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Q&A: Why does it take so long to slow or stop population growth? find out answer

Aside from the total size, the most important demographic characteristic of a population is its age and sex structure, or the proportion of people at each age, by sex. The age-sex structure determines potential for future growth of specific age groups, as well as the total population. For these reasons, the age structure has significant government policy implications. A population of young people needs a sufficient number of schools and, later, enough jobs to accommodate them. Countries with a large proportion of older people must develop retirement systems and medical facilities to serve them. Therefore, as a population ages, needs change from childcare and schools to jobs, housing, and medical care.

Population Pyramids

The age-sex structure of a country can be studied through population pyramids. The overall shape of the pyramid indicates the potential for future growth. The four representations of population age-sex structure provide an overall example of what a pyramid for different levels of population growth would look like — rapid growth, slow growth, zero growth, and negative growth. The horizontal bars show the percentage (or in some cases the actual numbers) of males and females in each age group.

The country pyramids shown on the chart "Three patterns of population change" also represent different stages of population growth going on today. The first pyramid, representing the population of the Democratic Republic of Congo, with its wide base and narrow top, is typical of a young population. This shape is the result of high birth rates that feed more and more people into the lowest bars and in turn shrink the relative proportion at the oldest ages. As the death rate declines, more people survive to the reproductive ages and beyond. The births they have further widen the base of the pyramid. This shape is common in many less developed countries that have experienced improvements in life expectancy but continue to have high birth rates. It reflects both a history of rapid population growth and the potential for future rapid growth.

The second age-sex pyramid is typical of a slowly growing population. The United States is an example of a country in slow growth. The United States has had declining fertility and mortality rates for most of this century. With lower fertility, fewer people have entered the lowest bars of the pyramid, and as life expectancy has increased, a greater percentage of the "births" have survived until old age. As a result, the population has been aging, meaning that the proportion of older persons in the population has been growing. This trend was interrupted by the postwar baby boom, 1946–1964, when birth rates climbed again. (The bulge of the baby-boom generation can be seen in the pyramid for ages 35–54 in 2000.) After 1964, birth rates continued their downward trend until the late 1970s. As the last members of the baby boom approached their childbearing years during the 1980s, the number of births rose again, peaking in 1990. These children, the youngest generation, are represented by the slightly widening base of the pyramid. Even though the number of births per woman is lower than ever before, the population continues to grow because of the children and grandchildren of the huge baby-boom generation.

Age-Sex Structures in Transition

A few countries have reached zero population growth or are experiencing negative growth because of low birth rates and an old age structure coupled with minimal net migration. While Germany's death rate exceeds its birth rate, its population continues to grow because of net migration. Pyramids in which the proportions of the population are fairly evenly distributed among all age groups are representative of many highly industrialized societies. Germany's old population reflects an extended period of low birth and death rates. While fewer children have been born, most of those born survive through to old age. The net effect is zero growth or no natural increase. Germany's pyramid also shows the effect of higher mortality among males. In an industrialized society, females generally outnumber males after age 40. This trend is particularly evident in Germany's oldest age group.

While birth and death rates usually determine the basic pyramid shape, migration also affects it. Typically, most migrants are in the working ages, and often more males than females migrate across national borders. In some Middle Eastern countries a large number of men migrated to work in the oil fields, which caused a bulge in one side of the pyramid, while it took a "bite" out of the pyramid of some of the countries from which they came.

Short-term fluctuations in birth and death rates that produce unusual bites or bulges in population pyramids, such as the baby boom, often can be traced to such historical events as wars, epidemics, economic booms, or depressions. The decline in the birth rate during the Great Depression caused a small bite in the U.S. pyramid for the group born between 1930 and 1934. World Wars I and II caused a deficit of older men in Germany. The impact of these events emphasizes the interrelationships among population change and economic, social, political, and health factors.