Chapter One: The Shape of Things - The Meaning of Population Age Structures

"Are we really going to be able to give these extra people jobs, homes, health care and education?" Official in Uganda's Ministry of Finance, discussing population growth in The Guardian, August 25, 2006¹

"Germany is becoming the world's nursing home…Germans really are a dying breed."
The Economist, March 18, 2006²

All national governments inherit the legacy of population dynamics shaped by demographic history stretching back into the dimmest human past. The structure of age within a population is among these dynamics. Along with population growth or decline, density and distribution, age structure can help a government's policies succeed or challenge its resources and undermine its legitimacy.

Fortunately for those who understand the connections among demographics, national stability and development, age structures can be influenced through population-relevant policies and programs. International actors can improve the efficacy of such policies and programs by helping countries establish and maintain public health and education systems.

Snapshot: World Population Today

World population today is moving in several directions that may seem contradictory. Its growth rate is slowing, but it is still gaining almost as many people each year—76 million—as ever. Humanity's median age is rising (it was 28 in 2005), yet every year there are more people under the age of 30. And finally, while analysts and policymakers in much of the industrialized world fret about falling birthrates and the reality or possibility of population decline, world population continues to climb with no certain end in sight. In 2006, the number of human beings on the planet surpassed 6.5 billion, and projections indicate it will reach nine billion by 2050. Only significant further declines in fertility and unexpected decreases in life expectancy could prevent the addition of another billion people over the next two decades.³

Demographically, the world is more diverse than ever before. Fifty-five percent of the world's people live in countries whose birthrates guarantee indefinite future growth. Nearly one billion people live in countries—Afghanistan, Iraq, Nigeria, Pakistan, the Palestinian territories, Somalia and Sudan among others—in which fertility averages four or more children per woman. Assuming no changes in family size or life expectancy, these countries' populations will double in fewer than 35 years.

Simultaneously, population aging and decline have risen in prominence on the public policy agenda in many regions. Nearly 45 percent of the world's people live in countries in which the average family size is below the number needed over the long term (usually slightly more than two children per woman) to avoid population decline. Populations with higher median ages have experienced smaller family sizes and, thus, slower growth or even shrinkage in recent years—usually along with increases in life expectancy. Most of these countries are developed, but not all—among them are Chile, China (with 1.3 billion people), Iran, South Korea and Thailand. Population decline has not yet begun in most of these countries and is likely to proceed gradually when it does, in contrast to the still-rapid pace of growth in the dozen or more fastest-growing national populations. So far, despite fears about its future impacts, population aging has not caused demonstrable harm to the economy, security or environment of any country.

Rapid population growth correlates closely with young populations. Countries with high proportions of young people tend to have, or to have had recently, a preponderance of large families. Moreover, existing high proportions of young people build significant growth into a country's future, even if members of the rising generation have smaller families than their parents did. Countries with high rates of population growth often experience increased strain on environmental and social resources. Unless economic growth steadily keeps apace with an increasing population and is equitably distributed, per capita incomes also shrink.

Because young populations grow, any government efforts to slow aging by increasing birthrates or encouraging youthful streams of immigrants also induce growth in population size and density. For most of the world's countries, in any event, aging is a future rather than a present worry. High birthrates and low life expectancies in these countries all but assure youthful population age structures for many years to come.

Favorable and Adverse Age Structures

When a country's age structure is favorable, its government and institutions stand to experience a relatively easy time (under average global and regional conditions) improving the welfare of the country's people and solving economic and political problems. When a country's age structure is adverse, it is more difficult to meet these challenges, resulting in lost opportunities and sometimes state failures.

Favorable age structures, though they vary depending on a country's economic prospects and its government's security intentions, generally include a large proportion of the population comprised of working-age adults, with smaller proportions and slower growth among dependent children and older adults. A favorable age structure provides a sufficient tax base for government services and a social safety net for the more dependent age groups. Adverse age structures, in contrast, often develop when there is a bulge in the proportion of young, dependent age groups or under conditions of economic stagnation and high unemployment.

The notion of a favorable or unfavorable age structure, however, is static—a snapshot in time. Population age structures can shift relatively rapidly and sometimes dramatically in response to policies and programs, to epidemic diseases and to other natural and human-induced events. Economically and socially significant shifts in age structure can occur in under a decade, and profound reconfigurations of the profile can occur in 25 years, as happened in Iran (Figure 1.1). Two examples of such rapid changes are Iran's recent decline from high to low birthrates and the ongoing toll of AIDS-related deaths in southern Africa.

Such rapid changes in age structure, and the gradual ones that are more common, are often the result of one or more modern demographic forces. These include the demographic transition, AIDS-related mortality and international migration. High rates of death from AIDS, which have been most profound in southern Africa, shrink the number of working-age adults and leave dependent children with fewer economic resources. Relatively high levels of inward-migration have benefited the economies and kept the populations of countries such as the U.S. relatively youthful compared to many European countries, where fertility rates are lower and restrictions on immigration are tighter. Even more dramatic effects are found in the Persian Gulf, where extremely high immigration keeps various economic sectors growing and makes working-age adults the most prominent segment of the population.

Figure 1.1
Iran's Population Age Structures: 1980 and 2005

These two population profiles, showing Iran 25 years apart, illustrate how a decline in fertility rates can make a dramatic impact on age structure in a relatively short amount of time. In the case of Iran, fertility rates declined from 6.6 children per woman in 1980-1985 to 2.1 children per woman in 2000-2005, producing a more balanced and favorable age structure. Over the 25 years, the share of Iran's population under age 30 decreased from 71 to 64 percent. Chapter Three discusses Iran's transition in detail.

The demographic transition—the transformation of a population characterized by large families and short lives into a population of small families and long lives—is one of the seismic shifts that have shaped the world in which we live. Others include the agricultural transition, by which farming in developed countries evolved from a household livelihood to a specialized industry in the 20th century, and the epidemiological transition, by which the dominant cause of death shifted from infectious to noncommunicable disease, also in the 20th century.

Like these other great changes, the demographic transition unfolds distinctly in each population and is rarely continuous or smooth. The general trend in recent history, nonetheless, has been similar and nearly universal in overall changes in death and birth rates despite differences in time periods. The transition began in some European populations as much as 250 years ago, but in many African populations death rates only began to decline after World War II.