The Impact of Migration on Population Ageing in Asia 1990-2020: A Decomposition Analysis Using Prospective Age The Impact of Migration on Population Ageing in Asia 1990-2020: A Decomposition Analysis Using Prospective Age Markus Dörflinger Abstract: Population ageing has become a global trend, which unfolds at different speeds across world regions and countries. In Asia, there are countries with rapidly ageing populations and those that continue to maintain a younger age structure. One potential driver of this difference is international migration. In this study, I assess the impact of migration on population ageing in Asian countries over the period 1990-2020. To do so, I propose a refined decomposition method, applying a prospective view on population ageing that accounts for variation in life expectancy. Using data from the United Nations World Population Prospects 2022, changes in the prospective old-age dependency ratio in 51 countries are decomposed into the effects of cohort turnover, deaths, changes in life expectancy and net migration. The results reveal that cohort turnover and deaths have had the largest impact on changes in the prospective old-age dependency ratio over the last three decades, whereas the impact of international migration and changes in life expectancy was smaller in all countries. However, in countries with either highly negative or highly positive net migration, the effect of migration on the age structure is substantial. As migration largely occurs at younger ages, high immigration has decelerated or even halted the process of population ageing in countries such as Bahrain, Macao, Oman and Singapore. The opposite effect is observed in emigration countries such as Armenia, Georgia and Timor-Leste. Hence, the large differences in the current level of population ageing across Asian countries can at least partly be attributed to international migration in the last decades. Keywords: Population ageing · Asia · Prospective age · Migration · Decomposition analysis 1 Introduction Population ageing, characterized by increasing shares of older individuals in a population, is an inevitable part of the demographic transition (Goldstein 2009) and linked to various opportunities and challenges for economies and societies worldwide (United Nations Department of Economic and Social Affairs (UN DESA) Comparative Population Studies ‒ Demographic Trends Around the Globe Vol. 50 (2025): 1-40 (Date of release: 27.03.2025) Federal Institute for Population Research 2025 URL: www.comparativepopulationstudies.de DOI: https://doi.org/10.12765/CPoS-2025-02 URN: urn:nbn:de:bib-cpos-2025-02en2 • Markus Dörflinger2 2023). However, the timing, speed and extent of population ageing vary across countries. This is attributable to different fertility and mortality trajectories as well as to migration. A large body of research exists on the impact of these factors on the age structure using stationary or stable population models (e.g. Alho 2008; Coale 1957; Espenshade et al. 1982), counterfactual population projections (e.g. Blanchet 1989; Coale 1986; Coleman 2008; Espenshade 1994; Fihel et al. 2023; Lee/Zhou 2017; Lesthaeghe et al. 1988; Lutz/Scherbov 2007; McDonald/Kippen 2001; Murphy 2021), decomposition approaches (e.g. Caselli/Vallin 1990; de Beer et al. 2011; Fernandes et al. 2023; Horiuchi 1991; Kashnitsky et al. 2017; Murphy 2017; Preston et al. 1989; Preston/Vierboom 2021), cointegration analysis (Santis/Salinari 2023) or the variable-r method (Canudas-Romo et al. 2021). Although the results of these studies may vary in detail depending on methodologies, two main conclusions can be drawn. First, while fertility decline is the main driver of population ageing, mortality improvements play a substantial role at later phases of the demographic transition. Second, migration can have a rejuvenating or an ageing effect on a population, depending on the age structure of the migrants and of the resident population. In addition to the direct impact at the time of migration, migrants’ fertility indirectly affects the age structure in the long term (Sobotka 2008). As shown for Europe, different patterns of population ageing across countries and regions can be attributed partly to migration (de Beer et al. 2011; Ghio et al. 2022; Kashnitsky et al. 2017). Similar conclusions about the role of migration in ageing countries are drawn from studies on replacement migration, indicating that realistic levels of immigration cannot offset population ageing completely but can to some extent slow down the process (e.g. Bijak et al. 2008; Billari/Dalla-Zuanna 2011; Craveiro et al. 2019; Huguet 2003; UN DESA 2000). While many empirical studies on the drivers of population ageing focus on European and North American countries, Asia presents a particularly intriguing world region for a comparative analysis of the impact of migration on age-structure changes. First, the level and speed of population ageing vary substantially across Asian countries (Balachandran et al. 2020; Gietel-Basten et al. 2016). Aside from the prominent example of Japan, which has one of the oldest populations in the world, population ageing is rapidly advancing in several other countries (e.g. South Korea), while other countries continue to maintain a relatively young age structure. Life expectancy has also developed very differently, with some countries (e.g. Hong Kong) having some of the highest life expectancies worldwide (UN DESA 2022b).1 Second, Asian countries demonstrate quite diverse migration patterns, ranging from high immigration (e.g. United Arab Emirates) to high emigration (e.g. Georgia) (ibid.). Moreover, migration patterns and policies in Asia have been subject to profound changes over time (Ali/Cochrane 2024; de Haas et al. 2018; Oishi 2021). In this context, understanding the impact of migration on population ageing is crucial to comprehending Asia’s heterogeneous demographic landscape. 1 When referring to the statistical units provided by the UN simply as “countries”, no opinion on the legal status of these countries, territories or areas, its authorities, or the delimitation of its frontiers or boundaries is expressed. The Impact of Migration on Population Ageing in Asia 1990-2020 • 3 Within the large body of research on the drivers of age-structure changes, age is predominantly measured as chronological age, i.e. the number of years a person has lived.2 Chronological indicators of population ageing, such as the old-age dependency or the total support ratio, are based on a fixed threshold to define who is considered old, mostly at age 60 or 65. However, as noted by Scherbov and Sanderson (2020), chronological age is only one dimension of ageing and many characteristics of individuals are ignored when using these conventional measures. Several new approaches have been elaborated recently to account for additional characteristics beyond chronological age, using risk of death (Alvarez/Vaupel 2023; Zuo et al. 2018), health care needs (Spijker 2023), distribution of ages within a population (d’Albis/Collard 2013), physical health (Demuru/Egidi 2016; Muszyńska/ Rau 2012; Sanderson/Scherbov 2010), cognitive functioning (Skirbekk et al. 2012) and active life expectancy (Manton et al. 2006). Prospective age, likely the most common alternative measure of population ageing, links a person’s age to the average remaining life expectancy (Sanderson/Scherbov 2005). The central idea of this approach is that a person’s characteristics, such as physical and cognitive health, depend more on the expected remaining years of life (prospective age) than on the number of years lived (chronological age). Hence, prospective measures may better capture many of the economic and social implications of population ageing. For instance, medical expenditures are concentrated in the final years of life (Sanderson/ Scherbov 2007). Prospective age becomes particularly relevant as (healthy) life expectancy tends to increase over time in most countries (Global Burden of Disease Collaborative Network 2020; Rau et al. 2008; Vaupel et al. 2021). Thus, a 65-year-old today is on average healthier and lives longer than in 1950. In fact, global remaining life expectancy at age 65 has increased from 11.3 years in 1950 to 17.5 years in 2019 (Scherbov et al. 2022). Considering these life expectancy increases and health improvements, prospective measures indicate a slower speed of population ageing than chronological measures (Sanderson/Scherbov 2008; UN DESA 2019). Life expectancy varies not only over time but also across countries and regions. For instance, the characteristics of an average person at a given chronological age in a low-mortality country (e.g. Japan) are barely comparable to those of an average person at the same chronological age in Afghanistan, where mortality is much higher and (remaining) life expectancy much lower. These differences become explicitly relevant when defining who is considered “dependent” in a population. The widely used fixed old-age threshold of 65 years, which marks an historically and contemporarily common retirement age in many countries, is based on the assumption that the retired population is entirely dependent on the tax contributions of those who work (Gietel-Basten et al. 2015). In many Asian countries, however, the coverage of the public pension systems is low (Organisation for Economic Co- operation and Development (OECD) 2022), and informal employment rates are high (International Labour Organization (ILO) 2024). In this context, prospective measures 2 An exception is the study on replacement migration by Craveiro et al. (2019), which uses prospective age. • Markus Dörflinger4 appear to be more suitable for a comprehensive understanding of population ageing in Asia. However, this new perspective is rarely incorporated into the analysis of the drivers of population ageing. In this paper, I present an approach to incorporate the prospective age concept into a decomposition of age-structure changes. The study aims to assess the drivers of population ageing in Asian countries over the period 1990-2020, with a particular focus on the role of migration.3 2 Data and methods 2.1 Data This study uses a) annual population and deaths counts by single age and b) life expectancy by single age (both sexes) for the definition of prospective age. These data are drawn from the United Nations’ World Population Prospects (WPP) 2022 estimates (UN DESA 2022b). The UN WPP 2022 estimates are based on population and household censuses, vital and population registration systems, surveys and other sources (e.g. UNHCR data on refugees) (UN DESA 2022a). Following the UN definition of geographic regions, 51 Asian countries are included in the analysis: 5 from Central Asia, 8 from Eastern Asia, 11 from South-Eastern Asia, 9 from Southern Asia and 18 from Western Asia. 2.2 Indicators of population ageing The old-age dependency ratio of the (dependent) old-age population relative to the working-age population is a common measure of the potential burden of population ageing. This ratio is used as the key indicator in this study.4 Conventionally, individuals are considered to enter old-age at 65, while the working-age population includes the population of age 15 to 64. Therefore, the (chronological) old-age dependency ratio is defined as follows: In contrast to the chronological approach, which uses a constant old-age threshold at age 65, the prospective old-age threshold (POAT) is determined by the average 3 In this study, when referring to migration, international migration is implied unless otherwise specified. 4 Other studies on population ageing use the total support ratio or potential support ratio, which is the reciprocal of the old-age dependency ratio. However, the use of the term “support ratio” is sometimes ambiguous (Kashnitsky et al. 2017), so I use the old-age dependency ratio in this study. (Chronological) old-age dependency ratio (OADR) = old-age population working-age population = ∑ Pop.age=100+ age=65 ∑ Pop.age<65 age=15 The Impact of Migration on Population Ageing in Asia 1990-2020 • 5 remaining life expectancy. The most common definition of the prospective old-age threshold is the age when the remaining life expectancy equals 15 years (Sanderson/ Scherbov 2013). Since life expectancy varies over time and across populations, this threshold is dynamic. However, Sanderson and Scherbov (2020) have shown that death rates, as an indicator of health, are approximately constant at this threshold across populations over time and space. The prospective old-age thresholds by country for the years 1990 and 2020 are reported in Appendix Table A1. The old- age population is defined for each country and year as the population at age=POAT and older, while the working-age population includes the population aged 15 and over but younger than the prospective old-age threshold. The prospective old-age dependency ratio (POADR) is then the ratio between the two age groups according to the prospective definition: 2.3 Decomposition analysis of changes in the prospective old-age dependency ratio To assess the drivers of age-structure changes in Asian countries, I conduct a decomposition analysis of changes in the prospective old-age dependency ratio over the period 1990-2020. The end of the period is set at the beginning of year 2020 to avoid any effects of excess mortality during the Covid-19 pandemic starting in 2020 (World Health Organization (WHO) 2023). Life expectancy for 2020 is obtained from the 2019 life table. Similar to decomposition analyses based on chronological age (de Beer et al. 2011; Ghio et al. 2022; Kashnitsky et al. 2017), cohort turnover, deaths and migration effects are taken into account. First, the cohort turnover effect depends on the number of individuals entering working-age and the number of individuals leaving working-age and entering old-age. Thus, the cohort turnover effect is an indicator of the age structure, which in turn is the long-term result of fertility, mortality and migration. Second, the deaths effect is defined by the number of deaths in working-age and old-age, respectively. Third, the migration effect is based on the number of net migrants of working-age and in old-age, respectively. When using prospective age, the definition of the relevant age groups shifts with changing life expectancy. Hence, I extend the decomposition method by adding an effect which accounts for the additional cohort turnover due to changes in the prospective old-age threshold. In case of life expectancy improvements, the prospective old-age threshold – the age at which the average remaining life expectancy equals 15 years – increases. In such instances, the effect of changes in life expectancy (LE) is determined by the additional number of individuals that “remain” in the working-age population and do not enter old-age due to the increase in the prospective old-age threshold. When life expectancy decreases, as observed at least temporarily in some countries, the effect is defined by the additional number of individuals that leave working-age and enter old-age due to the reduction in the Prospective old-age dependency ratio (POADR) = prosp. old-age population prosp. working-age population = ∑ Pop.age=100+ age=POAT ∑ Pop.age