Population Composition: Pyramids and Ratios

By the end of this lesson, students will be able to:

• Identify the three classic population-pyramid shapes (expansive, stationary, constrictive) and the demographic conditions each represents.
• Calculate the dependency ratio from age cohort data and explain what a high or low ratio means for a country.
• Calculate the sex ratio from male and female population counts and identify likely causes of skew.
• Read any age-sex pyramid and predict whether the country is growing, stable, or shrinking.
• Connect a country's pyramid shape to its likely Demographic Transition Model stage (preview of Lesson 4).

A population pyramid is a back-to-back bar chart that shows a country's population by five-year age cohort, split by sex (male on the left, female on the right). The shape is the country's demographic fingerprint. Three patterns recur, and any country's pyramid can be assigned to one of them.

The two ratios derived from the pyramid carry most of the AP exam weight on this topic.

A ratio of 75 means 75 dependents per 100 working-age people. High ratios pressure the working-age population to support both children (in expansive countries) and elders (in aging countries). The youth dependency ratio isolates the under-15 share; the elderly dependency ratio isolates the over-64 share.

A ratio of 100 means equal numbers. The biological norm is about 105 males per 100 females at birth (a sex ratio of 105). Higher male mortality at every age means the ratio drops with age. National averages can swing dramatically when sex-selective abortion (China, India) or skewed labor migration (Qatar, UAE) is in play.

Reading any pyramid in 30 seconds: look at the base. A wide base means many young children, which means high fertility and likely growth. A narrow base means low fertility and likely shrinkage. Then look at the top. A wide top means many elderly, which means a long life expectancy and the political economy of an aging society. Finally, look for asymmetries — cohorts that are missing on one side (war casualties, sex-selective practices) or that are abnormally narrow (a wartime baby bust, a famine, a one-child policy).

1. Why does Japan's high dependency ratio create different policy problems than Niger's, even though their ratios are similar in absolute terms?
2. The biological sex ratio at birth is about 105 (males per 100 females). What forces would you expect to push a national sex ratio above 110, and what social consequences typically follow?
3. Look at the US pyramid. Identify two cohort bulges and explain what historical event likely created each one.
4. Should governments respond to constrictive pyramids by promoting fertility, accepting more immigration, or planning for managed decline? Use one specific country as your case.
5. A pyramid does not show race, ethnicity, religion, or income. What additional layers of population composition might a geographer want to add, and why does the AP CED group these under "composition"?

Draft alignment — pending educator review. AP HG codes correspond to the official College Board Course and Exam Description (Effective Fall 2020, V.1). Statements below are paraphrased in CountryReports' own voice; refer to the College Board's published CED for verbatim wording.

AP® and Advanced Placement® are registered trademarks of the College Board. The College Board was not involved in the production of this material and does not endorse it. Standards statements above are paraphrased; codes refer back to the official College Board CED, the NCSS C3 Framework, the Common Core State Standards, and other cited frameworks.