Does income affect fertility or does fertility affect income?

"This paper tests for the dynamic causal connection between real income per capita and the birth rate for a subset of developing countries. These countries are Costa Rica, El Salvador, Guatemala, Mexico, and Uruguay. Our empirical findings show that, for the historical period under review, in several countries real income per capita affected the birth rate. Virtually no evidence is found to support the hypothesis that the birth rate affected real income per capita." excerpt