How many people within a given population could reasonably be "noble" without inbreeding becoming an issue?

I have searched around the site and seen several population-related questions, but not one that is getting at what I'm specifically wondering (or that is at least explaining it in a way that my mathematically challenged brain can get around). Apologies if I missed one though.

So I have a kingdom with a population of approximately a million people in 24,000 sq miles. The kingdom has existed for approximately 1600 years. This kingdom is probably analogous, roughly, to medieval Europe in the 1400s. I've worked out that there are 159 noble families within this population. (using this site and tweaking things here and there: http://donjon.bin.sh/fantasy/demographics/)

Due to various events, the nobility is predominately killed off, save for a single child from each family (some families without children and some mishaps meaning not every family), leaving 113 children of the noble class left. The population is basically human for all intents and purposes, but there is magic-type element at play that manifests in one member of this noble class at a time and does not pass directly from parent to child-- it just appears to randomly select a new host within the group when the previous has died.

I am trying to figure out if:

159 noble families is enough that there wouldn't be significant risk of inbreeding through the 1600 year history (safe to assume the number of noble families may fluctuate throughout the centuries, but 159 is the number prior to them being massacred).

And how many individuals would be a part of these 159 families, realistically. Again, overall population is around 1 million.

ETA: second part question removed because it's a moot point and I just blanked on an obvious point in my story. haha

Thanks!

• If a nobleman sleeps with a "normal" human, is the illegitimate offspring considered noble or not for purposes of the magic? Nobles aren't notably discriminating with their seed. If two such illegitimate children interbreed, can they recapitulate the minimum magic requirement? Does the magic depend on percentages of noble genetics? Is there a way to test for actual nobility (so the Duchess that sleeps with her bodyguard doesn't have heirs to the throne that are non-noble?) May 28, 2020 at 22:10
• In-universe, the nobility would not believe illegitimate children as viable vessels for the magic and none of the individuals who have had it in the past 1600 years have been outside the nobility or known to be illiegitimate. However, does the magic actually require a certain level of blood to receive it...no. The magi is effectively given by another being that has chosen the nobility of this particular people, or rather the first person chosen belonged to that group, and then they just stuck with it. But this isn't common knowledge that would inform choices in-universe. May 28, 2020 at 22:18
• centuries later, heart as big as peppercorn, single testicle and black as coal, corroded lung, intestine rotten and gangrenous, head full of water, short, lame, epileptic, senile and completely bald before 35, always on the verge of death but repeatedly baffling Christendom by continuing to live......but hey, you are a wizard El Hechizado....and at least you has the best chin! May 29, 2020 at 2:36
• What if the noble married a "normal" human? Is the child considered noble? Can "normal" humans be raised to the nobility? May 29, 2020 at 16:41

If we think of the nobles as an isolated (by choice) population, then what we need to look at is the human Minimum Viable Population (MVP).

From this question about a sustainable space colony, we get this source which says the number is 160, or maybe 80.

The “magic number” of people needed to create a viable population for multi-generational space travel has been calculated by researchers. It is about the size of a small village – 160. But with some social engineering it might even be possible to halve this to 80.

This source argues that 98 is fine, if we're only talking about genetic diversity and assume no other catastrophe.

The survival of a genetically healthy multi-generational crew is of a prime concern when dealing with space travel. It has been shown that determining a realistic population size is tricky as many parameters (such as infertility, inbreeding, sudden deaths, accidents or random events) come into play. To evaluate the impact of those parameters, Monte Carlo simulations are among the best methods since they allow testing of all possible scenarios and determine, by numerous iterations, which are the most likely. This is why we use the Monte Carlo code HERITAGE to estimate the minimal crew for a multi-generational space travel towards Proxima Centauri b. By allowing the crew to evolve under a list of adaptive social engineering principles (namely yearly evaluations of the vessel population, offspring restrictions and breeding constraints), we show in this paper that it is possible to create and maintain a healthy population virtually indefinitely. A initial amount of 25 breeding pairs of settlers drives the mission towards extinction in 50 +/- 15% of cases if we completely forbid inbreeding. Under the set of parameters described in this publication, we find that a minimum crew of 98 people is necessary ensure a 100% success rate for a 6300-year space travel towards the closest telluric exoplanet known so far.

So your population of 113 might just be viable if they breed very carefully, assuming they have a near 50-50 male to female split. And of course, they aren't on a spaceship; their genetic isolation is voluntary. So they still have the option to marry the occasional maid or whatever and improve the situation.

And regarding the pre-massacre period, if ~100 individuals can be made to work with careful planning, then 159 families should be able to get on just fine as long as they avoid obvious inbreeding.

• Good answer, but probably worth adding that the gender ratio of the 113 children matters - if there is not a 1:1 ratio (obviously not exactly) of male to female then the problem becomes stickier. May 28, 2020 at 23:07
• It's a good thing you pointed this gender ratio out because it made me realize the whole point is moot because, with a single exception, it is only females that were left alive. So they obviously have to breed out. Ha, completely spaced that aspect when writing up my question. I will go edit my original ask. May 28, 2020 at 23:18
• @taijhin I suddenly see a large problem related to medieval societies: All 113 of these nobles are children. Who is going to handle their families' responsibilities for them? Who is going to be their equivalent of regent (I know that's for kings, I don't know the title for your nobles). Frankly, I expect to see only a handful of these children surviving to be able to breed without having the gods change human nature. May 29, 2020 at 7:28
• @NomadMaker "Regent" would still be the correct term here. When talking about them obliquely, this may be prepended with the Noble rank (e.g. the Regent for a Duchy would be a "ducal regent", with their young ward being the Duke / Duchess) In absence of a living elder relative, this could be the family's Majordomo / Seneschal (who, let's be honest, probably did most of the work and decision-making already, just without a target on their back!) - getting the position would mean they had probably been with the family enough to develop reasonable loyalty. Basically, a bunch of Alfred Pennyworths May 29, 2020 at 14:22
• @taijhin Actually, your scenario of 1 boy and 112 girls is - from a strictly biological standpoint mind you - far better than the reverse of that. As long as you allow polyamory, one man can impregnate many women in a short time, while a women may (with rare exceptions) be pregnant by only one man at a time. Of course, all of the next generation would be half-siblings, but in the reverse scenario (1 girl, 112 boys), there might not even be a next generation to speak of. (An even split would probably be better though.) May 29, 2020 at 14:40

Nonpaternity saves the day.

Consider the Duke. He is a jerk. He is rough and mean, and prefers the company of prostitutes over the Duchess, who does not really know what she is doing in bed. A large part of that is lack of enthusiasm on her part - the Duchess does not like the Duke, their marriage having been arranged by their family.

The Duchess does like her coachman, who is attentive and kind.

The Duchess bears an heir! He looks a lot like the Duchess. There is not genetic testing. The Duke is satisfied that his heir is a healthy baby boy and does not give the matter a lot more thought.

The coachman remains attentive and kind. He teaches the young marquess how to ride, and hunt, and how to not be a jerk.

Not necessarily a full answer, but worth considering: many of the "horror stories" result from the historical example of the Hapsburgs.

However, that particular situation was exacerbated by the fact that "unfit" genes were kept in the pool by dint of the people possessing them being Hapsburgs, even if they would otherwise have died childless in an unregulated scenario, causing a build-up of recessive defects. Once these were in the main line of inheritance, it escalated in a "snowball effect" (i.e. at least one parent was almost guaranteed to carry the "bad genes").

If Magic is a factor here, it can correct for that simply by excluding these cases from consideration for inheriting the "Family Magic". This does not eliminate the risks of a small gene-pool, but it does help to eliminate the political power of genetically-weak lines, and ensure that at least one parent doesn't carry the "bad genes"

(And, occasionally, that obscure branch-family who were having to stoop to marrying commoners suddenly becomes the new Main Line when their child manifest the Magic)

• The key thing about Charles II of Spain is that he was effectively the result of inbreeding with a starting population of six people -- and three of those six were siblings. It's a very different situation from a starting population of 113 vaguely-related individuals.
– Mark
May 30, 2020 at 2:07

While the answer based on space travel by Harabeck is excellent, your scenerio has more in common with a population bottleneck because there is no need to maintain the population limit of 168 past the event which kills off most family members. The population of nobility will naturally begin to grow again afterwards.

Population bottlenecks permanently alter the genetic diversity of a population. To quote the Wikipedia article

The chances of inbreeding and genetic homogeneity can increase, possibly leading to inbreeding depression. Smaller population size can also cause deleterious mutations to accumulate.

However, they can also select for genes that help the population survive the disaster which triggers the bottleneck. For example, a previous bottleneck event might explain why the nobility has magic.

As to the size of a human population which can survive a bottleneck event, a study suggests that pre-1492 North Americans were all descendants of 70 individuals

The previous answers pointed out that, with the initial number of mostly unrelated individuals you are starting out with, that long term viability is plausible.

I would like to point out that, depending on what 'magic' can do, you can turn plausible into a certainty.

With technology (genetic testing), it's easy to detect when two particular individuals are unsafe to breed (by comparing genes); the chief problem we moderns would have would be the ethics (and whether two incompatible individuals in love would actually abide by our pronouncements).

Now, you don't have technology, you have magic. Magic could plausibly do what technology does (even if it's not spelled out); just have the couples undergo a magical ceremony by the magical patriarch/priest/wizard to see if the pairing is 'blessed'.

Of course, if you (for story purposes) want to have severe inbreeding problems, you can still have this magical ceremony, but just have it have the opposite effect; blessing those couples that actually do share recessive genes...