Many people have probably heard of this handsome fella, Charles II, King of Spain. He’s pretty much known as “that really inbred king”:

His parents were uncle and niece. His father’s parents were first cousins, once removed. His mother’s parents were cousins. And this continues for some way up the family tree.

There’s plenty of information out there about this guy. Needless to say he was pretty profoundly disabled. But we’re not here to discuss how his tongue didn’t fit into his mouth or how he drooled incessantly, we’re here to do some data analysis!

I’d like to investigate: is Charles II the most inbred royal out there? Is his inbreeding exceptional? And just how inbred are royals, in general?

Getting Started

I decided to look at the amount of inbreeding in relatively well defined, stable monarchies. I limited it to just five countries: Britain, Spain, France, Russia, and Portugal. This is absolutely a limited sample, but a trip down Wikipedia Lane was enough to convince me any more would make the project a lot less manageable.

As a starting point, I began putting together a family tree for the monarchs of Britain, but luckily found the wonderful website Roglo before I fell too far down that particular rabbit hole. Roglo is a database of over 7 million individuals based off the ancestry software Geneweb. You can get extremely detailed family trees for most historical figures. It’ll also tell you the coefficient of relationship, or exactly how related two given people are, expressed as a percentage. See here for a more detailed explanation on how that can be calculated.

If you know the coefficient of relationship between two people, you can then divide it by two to estimate the coefficient of inbreeding (F) for their offspring.

Overall Results

(Note: the data in this post can be found here)

With that method, I made the following chart to quantify how inbred the monarchs of our selected countries are. The creepy relationship cutoff comes from a very weird informal poll of my friends and family.

The median monarch is a bit more inbred than the offspring of third cousins (F = 1.86). The average monarch is more inbred than the offspring of second cousins (F = 3.47), and the standard deviation is 4.9.

Results by Country

We can look at it by individual monarch:

Charles II is a very exceptional case: he is more inbred than the offspring of a brother/sister union! However, he is less inbred than Alfonso XII, a totally unknown contender.

Alfonso’s parents were first cousins and both his sets of grandparents were uncle and niece. He did get lucky, though, and it seems that he was neither hideously ugly nor massively impaired by his inbreeding. He did try to get some new blood in by marrying his third cousin–guess that’s what counts as unrelated in the Spanish royal family.

We can see that Spain and Portugal had a lot of funky stuff going on. It’s hard to tell because all the dots are stacked on top of one another, but most Russian monarchs since Ivan the Terrible were actually born to unrelated parents. Britain and France land somewhere in the middle.

Results Over Time

We can also take a look at inbreeding over time:

Things really got going in the 1400s! But the more modern monarchs in my sample seem to be acceptably inbred.

Results for Current Monarchs

Which raises the question: all this historical inbreeding is all well and good, but what’s going on with our current monarchs?

Let’s take a look:

By modern standards it looks like most of our current monarchs are a-ok. They’re likely more inbred than your average Joe, but not creepily so! (King Harald excluded. Sorry, Norway)

And there you have it.

Was Charles II the most inbred? Nope. Was he more inbred than average? Very much so. How inbred are royals in general? Relatively! But there’s a very wide spread.