Population of the United States, by Age and Sex,
1950-2050 (millions)

(borrowed from http://www.ac.wwu.edu/~stephan/Animation/pyramid.html)

Population Pyramids

A population pyramid is a graphical tool to look at the age and gender (sex) patterns of population in a particular place. The graphic tool compares men and women of increasing ages by comparing percentages of the total population. A typical chart will have a pyrmid shape to it for places that are in stages two or three of demographic transition. Countries in these stages tend to have higher birth rates and small death rates. As people get older, both males and females, they die off at increasing numbers thus making the higher levels of the pyramid appear compartively small at the top and larger at the bottom because of higher birth rates. Stage two and three countries experience high and moderate population growth, respectively. Recently, more developed countries, specifically those in Europe have entered into stage four of demographic transition, which are characterized by declining birth rates and stablized death rates, and thus stabilizing population growth. In some countries, such as Denmark, a negative natural increase rate, has resulted in a popluation pyramid that is decidedly more rectangcular than pyramid shaped.

The power of population pyramids lies in the ability to compare longitudinally within a location, such as the example above, and to compare communities or countries make up. The population pyarmid also allows the analyzer the ability to apply concepts such as dependency ratio and sex ratio and discuss the impacts on the communities and socities under research. Why are there more women than men? Why do women, almost universally, live longer than males? What will the dependency ratio be in fifteen years when I retire? These are some of the questions that can be answered and analyzed through this graphical tools.