Also wanna point out: as you get richer, the social expectations for how you raise your kids get higher in terms of $$$, too. Go upper class and suddenly you need to get your kids into fancy private schools, you need to send them to get horse riding lessons, you need to go on nice vacations overseas, etc.

Obviously these are not true needs, but if people feel like they're needs, they can still become factors in how many kids you have.