You may have heard that the U.S. Social Security system, which provides support to retirees, is under threat. How is it that every working member of the population is required to give part of his or her salary to Social Security, but the fund is still running dry? The answer comes back to demography. Because people are living longer and having fewer children, the number of workers supporting retirees has been decreasing over time. In 1960 there were approximately five workers for each person receiving Social Security benefits. By 2005 the ratio of workers to beneficiaries had fallen from 3.3 to 1, and demographers predict that the ratio of workers to beneficiaries will fall to 2:1 by 2060. Today, about 12% of our population is over the age of 65, but by 2080 that number will almost double. The only way to pay for these retirees is to increase taxes on the working, decrease benefits for retirees, or reduce the number of retirees by raising the retirement age.

As you can see, demography is critical to planning for the needs of any population. In an aging population, that could mean Social Security and elder-care facilities. In a young, growing population, that might mean building more schools. To predict future trends in population growth, demographers study the age structure of a population. Age structure is a tally of how many males and females there are at different ages in a particular population, region, or country. The distribution of ages of a population reflects whether a population is growing, stable, or declining, and we can visualize it with an age structure diagram (Figure 5.4).

One way to predict growth trends is to compare the relative number of children in a population with the relative number of reproductive-age adults. Consider the triangular-shaped age structure of the population in Yemen (Figure 5.4), a country on the southwestern tip of the Arabian Peninsula. The age structure diagram reveals that young children far outnumber adults. As these children grow up, they will reproduce and contribute to a future population boom. In contrast, the age structure in Ukraine, a country southwest of Russia bordering the north shore of the Black Sea, is constricted at its base, indicating a relatively small proportion of children—too few to replace the adults in the population. That population is in decline. Finally, the straight-sided age structure of Iceland’s population indicates that the number of children is just sufficient to replace the adults in the population—this population is nearly stable.

The contrasting age structures of Yemen, Iceland, and Ukraine result mainly from differences in total fertility rate, which is an estimate of the average number of children that a woman in a population gives birth to during her lifetime. A fertility rate of fewer than 2 births per woman tells us that a population is not at replacement-level fertility. The total fertility rate in Yemen (4.5) far exceeds that of Iceland (1.9) and Ukraine (1.3), meaning that a woman in Ukraine is not producing the two babies necessary to replace each pair of adults in the population. In fact, true replacement-level fertility tends to be slightly higher than 2. That’s because some children die before reaching reproductive age. In developed countries, such as Iceland and Ukraine, replacement-level fertility is approximately 2.1 births per woman. In developing countries, replacement-level fertility is 2.5 births per woman or higher. These differences in total fertility rate translate to the rapidly growing population of Yemen, the stabilizing population of Iceland, and the declining population of Ukraine (Figure 5.5).

With a replacement-level fertility rate of 2.1, the United States seems like a perfect example of a stable population (Figure 5.6a). However, the U.S. population is still growing at a rate of 0.9% annually, and demographers predict it will reach nearly 400 million people in 2050 (Figure 5.6b). What’s going on?

The tendency of a population to grow even though fertility has fallen to replacement levels or lower is called population momentum. A lag period occurs because a large proportion of individuals remain in their childbearing years, most of whom will still be living long after their children are added to the population. In 2015 the annual birthrate per 1,000 individuals in the United States was 12; the death rate was 8 per 1,000. Population momentum in the United States will eventually stop once the birthrate equals the death rate, but it will be especially significant in a nation such as Yemen (see Figure 5.4), which has a large proportion of children in the population. The same is true of almost all the countries of sub-Saharan Africa.

Reproduction at replacement rate should eventually cause the U.S. population to stabilize—but this assumes no migration into or out of a population. We know that the U.S. population is not closed and has long received large numbers of immigrants. Immigration, which was responsible for approximately 48% of the growth in the U.S. population in 2015, will continue to add significantly to U.S. population growth well into midcentury.