Subsistence farming has been a marginal activity even in colonial North America, and that was even more true in United States. It was mostly restricted to native Americans. The rest were overwhelmingly engaged in market farming, not subsistence — either as owners, hired laborers, or slaves. In any case, by 1960, US society was overwhelmingly urbanized, and subsistence farming has faded from living memory.

I'm just saying that if you look at the birth rate chart, the further away you get from modernity, the higher the birth rate goes. Substitute whichever more accurate words you prefer.

This might seem like nitpicking, but it is hugely important distinction for people interested in demographics. The simple fact is that societies engaged in subsistence farming are, in fact, stable instead of growing, and their TFR is not very far from 2. This has been true about, say, most of pre 1800s Europe, or pre 1900s sub Saharan Africa, but has not been true about North America for last 400 years until very recently.

> I mean, in the sense that the birth rate used to be enormous because children were part of the subsistence farming labor force, yes.

No, its true among countries in the developed West, both across countries at the same time and within countries over time, without subsistence farming being a significant factor. There's theory as to why speculating in the role of family as disability/unemployment/old age support being more critical with relatively weaker social support networks and less so with stronger ones.

(My personal pet speculatio is that its more subrational, and that humans are loosely biologically programmed in a way that generalized insecurity — perhaps signalled through accummulated effext of stress hormones — mitigates the degree to which humans are inclined to K-strategy in reproduction, and that’s an important factor in adaptability to environmental change on timescales shorter than evolution; lots of things in nature have a stress/reproduction response like that.)