The time-scale of momentum effects (cont.)

The three plots below illustrate the population dynamics in the Most Developed Countries, in the Less (but not least) Developed Countries and, finally, in the Least Developed Countries following a sudden change in fertility rates to replacement level. The plots for the Least Developed Countries are similar to those shown earlier to illustrate population dynamics in a growing population.

The population age structure that becomes stationary fastest is that of the Less (but not least) Developed Countries (|CGR|<0.01% after 80 years). In the two other groups of countries, it takes about 100 years for the growth to halt. Population momentum for the collective of Most, Less and Least Developed Countries is 0.94, 1.29, and 1.53, respectively.

The group of Most Developed Countries have experienced declining fertility rates in the recent past and were characterized by below replacement level fertility rates in 2010. Therefore, the short to medium term trend in the CBR is the opposite of that in the Least Developed Countries. The Less Developed Countries already had fertility rates in 2010 around or approaching replacement level and that is why they experience a relatively smooth transition in terms of CBR in the first projection interval.

Figure 3: Population dynamics following the adoption of replacement level fertility rates in the Most, Less, and Least Developed countries (2010-2135)

¹Source (data and country classifications): United Nations, Department of Economic and Social Affairs, Population Division (2011). World Population Prospects: The 2010 Revision, Volume I: Comprehensive Tables. ST/ESA/SER.A/313

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