I agree I suspect this is a big one. 100% two income families are going to have less kids, and less time, and more income (hence as countries get richer they have fewer children)

But a career is less and less a woman's choice and more and more it's a requirement.

If average families could get by on one income with a decent standard of living I'm sure more women would decide to stay at home or work part time. I know at least one that would anyway..