The prosperity explanation is one that economists have been studying for decades. It is consistent across nations, races, cultures, and even religions, and can be show to be relevant over time as well. All prosperous nations end up with lower fertility rates. In the US, the two most prosperous ethnic groups, Whites and Asians, have lower fertility rates than Blacks or Hispanics. And fertility rates in the US have been declining since the 50s, and is actually higher today than it was in 1979.