Dynasties live three generations

Ibn Khaldun's rule is that a dynasty mostly does not exceed the lifespan of three generations. A generation is the lifespan of one person of middle age, which is forty, that is, the point at which growth and development reach their end. He holds that the wisdom of the wandering of the Children of Israel supports this: forty is meant as the passing of the living generation and the rising of a generation that had not known humiliation. ²

The first: they still keep to the manners of the desert, its roughness and its savagery, the hardship of life, valour, and sharing in glory, so the force of solidarity is preserved among them. ²

The second: their condition changes through kingship and comfort, from the desert to civilisation, and from sharing in glory to one person's monopoly of it and the idleness of the rest. The force of solidarity is broken somewhat, and much of it remains with them because they knew the first generation. ²

The third: they forget the age of the desert as though it had never been, and luxury reaches its peak among them, so that they become dependants of the state. Solidarity falls away entirely, and the holder of the state needs to lean on clients until God permits its extinction. ³

These three generations amount to a hundred and twenty years, and this is the meaning of the common saying that the life of a state is a hundred years. From it comes the rule for correcting a lineage: for every hundred years, three fathers. If a generation is missing, the count is corrected by adding one, and if there is an extra one, one is dropped. ³