Population Pyramid

Except for total size, the most important demographic feature of a population is its age-sex structure. The age-sex structure affects the needs of a population as well as the supply of labor; therefore, it has significant policy implications. A rapidly growing population implies a high proportion of young people under the working age. A youthful population also puts a burden on the education system. When this cohort enters the working ages, a rapid increase in jobs is needed to [Page 2250]accommodate it. In contrast, countries with a large proportion of older people must develop retirement systems and medical facilities to serve them. Therefore, as a population ages, its needs change from schools to jobs to medical care.

Figure 1 Population pyramids for developing countries typically exhibit a wide base, reflecting high fertility rates, while those in economically developed parts of the world portray a population more evenly distributed across age groups.

Source: U.S. Census Bureau, International Data Base. http://www.census.gov/ipc/www/idb/country.php.

The age-sex structure of a country or region can be conveniently summarized or described through the use of population pyramids. They are divided into 5-year age groups, the base representing the youngest group, the apex the oldest. Population pyramids show the distribution of males and females of different age groups as percentages of the total population. The shape of a pyramid reflects long-term trends in fertility and mortality and the short-term effects of “baby booms,” migrations, wars, and epidemics. It also reflects the potential for future population growth or decline.

Two basic, representative types of pyramid may be distinguished (Figure 1). One is the squat, triangular profile. It has a broad base, concave sides, and a narrow tip. It is characteristic of developing countries having high crude birth rates, with a young average age and relatively few elderly. Natural growth rates in such societies tend to be high.

In contrast, the pyramid for economically developed countries, including the United States, describes a slowly growing population. Its shape is the result of a history of declining fertility and mortality rates, augmented by substantial immigration. With lower fertility, fewer people have entered the base of the pyramid; with lower mortality, a greater percentage of “births” have survived until old age. In short, the structure of the population pyramid closely reflects the stage of the demographic transition in which a country is positioned.