Isn't this simply because the political realignment in US resulted in an arrangement where higher incomes generally correlate with left-wing political views? An

Isn't this simply because the political realignment in US resulted in an arrangement where higher incomes generally correlate with left-wing political views? And it's not exactly a new thing that higher incomes also correlate with fewer children. Indeed, they also point out that the finding only holds true for US whites but not for blacks. Which is also consistent with this being just a reflection of economic status.