Time and resources to make and support babies is a big constraint.
Most parents want to provide a standard of living to their kids that is better than what they grew up in, which costs significantly more to provide.
Kids didn't always have their own bedrooms or cellphones, and in an agrarian society kids were useful as extra "free" labor. The shift to cities has rewritten the economic calculations potential parents do.
Children used to be a way to support oneself in retirement. The more children you had, the better chance that at least one or two would be around in adulthood to take care of you.
Affluence means you have the means to take care of yourself, so therefore don't need to spend the effort of raising kids for this reason.
Re:The Population Bust
by lotsofpulp on Saturday August 17, 2019 @04:24AM
(#20722877)
In the US, I would say affluence actually allows you to have children, if you want to provide them with life in a good school district and a single family home in a suburb close to burgeoning city. You have to borrow student loans, then work to save for a down payment, then save for childbirth and health expenses. Now you have structural costs of student loan debt repayment, mortgage debt repayment, daycare, property tax, retirement savings debt, and possible vehicle debt repayment. And now you’ve advanced in age, and you can only have 2 kids.
Thus the constraint may just be "time to make babies".