The International Institute for Applied Systems Analysis (IIASA) is an interdisciplinary, highly-respected research institution near Vienna, Austria. The IIASA population projections are the most cited alternative to the UN work.
Interestingly their medium projection – which they consider most likely – stands in stark contrast to the UN projection. While the medium variant of the UN projection sees the world population growing continuously throughout the 21st century the IIASA medium (most likely) scenario see the world population increase to 6.54 billion in 2070 and then starts a slow decline to below 6 billion by the end of the century.
As the IIASA researchers explain there are important methodological differences between the UN and IIASA projections. The UN uses probabilistic population projections to extrapolate populations for each country over the 21st century and is applying the same statistical methodology to all countries. The IIASA researchers in contrast have a country-specific approach that takes into account the knowledge of experts for each country separately.
The other important feature of the IIASA projection is that it takes into account the education of the population. As we show below in this data-entry there is a strong link between education and fertility – more educated women have fewer children. This link is taken into account in the model of the IIASA researchers and contributes to their projection of a much smaller total world population over the course of the 21st century.
The breakdown by education level is especially interesting: It shows that our world will be inhabited by more and more educated people. In the visualization below you see that in 1970 there were only around 425 million people in the world that had secondary or post-secondary education. By the end of this century, the number of people with secondary or post-secondary education will have increased 10-fold and will reach 4 billion people!