You do realise that poorer people tend to have more children, especially in developing countries? But even then, these countries are gradually developing and their population growth is also slowing down thanks to more education and growing economy. The more educated people are, the more likely they will think first and plan before having a family. Educated people also have higher social mobility with higher disposable income, and likely try to enjoy more themselves than settle down as quickly as possible. So economic factor is a consideration with the slow down on population growth, but not the way you think it does.