“Afford” is a tricky word here. If the goal is simply to avoid bankruptcy, at the expense of the life satisfaction of the main child rearer (usually the wife), that isn’t so difficult for most Americans and Europeans. But of course people wish to maximize utility. And so here are some trends operating against having large numbers of children:
1. Jobs for women are higher-paying and more satisfying than ever before, and that raises the opportunity cost of having large families.
2. Divorce is these days socially imaginable, and for many people desirable if feasible. The larger the number of children, the harder it is to take advantage of the divorce option, and so that too encourages smaller families.
3. Living space has become especially costly in so many of the major Western cities and suburbs.
4. Given the connection between where you live and your public school system, the very best neighborhoods have become very costly positional goods, in part because of their school systems and the embedded social peers for your kids (even if they bus away to private schools.)