Demography is the study of the characteristics of a population, such as age structure and change in size over time. Demographers use population pyramids to visually represent these characteristics. The three main types of population pyramids represent growing populations, declining populations, and stationary or stable populations (those that are neither growing nor declining).
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Elements of Population Pyramids
A population pyramid consists of bars plotted on X and Y axes. The Y axis, ranging from 0 to some positive number, denotes age groups, so that the youngest age groups are at the bottom of the pyramid, and the oldest at the top. Two back-to-back bars extend along the X axis, representing the number of people in each age group, with the height of the bar indicating the age range (e.g., 0 to 5; 5 to 10; 10 to 15, etc.). According to the Population Reference Bureau, by convention, the bar representing the number of males in a given age group extends left from 0 on the X axis, and the bar representing the number of females in the same age group extends to the right of 0.
In growing populations, there are more people in younger age groups than in older age groups, meaning that most of the population will come of age and produce their own children, causing growth in the future. Charts for growing populations thus have a wide base of long bars that grow progressively shorter toward the top, giving the chart a very characteristic pyramid shape. The wider the base is relative to the top of the pyramid, the more rapidly the population is growing. According to Peter A. Kwaku Kyem, a professor of geography at Central Connecticut State University, most developing nations have growing populations.
Declining populations have more people in older age groups than younger; thus, fewer people will be having children and contributing to the population in the future. The chart for declining populations looks like a top-heavy or inverted pyramid because the bars higher up on the Y axis are longer than those lower down, resulting in a narrow base that gets wider higher up. According to a 2007 Public Policy & Aging Report report by Carl Haub of the Population Reference Bureau, many European nations entered a period of population decline at the end of the 20th century.
Stationary or Stable Populations
In populations that are neither growing nor declining, age groups are relatively equal in size. This results in a chart that looks more like a column than a pyramid, except at the very top where the size of older age groups naturally diminishes. According to the University of Michigan's Global Change program, stable population pyramids were characteristic of affluent European nations in the mid- to late 20th century, before they entered periods of decline as the century ended.
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