seems like an odd choice to so emphatically phrase this as a quality of "the left" when there are so clearly many plausible confounding factors (education, wealth, employment status, do you live in a small city apartment, etc.)
With academic social studies you can usually assume they've controlled for the basics like that, especially if the conclusions are unfortunate for the left which always means rigor gets ramped up significantly. They're working from large scale surveys and controlling for simple confounders is easy. Sure enough: > Arpino & Mogi explored mechanisms behind this reproductive pattern, finding that extreme right individual…
Arpino & Mogi are different authors that wrote a different paper that I didn't read. _This_ paper does not appear to have controlled for much at all in its primary contribution of analysis of US GSS data, and all the conclusions reached appear to me to be purely observational.
Another nuance is that since their analysis consists of filtering to people already over 40, and then asking about political leaning, there is no way to infer anything about causal association of left - vs - right ideology and fertility. For example, "having children makes you more right wing" is an equally plausible explanation for this data as "being right wing causes you to have more children" (or maybe neither of these statements are true at all, and it's caused by a variety of other confounding factors).