Reading a Population Pyramid

Each bar in a population pyramid represents an age cohort, usually in five-year bands, with males on one side and females on the other. Bar length shows what percent of the total population falls in that band. Stack all the bars from youngest at the bottom to oldest at the top and you get a silhouette. That silhouette is really a record of births, deaths, and migration going back decades, since every bar was once a newborn cohort that has survived and aged upward.

Three shapes do most of the work. A wide base means lots of young children, which comes from high recent birth rates, and points toward future population growth as those children reach reproductive age. A bulge somewhere in the middle marks a large cohort, often a past baby boom, moving through the age structure like a wave. A narrow top means short life expectancy or a small elderly cohort relative to younger groups, while a pyramid that narrows sharply at the base instead signals falling birth rates and a population headed toward aging or decline. Compare Japan, whose pyramid has a narrow base and a bulge in the older ages, against Niger, whose pyramid has a very wide base and shrinks steadily toward the top: Japan is aging and shrinking, Niger is young and expanding fast.

Run time forward and a bulge climbs the pyramid bar by bar every five years, widening the middle and eventually the top as that cohort ages and eventually dies off, which is exactly what happens to baby boom generations over sixty years. This links directly to the demographic transition model, since each stage of that model produces a characteristic pyramid shape, from the wide-based triangle of high-growth stage two countries to the rectangle of low-growth stage four and five countries.