http://audiovideo.economist.com/?fr_chl=37b523f6e1fab620d03697edb18b1216db2961ca&rf=bm

which discusses the fact that fertility falls once prosperity rises. In summary the argument is that when people are poor, they see children as a pension fund for the future, but once they are secure in their prosperity, they become able to trust that they don't need lots to be secure. In fact fertility rates are already falling enough to ensure that the population of the world will peak in the middle of this century; we merely need to survive through that peak.