
What causes low birth rates? Ask yourself what would it take for your child not be a hikikomori or working at a McDonald's.
Children have to compete in a competitive educational system and then in a competitive job market. Failure means being a hikikomori or working at a McDonald's. People cannot easily get a decent job with only modest effort. Parents have to invest in their children's education so they can be on a good career track or else their children are doomed to flounder.
High investment means less children.
Essentially, this is the elite overproduction hypothesis. It also validates the "lump of labor fallacy" since there are a finite number of elite spots.
So it is not about societal wealth (although my hypothesis would say that more unequal societies would have lower birth rates if other factors are controlled) or how generous social benefits are. Rather it is the ease that someone can live a dignified life. If people are constantly "upskilling", then that is a society with a low birth rate.
https://www.youtube.com/watch?v=ZPe7tbysc2g (Why Education is Destroying East Asia)