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There are many different ways to graphically present population data. The most important demographic characteristic of a population is its age-sex structure, and the use of an age-sex pyramid, also known as a population pyramid, is considered the best way to graphically illustrate the age and sex distribution of a given population.

An age-sex pyramid consists of two horizontal histograms joined together. It displays the percentage or actual amount of a population broken down by gender and age. The five-year age increments on the y-axis allow the pyramid to vividly reflect both long-term trends in the birth and death rates, and shorter-term baby-booms, wars, and epidemics.

The fertility rate of a population is the single most important influence on the shape of a population pyramid. The more children per parent, the broader will be the base of the pyramid. The median age of the population will also be younger. While mortality will also have an influence on the shape, it will be far less important an influence than fertility, but somewhat more complex. One would assume that lower mortality rates in a population would result in an older age distribution. However, just the opposite is true: a population with lower mortality rates will display a slightly younger age distribution. This is due to the fact that any disparities in the mortality rates of a population are more likely a result of variations within the younger age groups, usually infants and children.

There are generally three types of population pyramids created from age-sex distributions: expansive, constrictive and stationary. Examples of these three types of population pyramids appear at the end of this report. Definitions of the three types follow.

1.Expansive population pyramids show larger numbers or percentages of the population in the younger age groups, usually with each age group smaller in size or proportion than the one born before it. These types of pyramids are usually found in populations with very large fertility rates and lower than average life expectancies. The age-sex distributions of Latin American and many Third World countries would probably display expansive population pyramids.

The following figure is an example of such an age-sex pyramid. This pyramid of the Philippines shows a triangle-shaped pyramid and reflects a high growth rate of about 2.1 percent annually.

2.Constrictive population pyramids display lower numbers or percentages of younger people. The age-sex distributions of the United States fall into this type of pyramid.

In the United States, the population is growing at a rate of about 1.7 percent annually. This growth rate is reflected in the more square-like structure of the pyramid. Note the lump in the pyramid between the ages of about 35 to 50. This large segment of the population is the post-World War II baby boom. As this population ages and climbs up the pyramid, there will be a much greater demand for medical and other geriatric services.

3.Stationary or near-stationary population pyramids display somewhat equal numbers or percentages for almost all age groups. Of course, smaller figures are still to be expected at the oldest age groups. The age-sex distributions of some European countries, especially Scandinavian ones, will tend to fall into this category.

Germany is experiencing a period of negative growth (-0.1%). As negative growth in a country continues, the population is reduced. A population can shrink due to a low birth rate and a stable death rate. Increased emigration may also contribute to a declining population.

Population projections, or percentages of population growth or decline over periods of time, can also be plotted and displayed on a pyramid along with the current or historical population figures, thus allowing for easy comparison of future or historical trends. This type of pyramid is especially dramatic when large, consistent increases or decreases occur.

As an example, in the figure given at the beginning of this encyclopedia entry, the age-sex distribution of the population of Laos (Lao People's Democratic Republic) is given for 1995, 2005, and 2020 (the last being a demographic projection). The changes indicate that the population pyramid is becoming less expansive over time.