The poorest countries now have the fastest population growth, as they have yet to complete the “demographic transition” to the low-fertility regime that has taken hold in more developed societies. Demographers project that the world population will rise to about 9 billion by mid-century, and that it might thereafter plateau or decline as the poorer countries join the developed world in this low-fertility regime.12 Many rich countries already have fertility rates that are below replacement level; in some cases, far below.13
