> the steepest decline in births is among the poor, a population with, on average, worse impulse control and more issues with mental health, and since all qualities are at least partly heritable... Surprisingly, the fertility rate among the affluent does not appear to be nearly as impacted.

Generally, fertility rates are higher among poorer populations compared to wealthier populations. This pattern is observed both at the national level, with poorer countries generally having higher fertility rates than wealthier ones, and at the individual level, with poorer families tending to have more children than wealthier families.

https://www.stlouisfed.org/on-the-economy/2016/december/link...