Article

Population size and composition
United States population growth
Age-sex structure of the United States
Younger countries
Older countries
Age compositions of different countries
Populations age foretells the future

This brief, part of a series that analyzes population and housing data collected from Census 2000, compares the U.S. population in 2000 with those of other countries and major world regions, focusing on population size, growth, and age sex composition. It provides some explanation for major differences in population size and structure among countries. It also underscores differences in national population growth between the United States, other more developed countries (MDCs), and the world's less developed countries (LDCs),¹ and links these to the same population dynamics underlying age composition differences.

With a total of 281 million people, the United States had a larger population in 2000 than all countries except China and India. However, the size of a country's total population tells only a small part of its demographic story. A country's population growth rate and its age-sex composition indicate the challenges it faces in providing health care for its children and elderly, providing education to its youth, providing employment opportunities for its young adults, and supporting its elderly population. Figures 1 and 2 show the distribution of global population in 2000 — where people lived — and that part of growth during the 1990s that was accounted for by a handful of countries, as well as their contribution to change in the children (under 15) and elderly (65 and over) age groups.

Figure 1 shows the percentage of global population in 2000 living in the five most populous countries and the remaining countries of the world. The second and third bars show where the world's children and elderly populations lived, again focusing on the five countries accounting for the largest shares of these age groups globally. The United States ranked third in terms of total population size in 2000, with just under 5 percent of world population. It also accounted for large shares of the world's children (ranking fourth among all countries with 3 percent of the world's under-15 population) and elderly (ranking third among countries with 8 percent of the total).

The distribution of population of specific age groups across countries — where the world's children live, or the world's elderly, or any other age-specific population — depends entirely on the total population size of individual countries and on the age composition of each country. For example, because of higher fertility in the recent past, Pakistan and Indonesia had larger proportions of their population under age 15 than the United States in 2000. However, the United States' overall larger size offset the younger age distributions of Pakistan and Indonesia so that each country accounted for a roughly similar share of the world's under-15 population (Figure 1). Similarly, more developed countries such as the United States have somewhat lower fertility and mortality than countries in Africa, Asia, and Latin America, and consequently somewhat larger proportions of their populations were elderly in 2000. Countries with the largest shares of the world's elderly included three of these MDCs — the United States, Japan, and Russia — plus China and India which, because of their overall size, account for large shares of the elderly globally.

During the 1990s, the United States, along with India, China, Nigeria, and Indonesia accounted for nearly half of all global population increase (Figure 2). The importance of these countries to world population change is attributable to a combination of (1) their population size in 1990 and (2) their respective rates of growth during the decade. Each of these countries is large in both absolute terms and relative to other countries in their region. The similarity in size of shares of global population increase of the United States, on the one hand, and Nigeria and Indonesia, both which have smaller populations than the United States, on the other, is due to the much lower growth rates of the United States in comparison with Nigeria and Indonesia. (From 1990 to 2000, Indonesia's population increased by 19 percent and Nigeria's by 33 percent, compared with the United States' 13 percent.) The same effect explains the similar proportions of global population increase accounted for by China and the slightly smaller but more rapidly growing India. The distribution of global population increase is important because it indicates those parts of the world with the largest absolute increases in needs for food, housing, social services such as health care, and employment, as well as the potential for regional and global environmental impact.

At the turn of the millennium, populations are aging — median ages of national populations are rising — in every major world region. More developed countries (MDCs) with low fertility and low mortality are contributing heavily to this process, but so are some less developed countries (LDCs), such as China, which have seen their birth rates fall dramatically over the past two decades. China, Japan, Indonesia, and the United States are four of the five countries that contributed most to the growth of the world's elderly population between 1990 and 2000 by virtue of their relatively low mortality and relatively large overall populations (Figure 2, third bar).

At the other end of the age range, the United States and other MDCs contributed relatively little to the growth of the world's under-15 population during the past decade. Ninety percent of the world's children lived in LDCs in 2000, and most of the increase in this age group during the 1990s occurred in Africa, Asia, and Latin America. Five developing countries accounted for about 60 percent of the growth of the world's under-15 population during the decade (Figure 2, second bar). The United States ranked sixth among countries in contributing to the growth of the under-15 population during the 1990s (7 percent); no other MDC was among the top 50 countries in this respect.

For the past 50 years, the United States and other MDCs have differed from the world's LDCs in their fertility, their mortality, and in their overall growth. MDCs typically have had lower fertility, mortality, and population growth rates; LDCs have had generally higher levels of fertility, mortality, and population growth. This broad generalization is less true today than in the past. Some demographic differences are emerging between the United States and other MDCs combined. The U.S. contribution to total global population change discussed earlier partially reflects the fact that the U.S. population is growing faster than most MDCs: from 1990 to 2000 the U.S. population increased by 13 percent compared with 2.5 percent for other MDCs combined. As a result of its somewhat higher fertility — women in the United States give birth to more than two children on average over the course of their reproductive lives, while women in most other MDCs have lower fertility — and substantial immigration, the United States added more people to its population than all other MDCs combined during the decade. Eleven MDCs in Eastern Europe and the former Soviet Union actually lost population during the decade of the 1990s. The generally higher rates of population growth characteristic of the less developed countries of Africa, Asia, and Latin America during the 1990s and the generally lower rates of growth characteristic of MDCs are shown in Figure 3.

Notice that:
• About 20 percent of world population, but over 40 percent of the world's elderly, lived in the United States and other MDCs in 2000.

• Relatively small percentages of LDC regional populations were of working age (ages 15 to 64² —proportions ranged from 53 percent to 64 percent) compared with the situation in more developed countries 66 percent for the United States, 68 percent for other MDCs) with the smallest percentage being that of the world's least developed major world region, Sub-Saharan Africa.

• A greater number of elderly women than men were living in each region in 2000. In general, the largest percentage differences were found in the United States and other MDCs.

The U.S. population was older — meaning it had larger proportions of people in older age groups (ages 65 or older and 80 or older) — than much of the rest of the world in 2000 (Table 1, column 4). However, the United States also had a slightly younger population than most other developed countries for two reasons. First, U.S. fertility is slightly higher than that of other MDCs, as mentioned earlier. Second, about 10 million more people entered the United States than left the country during the 1990-2000 period, adding to the young adult age groups of our population.

Figure 4 shows the percentage of national population ages 65 and over. Countries with high percentages of elderly are essentially the world's MDCs — the United States, Canada, the European countries, Japan, Australia, and New Zealand — and a small handful of other countries with a history of relatively low fertility and mortality. The United States was among the top third of countries (ranking 41st out of 224) on percentage of population ages 65 and over in 2000.

Table 2 presents the child, elderly, and total dependency ratios⁴ for the United States, other more developed countries, and the world's developing regions in 2000. Most countries had a larger child dependency ratio than elderly dependency ratio, and this was uniformly true of the developing countries of Africa, Asia, and Latin America. Developing countries as a group had a larger total dependency ratio and a much larger child dependency ratio than the United States, reflecting the greater number of children being supported by working-age adults in Africa, Asia, and Latin America.

The distributions of the world's children and its oldest old (population ages 80 and above) indicate where in the world the needs for children and elderly health care services are greatest in the current time period, and where needs for schooling and elderly support will be greatest in the coming years. The United States' or any country's age structure and its support ratios, together with its national wealth, indicate the extent to which it is likely to be able to address those needs in the current period. Countries with rapidly growing under-5 and oldest-old populations are likely to face additional challenges in the decades ahead.

• Nearly 60 percent of the world's children under age 5 lived in just ten countries in 2000 (Table 3). The United States is the only MDC among these countries; in 2000 about 3 percent of the world's under-5 population lived in the United States. A majority of the world's population under 5 lived in less developed countries; over a third lived in India and China, and another 20 percent were found in the remaining seven developing countries shown in Table 3.

• In contrast, 65 percent of the world's oldest old lived in ten countries comprised primarily of MDCs. The United States accounted for 13 percent of the world's oldest old. China had the largest percentage of the world's oldest old (almost 17 percent) primarily because its overall population size dwarfs that of most other countries. Except for the world's two largest countries, no other LDCs made the top ten list for the oldest old.

Over the course of the next quarter century (from 2000 to 2025), world population is expected to increase by 29 percent, with nearly all growth occurring in less developed countries; to grow older (rising median age and larger proportions of population at ages 80 and above); and to become slightly less male-dominant (a lower male-to-female ratio⁵) than in 2000. Census Bureau projections indicate that:

• The U.S. population will be 23 percent larger by 2025, compared with 2000.⁶ Excluding the United States, MDCs as a group are likely to see a decrease in population of about 1 percent, and the age distribution of this group of countries will change substantially. LDCs as a group will grow by 35 percent over the 2000-2025 period.

• In 2000, there were about 101 men for every 100 women worldwide. The ratio of men to women was lower in the world's more developed regions (96 for the United States and 94 for other MDCs) than in LDCs (103). Associated differences in numbers surviving become more pronounced in relatively low mortality, older populations such as in the United States. By 2025, the male-to-female ratio worldwide and for LDCs will decline very slightly as the world's population ages. There will be approximately equal numbers of men and women globally and about 101 men per 100 women in the world's LDCs. Little change is expected in the ratio of men to women for the United States and other MDCs.

• Between now and 2025, as fertility continues to decline and relatively large adult cohorts grow older, the world's population will continue to "age," resulting in a higher proportion elderly and a smaller proportion children. By 2025, the number of elderly will nearly double, a percentage increase about three times that of the working-age population, while the number of children globally will be just 3 percent larger than in 2000. Population aging will occur in both MDCs and LDCs. The United States is expected to have nearly 80 percent more elderly than in 2000 but just 15 percent more working-age adults and 15 percent more children. Other MDCs as a group are expected to have a smaller but still substantial increase in elderly population (46 percent over 2000) combined with sharp decreases in child and working-age populations (17 percent and 7 percent, respectively). LDCs should see their elderly grow by 130 percent, or over twice as fast as the growth of the elderly in more developed countries, between 2000 and 2025. LDCs will experience an increase of 5 percent in under-15 population and an increase of 44 percent in working-age population.

• As a result of these shifts in age structure, the world's elderly dependency ratio will be about 50 percent higher in 2025 than in 2000 (16 compared with 11 dependents per 100 working age adults, Tables 4 and 2, respectively) but global total dependency will decline from about 58 to 53. Elderly support ratios will be higher in the United States and other MDCs in 2025 (30 and 34, respectively, compared with 13 for the LDCs). Over the coming quarter century, as a result of the aging of MDC populations, the historically higher total dependency ratios of less developed regions will be eclipsed by those of more developed countries.

During the coming quarter century, prevailing low fertility will account for declining proportions of MDC populations in the under-5 and under-15 age groups and, in some countries, for absolute declines in the size of the under-5 population. It also will account for the coming decline in working-age population in MDCs as a group.

The shifting age structures of the United States and other MDCs evident in the declining under-5 populations in the late 1990s mark the advent of a demographic challenge that will confront low-fertility countries during the coming decades.⁷

More industrialized nations as a group will be forced to support a growing elderly population even as smaller cohorts of workers enter the labor force. However, if present trends continue, the impact of population aging is likely to be less severe in the United States than in other MDCs because immigration and fertility both serve to replenish our younger population age groups to a greater extent than in other MDCs.

Endnotes
¹ More developed countries include all of North America and Europe, as well as Japan, Australia, and New Zealand. The remaining countries of the world are considered less developed countries.
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² While mean age at entry into the labor force, mean age at retirement, and thus actual labor force, or working ages vary from country to country, the convention of ages 15 to 64 is often used for international comparisons.
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³ The commonly used age dependency ratio is the number of people ages under 15 and 65 and over per 100 people ages 15 to 64. A dependency ratio of 50 means that there are twice as many people ages 15 to 64 as under 15 and 65 and over. This ratio is sometimes used as a surrogate for the economic dependency ratio, number of people not working per 100 people working in a population. The age dependency ratio is often used because it eliminates differences in definition between countries.
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⁴ The child dependency ratio (number of people under age 15 per 100 people ages 15 to 64) and elderly dependency ratio (number of people ages 65 and over per 100 people ages 15 to 64) are also useful in better understanding the age composition of the population.
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⁵ Male-to-female ratio refers to the number of people in a population who are male relative to those who are female.
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⁶ U.S. Census Bureau, Census 2000 Summary File 1; Projections of the Total Resident Population by 5-Year Age Groups, and Sex with Special Age Categories: Middle Series,
2025 to 2045 (NP-T3-F; January 13, 2000 Internet release).
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⁷ United Nations, 2000, Replacement Migration, ESA/P/WP/160, New York.
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