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As known as Age & Sex Pyramid.
A Population Pyramid is a back-to-back, pair of Histograms (for each sex) that displays the distribution of population in all age groups and in both sexes. The X-axis is used to plot population numbers and the Y-axis lists the age groups.
Population Pyramids are ideal for detecting changes or differences in population patterns. Multiple Population Pyramids can be used to compare patterns across nations or selected population groups.
The shape of a population pyramid can be used to interpret a population. For example, a pyramid with a very wide base and a narrow top section suggests a population with high fertility and death rates. Whereas, a pyramid with a wider top half and narrower base suggests an ageing population with low fertility rates.
Population Pyramids can also be used to speculate a population's future development. An aging population that is not reproducing would eventually run into issues such as having enough offspring to care for the elderly. Other theories such as the "Youth Bulge" state that when there's a wide bulge around the 16-30 age range, particularly in males, this leads to social unrest, war and terrorism.
This makes Population Pyramids useful for fields such as Ecology, Sociology and Economics.