Publications Summary

Document Type
Working Papers
Recommended Citation
Pullum, Thomas W., and Apoorva Jadhav. 2021. A Decomposition of Sources of Change in Population Size and Median Age, 1970–2020. DHS Working Papers No. 178. Rockville, Maryland, USA: ICF.
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Publication Date
August 2021
Publication ID


The median age of a population is an easily interpreted summary of an age distribution that has been used as an analytical tool in research on the structure of the labor force, environmental changes, and political movements. Changes in the median age are determined by fertility, mortality, and migration, the same factors that are responsible for population growth. This paper develops a relatively simple unified model to account for changes in population size and median age. We apply this model to publicly available data files, produced by the UN’s Population Division as part of World Population Prospects 2019, for the global population, and major SDG regions and subregions. The goal is to determine the relative importance of fertility, mortality, and migration for changes in population size and median age during the half-century from 1970 to 2020. Change is defined as the difference between the observed values in 2020 and what would have been expected in 2020 if fertility and mortality rates in 1965-70 had not changed after 1970, and if there had been no international migration. We find that in most regions, population size was much less and the median age much older in 2020 than would have been expected. Major declines in fertility were by far the most important source of change, with relatively minor countervailing effects from improvements in mortality.


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