According to the article, “Both groups [the United Nations and the IIASA] foresee India becoming the world’s most populous country, with its numbers peaking around 2070 and declining to around 1.5 or 1.6 billion by 2100. Where they differ most is in their estimates of the coming population decline in China and of the coming population explosion in Africa south of the Sahara—where most of the world’s growth is going to occur.” Why do the groups, the United Nations and the IIASA, differ on their projections for the growth of the world’s population, and how much uncertainty exists?