The changes in age and gender 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.
A population pyramid, also called an age structure diagram, is a graphical illustration that shows the distribution of various age groups in a human population (typically that of a country or region of the world), which ideally forms the shape of a pyramid when the region is healthy. It is also used in Ecology to determine the overall age distribution of a population; an indication of the reproductive capabilities and likelihood of the continuation of a species.
It typically consists of two back-to-back bar graphs, with the population plotted on the X-axis and age on the Y-axis, one showing the number of males and one showing females in a particular population in five-year age groups (also called cohorts). Males are conventionally shown on the left and females on the right, and they may be measured by raw number or as a percentage of the total population.
Population pyramids are often viewed as the most effective way to graphically depict the age and sex distribution of a population, partly because of the very clear image these pyramids present.
A helpful analogy to facilitate understanding population pyramids is that just as a builder employs a blueprint for showing a house´s structure, demographers and geographers employ population pyramids as a blueprint for showing population dynamics.
While all countries´ population pyramids differ, four general types have been identified by the fertility and mortality rates of a country.
Stable pyramid - A population pyramid showing an unchanging pattern of fertility and mortality.
Stationary pyramid - A population pyramid typical of countries with low fertility and low mortality, very similar to a constrictive pyramid.
Population pyramids for 4 stages of the demographic transition model
Expansive pyramid - A population pyramid showing a broad base, indicating a high proportion of children, a rapid rate of population growth, and a low proportion of older people. This wide base indicates a large number of children. Steady upwards narrowing shows that more people die at each higher age band. This type of pyramid indicates a population in which there is a high birth rate, a high death rate and a short life expectancy. This is the typical pattern for less economically developed countries, due to little access to and incentive to use birth control, negative environmental factors (for example, lack of clean water) and poor access to health care. Constrictive pyramid - A population pyramid showing lower numbers or percentages of younger people. The country will have a graying population which means that people are generally older, as the country has long life expectancy, a low death rate, but also a low birth rate. This pyramid has been occurring more frequently, especially when immigrants are factored out, and is often a typical pattern for a very developed country, a high over-all education and easy access and incentive to use birth control, good health care and a low number to no negative environmental factors.