A population pyramid is a graph that shows the distribution of ages across a population. The graph is divided down the center between males and females in the population. The graphic starts from youngest at the bottom to the oldest at the top. It is called a population pyramid because when a population is growing (there are more babies being born than there are people dying), the graphic forms the shape of a triangle. They also show the number of dependents (children and, sometimes, elderly people) and general structure of the population at any given moment.

There are three main trends in populations that affect the shape of a population pyramid.

1. Expansive. There are both high fertility and high mortality rates. The population does not increase much in total number, and it has many young people. An expansive population has a sharp triangle shape in the graph.
2. Constrictive. There is a lower mortality rate and the fertility rate remains constant. The population has many middle aged and elderly people, but fewer young people. Constrictive pyramids are wider in the middle of the graph.
3. Stationary. There are low mortality and fertility rates. The population will not change significantly unless there are sudden changes to fertility or mortality rates, such as casualties from armed conflict, high female mortality in childbirth, or the migration of young workers out of poorer regions. Stationary graphs have a square or “pillar” shape that represents a stable population.

Population pyramids can help officials develop government policy and private industry distribution of services based on a region’s population needs.

Source: Population Pyramid
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