I doubt it is one thing. That is probably the single largest factor, but I wonder how large a role other factors play: the internet as social vehicle, male fertility, female fertility, social safety nets, average educational attainment, cultural factors, etc.

A 0.2% difference in birthrate makes a massive difference on the timescale of civilization, so it is a very interesting question to me.

Cost of housing in first world countries requirung two full time incomes is a big pressure point.