I'm not sure the authors are examining the right variable. Very high individual or household income is actually a predictor of higher fertility than average, ev

I'm not sure the authors are examining the right variable. Very high individual or household income is actually a predictor of higher fertility than average, even in the developed world. Moreover, the fertility vs. income curve in the US at least is actually U-shaped, with higher values at both ends, but a depression in the middle. It is only really the case that higher income suppresses fertility when talking about average income over time for a country or other large place. Nevertheless, that trend is quite robust, even though zooming in reveals other things happening too. This is a classic statistical paradox of aggregation.