The Causes and Effects of Populations' Structures
Beyond describing the structure of a population at one point in time, age pyramids are especially important in that 1) they allow retracing the demographic history of populations, and 2) they allow projecting the future evolution of populations.
The bulges in age pyramids indeed contain the traces of past events such as wars, epidemics or economic 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.
By looking at population pyramids, one can also assess the potential for future growth of a population. The age pyramid of West African countries shown before has a lot of potential for future population growth: if the death rates decline, more and more of the numerous children in the population will survive to the age of childbearing. As a result, more and more women will bear children every year (even if on average each woman does not bear more children) and the births they have will further widen the base. As a result, the population will grow fast.
This is a very simple but critical point: today's children are tomorrow's mothers; today's 55 year-olds are tomorrow's retirees. Barring a few hypotheses about future trends in fertility, mortality and migration, demographers can thus use the information contained in population pyramids to project what a population may look like a few years down the road. For example, if we assume that women will keep baring children at the same rate and that people also keep dying at the same rate, then we can infer what the age-sex structure of the population will look like 10, 15 or even 50 years from now using simple arithmetic. An example of such projections is shown below.
This is tremendously useful information. If we can project what proportion of the US population will reach retirement age in 2020 or in 2050, then we can get a sense of the problems that face social security. If we can project how fast the proportion of people living in areas where malaria is endemic, then we can forecast the increases in budget allocated to fighting malaria that will be required to avoid increasing transmission rates. Demographic data and projections are thus a crucial component of decision making in public health.