14.14–14.16: The human population is growing rapidly.

High-density human habitations in Hong Kong.

The “baby boom.” It happened half a century ago, yet young people in the United States today may end up paying a price for it. What was it? And why does it still matter? Increasing birth rates caused the baby boom. Beginning just after World War II, and continuing through the early 1960s, families in the United States had about 30% more babies per month than they would have had if they had followed their parents’ pattern. Then the birth rates began to return to their earlier levels, where they have remained since. As the babies from the boom years grew to school age, schools had to increase in size to accommodate all the children. Then the schools had to downsize once the baby boomers had graduated. Now, as these individuals approach retirement age, the question of how their retirement and health care needs will be met is one of the biggest issues facing American society.

Populations often vary across space—dense in the cities, more sparse in rural areas. The baby boom illustrates that populations have an “age distribution” as well. Just as there may be more individuals in some areas and fewer in others, there may be more individuals in certain age groups and fewer in others.

Describing populations in terms of the proportion of individuals in each age group reveals interesting population features. A population can be divided into the percentages of individuals that are in specific age groupings, called cohorts, such as 0–4 years, 5–9 years, 10–14 years, and so on, in an “age pyramid.” People have radically different likelihoods of dying or reproducing, for instance, depending on their age. A 10-year-old isn’t going to produce any offspring, but a 30-year-old has a relatively high likelihood of reproducing. And a 10-year-old is less likely to die than an 80-year-old.

It can be useful, therefore, for a society to know the relative numbers of 10-, 30-, and 80-year-olds in its population. The age data can be used to determine whether a society would be better off investing in new schools or in fertility wards or in convalescent homes, among other social issues. Companies, too, rely on such demographic data to best plan their strategies for producing the goods that people will want and need.

Around the world, countries vary tremendously in the age pyramids describing their populations. If two populations are the same size but have different age distributions, they will have some very different features. Most industrialized countries are growing slowly or not at all; most of the population is middle-aged or old. In these countries, such as Norway, the age pyramid is more rectangular than pyramid-shaped (FIGURE 14-22). Because birth rates are low, the bottom of the pyramid is not very wide. And because death rates are low, too, the higher age classes in the pyramid don’t get significantly narrower. Instead, the cohorts remain about the same size all the way into the late sixties and seventies, at which point high mortality rates finally cause them to narrow. This gives the pyramid a more or less rectangular shape.

Figure 14.22: A visual representation of population growth. Age pyramids go from triangular to rectangular when population growth stabilizes.

A more triangular shape is seen in the age pyramids of developing countries. Kenya, for example, has very high birth rates, reflected as a large base in its age pyramid (see Figure 14-22). But high mortality rates, usually due to poor health care, cause a large and continuous reduction in the proportion of individuals in older age groups. In these countries, most of the population is in the younger age groups.

Notice the “bulge” in the U.S. age pyramid (FIGURE 14-23). The shape of the U.S. age pyramid has economists worried that the social security system (including Social Security and Medicare) will not be able to offer older citizens sufficient benefits in the next 10–30 years. Because of the unusually large number of babies born about 50–65 years ago, an unusually large number of people are now reaching retirement age. Since the baby boomers were born, there haven’t been any years with such a large cohort. This means that the current numbers of working individuals who contribute to the social security system are not sufficient to cover the payouts promised to the large number of retirees, and the baby boomers will be expensive to support as they reach retirement.

Figure 14.23: The retirement boom.