Abstract
Counterfactual population projections have been used to estimate the contributions of fertility and mortality to population ageing, a method recently designated as the gold standard for this purpose. We analyse projections with base years between 1850 and 1950 for 11 European countries with long-run demographic data series to estimate the robustness of this approach. We link this approach with stable population theory to derive quantitative indicators of the role of fertility and mortality; consider ways of incorporating net migration; and examine the effect of using alternative indicators of population ageing. A number of substantive and technical weaknesses in the counterfactual projection approach are identified: (1) the conclusions are very sensitive to the choice of base year. Specifically, the level of base year fertility has a major influence on whether fertility or mortality is considered the main driver of population ageing. (2) The method is not transitive: results for two adjacent intervals are unrelated to results for the combined period. Therefore, overall results cannot be usefully allocated between different sub-intervals. (3) Different ageing indices tend to produce similar qualitative conclusions, but quantitative results may differ markedly. (4) Comparisons of alternative models should be with a fixed fertility and mortality projection model rather than with the baseline values as usually done. (5) The standard counterfactual projections approach concatenates the effects of initial age structure and subsequent fertility and mortality rates: methods to separate these components are derived.
Highlights
The latest United Nations population projections suggest that by 2100 in Europe, one person in seven will be aged 80 or over and over 30 per cent of people in three quarters of European countries will be aged 65 or over (United Nations Department of Economic and Social Affairs 2019)
The standard interpretation of results for the countries presented here would be that fertility change was primarily responsible for population ageing in the period 1920–2015, while mortality change was responsible in the overlapping period 1930–2015
The conclusions about the determinants of population ageing over the past century or so are heavily dependent on an apparently arbitrary choice of base year
Summary
Preston et al (1989) developed a method based on the Preston-Coale synthesis (Preston and Coale 1982) that decomposes actual changes in population mean age between births, mortality and net migration They concluded that mortality improvement was the main driver of population ageing in two developed countries, USA and Sweden, in the period 1985–1990. Murphy (2017) has extended the Preston, Himes and Eggers model to incorporate fertility rates into the model and obtained estimates of the relative contribution of fertility, mortality and net migration to population ageing for 11 European countries across the whole of the twentieth century These results confirm that the importance of mortality decline on population ageing holds over an extended set of countries and time frames.
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