Okay, so a kingdom has this rule: Whoever is the oldest person who was the son of a previous king is the king. This mostly passes the throne between brothers and cousins. My question is, how do you prevent them from killing each other?
Here are the problems:
- If your dad is close to becoming king, but will likely die before so, you, your brothers, your sons, and your nephews all want to kill the current king. (If your dad dies before becoming a king, you can never become a king.)
- If you someone is about to become king, and his kids are a little older than you, you want to kill him before he becomes king (along with anyone else in your position, and your descendants).
- If you do not like a certain branch of the royal family (like, say, if you're the king and they are trying to kill you), you have an incentive to kill the patriarch (common living male ancestor) of it. Sure, they may want revenge, but any designs on becoming the king they had is now lost, and they lose a whole lot of political power.
- If the king and the next in line die around the same time (say in the same battle), it will be up to debate whether that guy was king before he died or not. This will lead to divisions in the kingdom.
- If record keeping for births is not good, it might not be clear who two people are older (you do not have to just track the king's sons, but also the sons (and their sons) of any previous king.)
- The problem is fractal in nature. All of the above applies at all levels.
This is ripe for backstabbing and killing and stuff. Although it will probably be fun, how do you lessen that?
Note: Although you would have a large pool of potential kings, it wouldn't grow out of control. When you get a large enough group of potential kings, they will be closer in age (due to pigeon hole principle), and when a king lives longer than average, a lot of the kings close to him will die before becoming kings. The effect is larger the larger the pool gets, so its self limiting (and also makes the system select for longer lived kings). Also, the backstabbing helps thin the pool (which in turns selects for backstabbier kings).