Introduction

Because of revolutions in family planning and life expectancy, in the late 20th century humanity reached age distributions never before seen in our history as a species, providing a rich set of puzzles about the political, economic, and social causes and consequences of population aging. Technically, any country in which fertility is falling and median age is rising is aging, and nearly all the world’s countries fell into this category at the turn of the 21st century. But analytically, this pattern becomes interesting and novel questions arise after these trends have continued for several decades and the proportions of young and old in the society shift—as is the case in a much smaller group of countries. At no time has aging been more remarkable to study than it is in the 21st century and the certainty that aging will intensify in many countries means it will only grow more important over time. In the discipline of International Studies, research mostly focuses on the consequences of such demographic change and its influences on broader questions of interest to the field, such as economic development, conflict, and political power. At the same time, it is impossible to understand population aging without also considering the causes of the trend itself, particularly low fertility, so research on population aging draws heavily on sociology, demography, and economics.

As a global issue, population aging is relatively new—for most of human history fertility and mortality were both high. The Demographic Transition Theory describes a society’s transition away from these high levels to low fertility and mortality. In the theory, a society transitions through four phases: high fertility and mortality in the first, pre-industrial stage; declining mortality in the second, industrializing stage; declining fertility in the third, industrial phase; and stable, low fertility and mortality in the fourth, postindustrial stage. In the world’s most developed states, mortality began to fall in the late 18th to 19th centuries, which improved chances of infant and child survival and yielded gains in life expectancy. Primarily in the 20th century, fertility followed suit. As our knowledge and implementation of health practices has continued, record societal life expectancy continues to increase, more than doubling since the 19th century. Globally, average life expectancy at birth has greatly improved as well: in 1950 it was only 47 years but in 2020 it was 72 (UN DESA, 2019).

Today, many countries have completed the demographic transition; others seem to be barely beginning. Thus, the set of countries scholars focus on when studying aging as a global issue is typically limited to those which have completed the four stages of the demographic transition and have moved to what some demographers call the fifth stage of the transition, and others call the Second Demographic Transition. Both terms capture the shift to a very low fertility society, with the concomitant changes to an older age structure (Lesthaeghe, 2014). Research on the Second Demographic Transition explores the influences on changing preferences about family size to rates far below the typical replacement rate of 2.1 children per woman, namely changing marriage patterns, secularization, incompatibility between motherhood and work outside the home, and changing family formations. The replacement rate refers to a level of fertility needed to maintain a steady population size, one each for the parents and a margin for those females who do not make it to reproductive age. Among the countries with the lowest fertility in the world around 2020 were South Korea, with 1.1 children born per woman on average, Singapore and Taiwan with 1.2, Bosnia & Herzegovina, Spain, Italy, Greece, and Ukraine with 1.3 (UN DESA, 2019). Many of these very low rates appear to be due in part to distortions arising from deferments in marriage and childbearing.

Globally, population aging is intensifying and spreading--states that are already considered “aged” are reaching record proportions of older persons. In 1950, the median age of developed countries was 29 years; in 2020, the median age of developed countries was 42. In the intervening years, Western Europe and Japan led the way with below-replacement fertility and low mortality, and thus the most extreme population aging. Japan has had low fertility since the 1970s, and by 2035, using the United Nations’ (UN’s) medium variant, Japan’s median age is projected to reach a remarkable 52.4 years (UN DESA, 2019). Aging is also spreading in the sense that more countries are achieving median ages over 35 years in geographic areas well outside of Europe and Japan. As this article will show, there are far more questions about the implications of population aging than there are answers, so researchers will find this a young and fruitful field, particularly when it comes to newly aging states. While the majority of studies of aging as a global issue are based in sociology, economics, demography, and their related disciplines, aging research is relatively underdeveloped in political science. There is much room for new analyses and new insights.

The following sections first describe typical methods and measurements for the study of population aging, then the major research themes involving political and military, economic, and social implications and disciplinary connections to economics and law. The article then describes the many controversies and debates over the impact of aging, including debates over the relative weight of demographic factors and differing perspectives as to whether population aging is a trend warranting celebration or alarm. The article concludes by outlining lingering questions in the study of aging, including interactions with migration and climate change.

Methods and Measurements

There are multiple theoretical perspectives and ways to measure population aging, and decisions about approaches, definitions, and measurements can make a dramatic difference on the results of studies of its impact.

Some scholars approach the study of aging through a generational lens. They may focus on cohort effects and assume that individuals are shaped by a shared life experience, such as a war or technological revolution. Other scholars may focus on period effects (examples include economic cycles, wars, droughts, or epidemics), which are experiences that shape all individuals alive at that time, regardless of age. Life-course theories examine how exogenous processes and events shape individuals in different periods of their lives, whereas life-span theories look at micro-level processes within an aging individual. In its most simplistic formulation, individual aging begins at birth. Both life-course and life-span theories encourage contextualizing “old age” by recognizing how it is shaped by experiences throughout life. Thus, most scholars who use these approaches argue that longitudinal data and multilevel analysis are needed to capture the changes throughout an individual’s life, rather than focusing on only what happens in a person’s oldest ages (Fulle-Iglesias, Smith, & Antonucci, 2009). In this sense, scholars recognize that each person passes through various ages—aging, with luck, is process that every individual can expect to experience.

Although much of the International Studies research on aging focuses on the state and societal level of analysis, such research still relies on assumptions about what aging means at an individual level—a person’s place in the family, the workforce, and the polity, just to name a few. Measures of chronological age can be problematic, though, as what it means to be 65 today is different than what it meant to be 65 in a different time (Sanderson & Scherbov, 2008, p. 4) or in different contexts, even at the same time. Globally, life expectancy at birth in the year 2020 ranged from 52 years in the Central African Republic to 84 years in Japan, with the lowest life expectancies driven by high infant and child mortality (UN DESA, 2019). Even in the least developed countries, some people live to reach their 70s and 80s. How might concepts like retirement, grandparenthood, or senescence differ in these contexts?

In addition to chronological age, researchers often use dependency ratios as measures of population aging, particularly old-age dependency ratio, which measures the proportion of those aged 60 and over (or 65 and over) to those of working ages, typically defined as 15 or 20 to 60 or 64 years. These ratios are meant to capture the economic effect of population aging, implying that the proportions of those who “contribute” to the economy—prime-age workers—are shrinking at the same time that the proportions of those who “take out”—older persons assumed to be retired—are growing. However, use of dependency ratios requires a firm age boundary and the assumption behind dependency ratios is that those under or over a certain age are dependent on those of middle ages, which may be empirically false. Cross-nationally, retirement ages can vary by at least a decade. In many cases the effective age of exit from the workforce differs significantly from the official retirement age, being either much younger or much older. In both cases, arbitrarily defining dependents at age 60 or 65 and above would be empirically inaccurate.

To skirt the problems with dependency ratios, some scholars have argued that median age is a better way to compare countries’ age structures over time or cross-nationally. Cincotta, in the article “The Age-Structural Theory of State Behavior,” introduces the concept of the age-structural transition and argues that “median age provides a simple and intuitive means for analysts to estimate and visualize the three most important analytical qualities in age-structural theory: a state’s position in the age-structural transition, its direction of movement, and its rate of change.” Cincotta defines the age-structural transition as “the continuous path of cohort reconfiguration that leads from a youthful population to one numerically dominated by middle-age adults and seniors.” However, as Cincotta concedes, focus on median age does not reveal potential differences in age distribution among populations. For more detail, scholars examine the “shape” of populations by looking at the various sizes of age cohorts—easily visualized in population pyramids or trees. As an example, there is a robust literature around the “window of opportunity,” when a society has an abundance of population in the middle parts of the age structure.

Another way scholars have added nuance to studies of aging is through the National Transfer Accounts (NTAs) approach, which measures aggregate intergenerational transfers, accounting for the economic flows among children, workers, and the elderly (Mason, Lee, Tung, Lai, & Miller, 2009). Although this approach still assumes that those in the youngest and oldest age brackets are out of the workforce and consume more than they produce, it does recognize that flows are multidirectional. Grandparents, for example, may be officially retired but provide occasional, unpaid childcare for their grandchildren, allowing their adult children to be more economically productive.

Although old age is culturally constructed and variable across place and time, most large-n quantitative studies, such as those in political economy, ignore this variance to simplify analysis, and there are few qualitative studies of societal-level aging. Thus, the field often lacks nuance. Scholars who have shifted away from using dependency ratios in favor of median age or other measures of age structure are able to correct for some of these shortcomings, but still may miss details about the distribution of population among various age groups when aiming for a single, comparative measure. Finally, measurement is a challenge because there is reason to believe that it is not the demographic trend itself that shapes an outcome, but perception of that trend that matters (Winter & Teitelbaum, 2013). Some studies of the intersection of migration and aging, for example, attempt to account for societal fears that as the majority population ages and dies, ethnic minority populations will become more prevalent and powerful, and trace how this plays out at the voting polls.

Implications

Studies of the implications of population aging fall into three major categories: political, economic, and social. Political demography studies often focus on the political power of various age groups and attempt to assess the degree to which intergenerational conflict is emerging as the sizes of age groups change and their demands on services like entitlements shift alongside. Political demography studies also look at voter behavior and preferences to assess possibilities for reform of age-related policies, like retirement, healthcare, and education. A separate branch of political demography examines the military implications of population aging, particularly its effect on the willingness and ability of a state to use force. These studies assume that aging affects both manpower and budgets and most of this scholarship focuses on the world’s most powerful states—the United States, China, Russia, and those in Europe—all of which are aging. Studies on the economic implications of population aging look at the changing nature of the labor market itself and on the possibility of macroeconomic growth in the face of demographic change. For example, countries may attempt to increase female labor force participation to make up for a smaller workforce. Finally, research on the social impact of population aging is mostly concerned with individual- and family-level well-being, as the care demands of an aging population create pressures on individuals, families, and social safety nets.

Political Implications

The major theme of political demography studies of aging is the political power of various age groups. Much of this work has concerned political dynamics in democracies with aging populations. As people are living healthier longer, their voting lives are longer, too (Matsubayashi & Lu, 2019), and to the extent that researchers believe the size of political influence moves in tandem with the size of voting blocs, they may also assume the aged are growing in political power at the expense of other generations. Scholars also examine the voting behavior of various age groups, and the feasibility of such groups being mobilized by political rhetoric surrounding age-related issues.

To some, population equals power and reforms to social welfare policies that favor the aged are politically infeasible. One camp expects more generous welfare policies in an aging electorate. A rational choice model assumes that seniors will only vote in their immediate interest, and much of the work on this research question is in political economy, which assumes this perspective, and indeed finds evidence to support it. Epple, Romano, and Sieg (2012) find a relationship between education funding and the age of voters in particular communities, starting from the perspective that older voters have less of an incentive to vote for generous education funding. Galasso and Profeta (2004) use the same perspective to argue that an older electorate means more generous pensions systems because it raises the profile of the issue on politicians’ agendas. Jäger and Schmidt (2016) find that because seniors tend to discount long-term investments there is a negative relationship between share of elderly people and public investment rates. Empirically, public-pension fiscal issues are related to 1930s definitions of pension eligibility ages—since that time life expectancy had risen greatly, particularly the remaining years of life left once someone reaches age 65. Because pensionable ages were set long ago, efforts to increase them can result in protests, as has happened in Russia and France. To minimize the electoral penalty to politicians, raising pensionable ages can be done only gradually, with substantial fractions nearer to retirement grandfathered into the old system.

But do older people vote only in their own interests? Others have looked beyond a rational choice model and found that seniors’ voting preferences are more complicated. More micro-level analyses show different findings. For example, Goerres and Tepe (2010) find that seniors’ experiences with other generations within their own families shape their attitudes toward public childcare, an issue not directly of interest to them.

There are also more nuanced and varied findings among studies that place emphasis on context: economic context such as the extent to which women are integrated in the labor market; cultural context such as attitudes toward working mothers; and political context, especially the extent of state spending on the family (Lynch & Myrskylä, 2009). Institutions partly determine how generous policies toward the aged are both within Europe, where the majority of research has been done, and in other contexts (Breyer & Stolte, 2001; Sciubba & Chen, 2017; Tepe & Vanhuysse, 2009, 2010). For example, differences between federal structures may explain patterns of old-age care in Germany and the United States (Campbell, 2003). In comparisons of various welfare states, Esping-Andersen and Sarasa (2002) find a lot of variation in bias toward the aged even among aging states because of differences in the institutional designs of their welfare states.

Certainly, there is little consensus in the literature on the relationship between demographic aging and politics; this is partly because data has been limited, as population aging is so new. In an early attempt to theorize about aging and politics, Binstock and Day (1996) were concerned about whether the political attitudes of older people would differ from younger generations and whether the generations would inevitably conflict. Although their studies found little reason to expect intergenerational conflict, they cautioned that this could change in the future. Taking this caution to heart, it is important to note that, arguably, the countries under study during the time frames of most of the studies published before 2010 were not “aged” at all, in the sense that their median ages were still relatively young. Such studies should be repeated since aging has intensified, and regularly in the future as aging continues to progress.

Not only have the data been limited, but researchers’ measures of aging have prevented a clear consensus from emerging in the literature. Fullerton and Dixon (2010) find that cohort effects, not age effects, explain American attitudes toward education, health, and Social Security spending, and thus fears of intergenerational conflict are overblown. Similarly, Goerres (2008, 2009) finds that generational effects, not age effects, determine party preferences in Germany.

The studies that start from a rational choice perspective often assume more generous policies toward the elderly. Other studies use a completely different set of assumptions and argue that aging can lead to more austere social spending because of the pressure on the median voter, who is of working age, and their desire to avoid higher taxation to pay for generous spending on the elderly—this is what some have termed the “fiscal leakage” hypothesis (Razin & Sadka, 2007). Sometimes contesting pressures within a society can cancel each other out: Sanz and Velázquez (2007, p. 917) find that aging “is the main driving force of the growth of government spending, followed by relative prices and population” but other age groups work to counter increases in benefits to retirees. They go on to note that “institutional reforms have been successful at reducing the impact of ageing on pensions in recent years.” There is still much work to be done in this area, particularly if age divisions supplant class divisions in some postindustrial societies and as demographic aging reaches countries without democratic institutions, such that the size of voting blocs ostensibly matters less (Sciubba & Chen, 2017).

Two areas of research on the politics of aging are relatively new but have fascinating early results. The first of these is the links between populism and population aging (Auerswald & Yun, 2018). The 21^st century return of populism to Western Europe in the midst of the continent’s dramatic demographic changes has led some scholars to surmise that immigration and aging have played a role in shaping attitudes toward European integration (Coleman, 2016). This dynamic comes together with aging because increasing immigration is often proposed as a solution to population aging and shrinking without adequate attention to possible social and political consequences of large-scale immigration into low-fertility societies. From this perspective, Brexit may not have been primarily about economic competition and sovereignty, but about “competition for the cultural values of traditional England” (Harper, 2016, p. 184).

The other potentially fruitful area for research is on social movements, global governance, and law, particularly the influence of the “old-age lobby.” There is very little work on elderly social movements (Pratt, 1976) but the existence of a social movement around the proposed UN Convention on the Rights of Older Persons has sparked a body of scholarship. One part of this looks at potential legal gaps on elderly rights at the international and national levels (Doron, 2006; Doron & Apter, 2010; Rodriguez-Pinzón & Martin, 2003). Another examines the discourse and dynamics of the social group surrounding the push for institutionalized elder rights (Herro, 2017; Sciubba, 2013, 2014b).

Military Implications

While the previous section, “Political Implications,” showed that there is a budding literature on the politics of aging, there is very little research on the national security implications of population aging. Typically, research on demographics and security focuses on youthful populations and their link to conflict, of which there is plenty of empirical data to study given the prevalence of states with youthful age structures. The dearth of work on aging is partly because the set of countries with advanced societal aging is so small. There are insufficient numbers of observations for quantitative studies that require a large n; even qualitative studies have only 10–20 years of data, and that is only for very few states. Given the limitations, many of the handful of studies in this area are prospective, based upon hypotheses about potential future effects rather than evaluations of empirical evidence to date. Of the few studies that show a link between aging and war, empirical results are inconclusive, meaning that it is just as likely a state with a high median age will be belligerent as not. Findings in the few empirical studies conflict, with one quantitative study finding that older societies are indeed more peaceful (Brooks, Brooks, Greenhill, & Haas, 2019), and one qualitative study finding the opposite (Sciubba, 2014a).

Theoretical links are also underdeveloped. Early International Relations literature discussed population as one of several foundations of national power (Sprout & Sprout, 1945), but all of the major International Relations theories of war were developed during a time of relative population youth and abundance. Scholars simply are not sure whether to expect an aging world to be more peaceful or more turbulent. Thus far, states with mature age structures have been some of the most peaceful in the world. Is this correlational or causal? We should also consider issues of operationalization. As we broaden our definition of conflict, we see more evidence among aging states. Aging China is militarizing the Indo-Pacific; aging Russia cut off gas supplies to Europe in 2009, invaded Ukraine in 2014, and began conducting airstrikes in Syria in 2015. Likewise, the United States, which is entering a mature age structure, has a large military footprint around the globe, involved in both overt and non-declared wars.

The majority of research concerns the influence of societal aging on willingness and ability to fight. Manpower and budget are the biggest concerns with population aging—namely, that there will not be enough of either. Some argue that aging states will be the most peaceful because they have lower capacity for conflict as defense spending is crowded out by spending on seniors. They surmise that aging societies have high per unit soldier costs as the number of age-eligible soldiers shrinks and the money invested in individual soldiers rises. States will therefore be wary of expending their valuable resources. Some also argue that aging states will be averse to military casualties because the personal and political costs of losing a child is higher the smaller the family (Brooks et al., 2019; Luttwak, 1994). Others, however, use power transition theory to argue that aging states may be more aggressive in the face of aging as a last gasp to grab global power (Sciubba, 2014a). Along these lines, alliances could, to some degree, help states compensate for declining manpower by providing “strength in numbers” and shared investments in personnel-saving technologies, which allow them to project power even in the face of aging. Yet some argue that alliances like the North Atlantic Treaty Organization (NATO) are set to implode because of internal demographic changes, which weaken their ability to effectively respond to threats (Ceccorulli, Fassi, & Lucarelli, 2017).

Given both the rapidity of population change as global aging intensifies and that aging is an issue shared by all of the world’s most powerful states, this research area is ripe for growth. The emphasis that scholars place on population as power has waned the more that the nature of conflict has changed to give the edge to advanced weaponry—at least in potential great power conflict—and the growth of cyber capabilities as a potential low-manpower arena is another way states might compensate for demographic changes. Studies that look comprehensively at the tradeoffs between guns and butter in an aging society are sorely needed.

Economic Implications

Studies on the economic and social implications of population aging are less connected to political science and thus will just be briefly discussed. On the economic side, these studies mostly question potential for economic growth, primarily because of shifts in both the labor market, which shrinks and ages, and in numbers above arbitrarily defined ages of “dependency,” those whom many analysts assume to be out of the workforce and beneficiaries of social welfare and high healthcare spending. Certainly, the number of youth entering the workforce declines over time in an aging society. Populations of “working age” have peaked in most aging countries, including China’s; the annual number of new entrants to Mexico’s labor force have peaked as well. However, the age boundaries of “working age” may be better understood as variable rather than constant, as mortality and morbidity are also declining. In addition to dependency ratios and other measures of age structure, how a researcher measures economic variables, whether by per capita or aggregate figures for example, affects results.

Early on, Coale and Hoover (1958) explored how fertility decline changes age structure and how that age structure affects economic growth. Following this, perhaps the most influential line of research has been that of the “demographic dividend,” stemming from Bloom, Canning, and Sevilla (2003), who argue that those of the youngest and oldest ages who are outside the workforce consume more than they produce, and economic growth is in part driven by an abundance of those in the working ages. Their models of “demographic dividend” attribute part of East Asia’s economic boom to dependency ratios that provide a favorable combination of labor supply, savings, and human capital. Of course, the validity of such a model depends on what proportion of economic growth is attributed to population growth (Matsutani, 2006).

Many macroeconomic and microeconomic questions remain. Scholars have tried to understand whether older workers must exit the workforce for younger workers to take their place. Many label this the “lump of labor fallacy” and argue that there is no fixed amount of work within an economy. Other research in this area concerns microeconomic issues with financial solvency in retirement, potential ways of bringing in large, underutilized segments of the potential workforce, including women and older people, and conditions that affect early retirement (Merkle, Schreiber, & Weber, 2017).

Although there is a strong research tradition on whether economic growth is possible in an aging society, there has been very little questioning of the economic growth model in general. This lack of reflection is certainly due to the neoclassical assumptions that drive much research in economics and political economy. From this perspective the ultimate goal of market economies is economic growth—continuous expansion in the aggregate. But there is room for a normative debate: should the goal continue to be economic growth even when a population is shrinking? While some environmental scholars have raised the issue, very few experts in aging have taken on this important question.

Research on the social implications of population aging mostly focuses on individual well-being. We know that individuals without a strong government safety net or family structure will have to provide for their own care and security in old age. Individual characteristics matter as well, such as good physical and mental health, and occupation, including self-employed status (Wahrendorf, Akinwale, Landy, Matthews, & Blane, 2016, p. 270). There is some research on the broader implications for social relations. For example, shortages of caregivers have spurred immigration schemes, which change the ethnic composition of receiving societies and introduce another element of complexity to demographic change.

Key Controversies and Debates

Concerns about demographic aging appeared among the French elite in the mid- to late 19th century as competition with Germany intensified (Teitelbaum & Winter, 1985), and policymakers in other countries and contexts have occasionally voiced similar worries. On the whole though, widespread population aging is new. Because of that newness and because scholars are studying a moving target, there are quite a few controversies and debates around the issue. At a foundational level, scholars continue to debate whether demography is destiny, or rather, to what degree aging is deterministic. States can overcome geography and project power—can they also overcome demography?

There are also debates over perspective, particularly whether population aging is inherently positive or negative. People are living longer than ever, but instead of celebrating this positive milestone, many express concern and even alarm. Aging is portrayed in the media, policy, and often in academic circles as challenging and even catastrophic. Both low fertility and long life expectancy are actually signs of societal progress. If people are living longer and having smaller families, that means they have the confidence to have fewer children and feel more assured those children will live to adulthood. But because widespread demographic aging is a new phenomenon there is substantial uncertainty about the implications, as this article has shown, and that creates fear.

A related debate is over the characteristics of an aging society, particularly whether or not an aging society can still be an innovative one. Those who are pessimistic about aging societies seem to dominate the debate. On the one side are those like Magnus (2009, p. 77), who says that “Today’s information economy . . . demands flexibility, innovation, and continuous improvement in skill levels and education standards; it rewards merit and achievement as opposed to seniority; it prizes individual creativity rather than machines—and it may be a tougher world into which to integrate millions of extra retired and old people.” On the other side are those who point out the economic opportunities of an aging society, and argue that they can spur innovation—partly out of necessity (Coughlin, 2017). Terms such as “young,” “youthful,” “aging,” “aged,” and “dependent” can lead to distorted interpretations when applied to populations as shorthand terms to describe shifts in age compositions resulting from fertility differentials or trends. Characteristics of “aging” individuals, that is, less energy, dynamism, or creativity, are often ascribed to populations that are “aging.” Yet some populations that have “aged” are the most energetic, dynamic, and creative—witness the technological superiority of Germany and Japan, for example. Conversely, populations that are “youthful” may be seen as having the characteristics attributed to “youthful” individuals: a high level of energy, optimism, creativity, enthusiasm. Yet, too frequently, such populations are mired in sclerotic and corrupt governance and an inability to break away from factors that impede development.

Finally, scholars are divided over whether or not demographics are influential at all. For example, scholars disagree on whether the demographic dividend is a real phenomenon or whether other factors that tend to accompany this stage in development, like investments in education and infrastructure, or growth in governance capacity, are actually responsible for driving the economic growth that these middle-age structures exhibit. Lutz et al. (2019) find that human capital, in the form of education, matters far more than age structure for economic growth. This debate is relevant for studies of population aging because if we cannot be sure that demographic structure drives economic growth, we similarly cannot be confident that when the “window of opportunity” closes and there are more old-age dependents relative to prime age workers, economic growth will come to a grinding halt.

Lingering Questions: The Future of Aging

As this article has shown, there are far more questions about the implications of aging than there are answers, and the projected development of this trend means that even more questions constantly arise. Lingering questions surround historically rapid demographic aging, new sets of aging states at different speeds, shrinking populations, the intersection between migration and aging, and the intersection between aging and climate change. The field is ripe for more comparative aging work in general.

First, while we look to Japan, Germany, and Italy as global leaders in population aging, they are certainly no longer alone as aged states. Increasingly, population aging is a worldwide phenomenon, touching more continents than ever and intensifying in places where the trend began decades ago. Just as researchers start to figure out what aging means in the vanguard of states, two trends are occurring: first, those countries are reaching record-high median ages and proportions of elderly; and second, a new set of countries with different histories, cultures, and institutions are experiencing demographic aging. Scholars will have to continue to trace the political, economic, and social implications of aging in both new contexts. One question that will drive this research is whether or not the past is precedent. While scholars have extensively studied aging in Japan and Europe, there is little work on aging elsewhere (Coulmas, 2007).

Second, population aging is happening at a much faster rate today than it did historically. The European demographic transition lasted at least 150 years, “giving societies and governments time to address and adjust as their populations transitioned from young to old” (Leeson, 2018, p. 112). Today’s transitions are much faster and are in part because of external interventions that change both fertility (through family planning access) and mortality (through health programs like vaccines). Some scholars point out that differences in the nature of the demographic transitions, whether from internal developments or external interventions, can affect the way a state prepares for resilience in the face of demographic aging and the health challenges that come along with it (Bollyky, 2018). With regard to China, many in the media—and a few scholars—have questioned whether it matters that the country will “grow old before it grows rich,” but China certainly is not alone on this path and many middle-income countries are following in its wake. Scholars would do well to consider context: the various cultures and institutions in these newly aging societies are likely to produce different political, economic, and social effects from demographic change than they did for the vanguard states.

Third, just as we are getting used to the issue of population aging, societies now have to confront the inevitable outcome of the second demographic transition: a shrinking population. Barring extremely high levels of immigration (Harper, 2016), aging countries will go the way of Japan and Italy, whose populations are already shrinking from low fertility and mortality. How might the fiscal consequences of population shrinking be different from an aging, but still growing, population (Clements, Dybczak, Gaspar, Gupta, & Soto, 2015)? What factors determine the outcome? To connect to a previously mentioned issue, Coleman and Rowthorn (2011) take a historical look and assume that the pace of decline matters, as does decline relative to others, particularly political or economic rivals. There is much more work to be done in this area.

Fourth, there is little work on the intersection of migration and aging, particularly of the aging of migrants themselves. There is some research on those who have retired internationally, and some on those who migrate even at older ages, mostly to take jobs in the care or home industries. There is even research on those left behind when younger people migrate (Toyota, Yeoh, & Nguyen, 2007). But the number of older people with a migration background is increasing in some countries, and given the diversity of younger generations, will continue to do so. Given the different characteristics—preferences, education background, and even citizenship status—of migrants versus non-migrants, more research needs to be done comparing differences between the two populations, as Anja Steinbach (2018) has done with native Germans versus those of a migration background.

One fruitful area for research is high-migration, low-fertility societies. Teitelbaum and Winter (1998) conducted an early study of this dynamic in relation to fears about population change and concomitant questions of citizenship and national identity in states including Germany, Russia, France, and the United States. To do such research well, one needs to trace the historical flows of migrants in relation to changes in age composition and examine issues specific to both older people and those with migrant backgrounds. The issue is also relevant to intergenerational transfers, if older generations differ from younger ones in terms of national origin, race, ethnicity, or language. Cultural context matters as well: origin and destination countries might have different ideas about age and aging. This might mean they have different value systems, collectivism versus individualism, ideas about family versus government support. As King, Lulle, Sampaio, and Vullnetari (2017, p. 182) discuss, the aging process is “socially constructed and culturally embedded.” As they move internationally, migrants will encounter different aging models, including at what age someone becomes elderly, or what it means to be retired.

Finally, there is room for research on the intersection between aging and the environment. How is population aging likely to change consumption patterns, and thus carbon emissions? What does the potential end of population growth mean for sustainable development (Lutz, Sanderson, & Scherbov, 2004)? How do patterns of human settlement, like urbanization and land change from urbanization, affect environmental degradation (Liddle, 2014; Weber & Sciubba, 2019)?

In general, although research on political, economic, and social implications of population aging is increasing as the issue grows more widespread, there is a dearth of comparative aging work (Silverstein & Attias-Donfut, 2010). The field has much room for growth and many questions that will only be answered as more states experience aging and as aging intensifies. This is good news for quantitative researchers, whose datasets will grow larger with observations. As the world annually sets records for high median age, there are developments we cannot anticipate. The field of political demography is growing, and given the interdisciplinary nature of the questions about demographic implications, there is ample room for collaboration across disciplines and specialties.