sects had different rules on which relationships were incestuous and thus forbidden.

The Roman Catholic Church in western and central Europe gradually prohibited marriages between people who were more and more distantly related. If two persons within the prohibited degrees wished to marry, they had to get (and pay for) a dispensation, which specified each and every one of the prohibited relationship they had.

And I think that it was a custom for kings and nobles not to mention one of their prohibited degrees of relationship when applying for a dispensation. So if they wanted to get out of a marriage, they could suddenly "discover" that prohibited relationship decreed null and void - paying a fee, of course.

I once read that about 990 King Hugh Capet of France sent a letter to the eastern Roman or 'Byzantine" emperor, requesting an imperial daughter as a bride for his son king Robert II, and said that he want the daughter of a monarch to be his son's wife but couldn't find any that were not related too closely to Robert.

And that was a problem in Catholic Europe in the Middle Ages. The number of kingdoms, and thus the number of kings who had available daughters, was small.

A number of kingdoms were created and destroyed during the Middle Ages, and it was common for two or more kingdoms to be inherited by a single man. And sometime if a king had two or more kingdoms his sons divide them after he died.

So the total number of separate Catholic royal families on the mainland of Europe and islands in the Mediterranean, plus the 3 Scandinavian kingdoms, and the Kingdoms of England and Scotland in the British Isles, was about 10 to 15 during the Middle Ages. Political changes during the 19th and 20th centuries resulted in the number of European monarchies increasing to over 20 and then dropping to 7 kingdoms, 2 principalities, and 1 grand duchy.

There were a number of other Catholic kingdoms in Medieval Europe. There were a few Kingdoms in Wales, and about 50 to 200 little kingdoms in Ireland. But apparently they were outside of the potential marriage market for the other Catholic monarchs in Medieval Europe.

So as a result, I guess that about half the kings in Medieval Europe married the daughters of other kings, and about half married the daughters of powerful nobles like dukes and counts. During the Renaissance, European royalty started to exclusively marry European royalty instead of nobles, and became a closed caste. But that was only possible because the powerful princely families in the Holy Roman Empire were considered to be equal to royalty for marriage purposes, while other nobles were not.

  • 4
    Members of all royal families in Europe were quite content to marry into any of the few hundred sovereign families of the Holy Roman Empire; and examples are trivial to find: Victoria married Albert of Saxe-Cobourg-Gotha; Peter of Russia married Sophie of Anhalt-Zerbst, later Catharine the Great; Maria Theresa Walburga Amalia Christina of Habsburg married Franz Stefan, Duke of Lorraine and Grand Duke of Tuscany and Nikolai II Alexandrovich Romano married Alix of Hesse. Oct 15, 2021 at 20:16
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    Can you clarify this sentence: "The Roman catholic Church in western and central Europe prohibited more and more distant relationships. " Oct 15, 2021 at 20:36
  • 1
    @PieterGeerkens in the case of Nikolai II and Alix , it is worth pointing out that the genetic disorder that affected their son was not directly due to inbreeding (although its recurrence in the royal families certainly was). Also, Alix was a granddaughter of queen Victoria - not such a low nobility after all.
    – Roger V.
    Oct 16, 2021 at 5:50
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    "I think that according to biology, relationships closer than 2nd cousins have an increased probability of inheriting bad genes, while relationships between 2nd cousins do not increase the probability of inheriting bad genes." This is a serious misunderstanding of the way consanguinity affects genetics.
    – Evargalo
    Oct 19, 2021 at 7:08
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    "the total number of separate Catholic royal families .. was about 10 to 15 during the Middle Ages" which is plenty of more than other regions. Considering that the archdukes of the Holly Empire and high nobility in most countries were eligible candidates to be kings, this number seems pretty low. Hungary alone had 10 royal families throughout history (most of them from outside, but you get the point), Poland had Piast, Přemyslid, Jagello, etc.
    – Greg
    Oct 20, 2021 at 2:27

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