Basically banding into groups and guarding against outsiders is the default human behaviour. It just works that way if you do a game theory analysis of our social structures. They usually don’t scale too well, but that’s what we revolved to do as social creatures.

It’s actually and very counter intuitively the Catholic Church that lead us to individualism, common laws, nationalism, even the Industrial Revolution and the scientific method.

It sounds bizarre but if you follow the historical logic, in a round about way it has paved the way for the modern world, which the rest of human civilisation was forced to adopt, either to compete or at gunpoint.

There are few books I read in a year that change the way I look at the world, “The Weirdest people in the world” was definitely one of them.

As far as 'population stability' though ... quite a lot of systems achieved this kind of stability without quite the same kind of social order.

But good point.