How to use the simulator
There are two ways to change the pyramid:
- Preset buttons. Expansive, Stationary and Constrictive set the slider to +18, 0 and −12.
- base width (growth/cohort) slider. Runs from −15 to +25 in steps of 1 (starts at +18). Positive values make each younger cohort larger than the one above it, widening the base. Negative values make each younger cohort smaller, pinching the base.
When you drag the slider, the highlighted preset follows it: Expansive above +10, Constrictive below −4, and Stationary in between.
The pyramid has 17 five-year age groups from 0–4 at the bottom to 80+ at the top. Males are on the left in navy and females on the right in coral, and each bar's length is that group's share of the total population. The female bars run slightly longer from age 65 up, because women tend to outlive men.
The stats panel shows:
- Young (0–14), Working (15–64) and Old (65+) as percentages of the population.
- Dependency ratio, given as dependents per 100 working-age people.
- A short interpretation of the shape with example countries: Niger and Nigeria for expansive, the U.S. and Sweden for stationary, Japan and Germany for constrictive.
The presets give dependency ratios of 89 (Expansive), 52 (Stationary) and 58 (Constrictive). At the slider's ends the figures are 115 at +25 and 65 at −15. Both ends are high, but for opposite reasons.
The key ideas
The dependency ratio compares the people usually too young or too old to work with the working-age group:
It splits into a youth part (0–14 only on top) and an elderly part (65+ only). Two countries can have the same total ratio but face very different problems.
- Expansive (wide base, narrow top). High birth rates, a young population and rapid growth. These are typically Stage 2 or early Stage 3 countries in the demographic transition model. The pressures are schools, child health care and creating enough jobs for each new cohort.
- Stationary (near-vertical sides). Fertility near replacement level, slow growth and a balanced age structure, typical of late Stage 3 or Stage 4.
- Constrictive (narrow base, wide middle or top). Fertility below replacement, an ageing and possibly shrinking population, typical of Stage 4 or 5. The pressures are pensions, elder care and a shrinking workforce.
A pyramid also shows a country's history. A bulge marks a baby boom, a notch can mark a war or a famine, and a working-age bulge that is mostly male often comes from labour migration. The simulator builds smooth pyramids from one growth setting, so it cannot show these irregular features. Look for them in real data.
Worked example
Problem: Compare the youth and elderly dependency of an expansive and a constrictive country, using the simulator's presets.
Step 1: Expansive preset (+18). The panel reads Young 44.0%, Working 52.8%, Old 3.1% and a dependency ratio of 89 per 100.
Youth part: .
Elderly part: .
Almost all of the dependency comes from children.
Youth part: .
Elderly part: .
Almost all of the dependency comes from children.
Step 2: Constrictive preset (−12). The panel reads Young 8.6%, Working 63.2%, Old 28.2% and 58 per 100.
Youth part: .
Elderly part: .
Now about three-quarters of the dependency comes from older people.
Youth part: .
Elderly part: .
Now about three-quarters of the dependency comes from older people.
Step 3: Interpret. The expansive country needs to spend on schools and young families now, and it faces population momentum: those 44% of children will become parents themselves, so growth continues even if fertility falls. The constrictive country needs pensions and health care, and may turn to immigration or pro-natalist policy to support its workforce.
Step 4: Find the low point. Press Stationary and the ratio falls to 52, the lowest of the three. A country moving through the demographic transition gets this temporary window with relatively few dependents, sometimes called a demographic dividend, before ageing pushes the ratio back up.
Common mistakes on the AP exam
- Assuming a high dependency ratio means young people. Japan and Niger can both have high ratios. Always say whether the dependents are mostly young or mostly old.
- Reading bar length as a count. On percentage pyramids each bar is a share of the total, so a country with a large population can still have short bars.
- Swapping the sides. By convention males are on the left and females on the right. Check the labels before describing a sex imbalance.
- Explaining shape with births only. Death rates and migration also shape a pyramid. A male-heavy bulge at ages 25–40 points to labour migration, not to birth rates.
- Forgetting population momentum. A wide base means continued growth for decades, even after fertility drops to replacement level.
When the AP exam uses this
Population pyramids are topic 2.3 (Population Composition) in AP Human Geography, and they connect directly to the demographic transition model (2.5) and to the challenges of ageing populations (2.9). Free-response questions often provide a pair of pyramids and ask you to identify each country's stage, explain one consequence of its age structure, or propose a policy response.