[xyzzyz]

In fact, Caplan discovered the so-called Hajnal line, and when it was first suggested by Hajnal, being to its south and east was associated with high fertility, not low, like today.

https://en.m.wikipedia.org/wiki/Hajnal_line

There is a lot of evidence that high rates of living with parents south of Hajnal line is largely about sociocultural practices instead of economic conditions, and it goes back centuries, see eg. Individualism and the Western Liberal Tradition: Evolutionary Origins, History, and Prospects for the Future, by Kevin MacDonald.