Elements of Population Composition (College Board AP® Human Geography)

Patterns of Age Structure & Sex Ratio

• Population composition refers to the characteristics of a given population group, such as age and sex ratio

• The age–sex ratio compares the numbers of males and females of different ages

• Age and sex ratios can have important impacts on public policy

  • An area with a very young population will likely invest in building schools and children’s services.

  • An area with an older population might focus on providing health care and transportation for seniors

• Sex ratios may reflect a gender imbalance in cases such as war, or if a population lives near a military base

Population Pyramids

Features of a population pyramid

• The age–sex distribution of a population can be displayed on a population pyramid

• A population pyramid is a graph of the population of an area by age and sex

  • A population pyramid will normally show the percentage of the total population in five-year age groups, with the youngest group at the base of the pyramid

  • Men are usually shown on the left and women on the right

  • Each age–sex group is called a population cohort

Image: Population pyramids for three different countries, illustrating rapid population growth, steady growth, and no growth or population decrease

• A pyramid with a wide base shows a rapidly growing country with a large portion of young people

• A more rectangular pyramid depicts a country with a relatively even number of age cohorts and a steady population

• A pyramid that resembles an inverted triangle reflects a country with an aging population and fewer young people

• Population pyramids can be used to analyze and predict future population growth, using population projection 

  • If a country has a population pyramid with a wide base it can be predicted that its population will continue to grow as those young people have children

• The dependency ratio refers to the percentage of people in a population who are either too old or young to work and are supported by others

  • The economically active age groups are in the vertical center of the pyramid, with the young and old dependent populations at either end of the vertical axis

• Though population pyramids only present age and sex cohort information, much more can be learned by analyzing the pyramid

  • Population pyramids are often symmetrical left to right, except at the oldest ages as women tend to have longer life expectancies than men

  • Cohorts that are not symmetrical indicate extraordinary circumstances, including war

  • A sudden change in cohort size is often a result of large numbers of deaths from war or famine

  • Cohorts that vary widely by sex may indicate location near a military base, as in the example below. In an example such as this, males in the military-age cohorts are over-represented
    

    Image: Population pyramid near a military base
    

  • A population pyramid that shows over-representation in the economically active age groups, especially among men, like the image below, usually represents a location with large numbers of guest workers

Image: Population pyramid reflecting large numbers of guest workers

• Other anomalies might include the following:

  • Large numbers of older cohorts, which would indicate a retirement community

  • Large numbers for ages 18–25, which might indicate a college or university town