POPULATION PYRAMID

The term "population pyramid" describes the shape of a diagram showing the composition, by age and sex, of a nation's population at the time of a census. It is also called a "population profile." It is a convenient way to display in visual form the national population composition, and it is widely used by demographers, vital statisticians, public health specialists, social policy planners, and the television and print media when issues of national population are being discussed. The numbers used to construct the diagram are derived from national census returns. Because of its pyramid shape, a population pyramid is most aptly applied to a population with high birth rates and high death rates in infancy and at all subsequent ages. The term was probably coined with this in mind, because it evokes an image of small numbers in the upper age ranges perched on top of much larger numbers of newborn infants and young children. The population pyramid of a typical developing country in the mid twentieth century had this appearance. In the Philippines and Mexico, high birth rates and high death rates in infancy and childhood preserved the pyramid shape into the 1960s.

The changes in age and sex composition of the population in many industrial nations in the twentieth century altered the shape of the population profile, sometimes dramatically. The most obvious changes are due to a decline in the number of children born, plus reduced death rates at all ages up to old age. This produces a diagram better described as a population profile rather than a pyramid. It has a narrower base, a broader middle, and a blunter apex. Sharp declines in the numbers born at times of crisis such as wars and severe depressions leave a legacy of a narrowing at the middle of the profile several decades later.

Nations that suffer severe losses of young men in major wars have a profile that shows the excess of females, and this too works its way through the age groups as the cohorts of young people grow older; this is demonstrated in the 1965 profile of the United Kingdom, with the smaller numbers of middle-aged and old men than women in the same age groups reflecting the losses of the two world wars.

The most dramatic changes in the shape of the population profile may be those that are now appearing in sub-Saharan Africa, where HIV/AIDS (human immunodeficiency virus/acquired immunodeficiency syndrome) has had a devastating effect, selectively killing young sexually active men and women and leaving nations with orphan children to be raised by aged grandparents. Other changes appear in nations where high proportions of young immigrants are rapidly absorbed, or conversely in nations where there is substantial emigration of able-bodied young adults.

Population profiles or pyramids of successive census populations are a useful tool for the visual display of the changing composition of any nation's population, such as the United States and Canada throughout the twentieth century—with changes reflecting immigration, losses in the two world wars, reduced birth rates in the depression of the 1930s, and the surging birth rates of the baby boom years.

John M. Last

(see also: Demography; Vital Statistics )