Why is that guy over there the king?
Because his father was king? How fair is that? His father was a good man, while he is an evil bastard. Why should he rule? It’s not fair.
Of course it’s not fair, but that’s not how human societies are arranged. Human societies are organized over Schelling points. Primogeniture is a Schelling point. Monarchy is a Schelling point. And primogeniture monarchy is another Schelling point. Schelling points happen through trial and error. Lots of error really. And they don’t assure there won’t be any more error. But given the huge and numerous constraints which exist, this was the best arrangement possible.
Schelling points are fixed in place not because of an explicit understanding of their origin and importance. They just stumble upon existence, after which the people involved come up with some bullshit that sounds right. Or may I rephrase, people see Schelling points and come up with elaborated post-rationalizations to justify them. How do you justify that some kid, who has done nothing of merit in his life, who is ugly, dumb, clumsy and of bad character, becomes king just because his father was? Well I don’t know. But it’s always been like that, so if sons have a right to the kingship… well, it must be about blood. Yes, the bloodline. That’s it. It is of utmost importance that all the kings must be of the same blood as the previous king, as this is the blood of the founder, which was awesome. We can’t afford losing this awesome blood. It doesn’t matter that the lawful heir of this bloodline is ugly, dumb, clumsy and of bad character. That’s… uh… the fault of his teachers. Yes, bad teachers. The kid shares the bloodline so he must be king.
On the face of it the theory is quite stupid. Let’s assume blood alone makes you awesome. Even if you keep the father’s line, sons have mothers too, so the ‘bloodline’ gets diluted each time. By the 5th generation the new king shares very few genes with the great founder. And that’s if you are lucky and you don’t have a slutty queen who fucks someone else and fathers his children. Surely someone must have noticed this fact, but of course the solution to it is even worse. You keep using women from inside the family to avoid diluting the precious bloodline, and what you get is a monster. So you gotta worship the bloodline while doing all you can to dilute it. Hypocrisy is by no means an innovation of our times.
Dynasties in most of the world changed quite often, especially in Europe. So the common rationalization for the legitimacy of kings was not so much the bloodline, as simply the law. There are inheritance laws that say who has titles, and sometimes kings agree to leave the crown to some guy. And law is sacred. Most of the time anyway, European history is full of succession wars where people didn’t quite agree on the sacredness of laws, and everybody with a big army always found a plausible legal claim to the throne they wanted. But the emphasis was still in the law, which is a funny thing to take as sacred. Surely there are more important things that some agreement reached at some point of time, in some state of mind, by some old guy who didn’t really know what he was doing. But hey, another Schelling point, can’t touch that. Perhaps the famous legalism of Europeans, which is quite distinct to other civilizations, comes from the fact that we had no other concept in which to base our politics.