A population pyramid is a way to visualize two variables: age and sex. They are used by demographers, who study populations. A population pyramid is a graph that shows the distribution of ages across a population divided down the center between male and female members of the population. The graphic starts from youngest at the bottom to 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. A population pyramid can be used to compare differences between male and female populations of an area. 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. The first is when there are both high fertility and high mortality rates among younger members. This type of population, known as “expansive,” creates a sharp triangle shape in the graph. Expansive pyramids mean that the population does not increase much in total number and has many young people. The second trend, known as “constrictive,” is when there is a lower mortality rate with the fertility rate remaining constant. These population pyramids are wider in the middle of the graph as the population has high numbers of middle aged and elderly people, but fewer young people. The third trend is “stationary” which is a population with low mortality and low fertility rates. These graphs have a square or “pillar” shape rather than a pyramid one. These population pyramids represent a stable population that will not change significantly barring any sudden changes to fertility or mortality rates.

Population pyramids are useful for studying the future of a region as well as examining historical and current population trends. If part of the population has been affected by sudden changes, such as casualties from armed conflict, high female mortality in childbirth, or the migration of young workers out of poorer regions, the graph will offer a way to visualize how the future population will be affected. They can also help direct government and private industry distribution of services for regions based on population needs.

Population Pyramid

Population pyramids help show how populations are composed and how they are changing. Here is a population pyramid from the Unite States in August 2016 showing various age groups: baby boomers, generation X, generation Y, and generation Z. Typically, there are three trends in population pyramids: expansive, constrictive, and stationary. These illustrate the trajectory of a regional population, which may be growing, shrinking, or staying the same.

demographic
Adjective

having to do with the social characteristics and statistics of a population.

Noun

capacity of soil to sustain plant growth; or the average number of children born to women in a given population.

gender
Noun

physical, cultural, and social aspects of sexual identity.

mortality
Noun

state or condition of death.

population
Noun

total number of people or organisms in a particular area.

stable
Adjective

steady and reliable.