Population Structure

Quick Summary

• Population structure = the breakdown of a population by age, sex, ethnicity, religion, etc. It's mainly shaped by fertility, mortality, and migration.
• A population pyramid (age-structure graph) plots age groups against male/female population size — it's the main tool for visualising this.
• Pyramids let governments plan spending on schools, hospitals, pensions, and housing based on what age groups actually exist.
• As a country develops through the stages of the Demographic Transition Model (DTM), its pyramid shape changes — from concave (Niger) → convex (Nepal) → column (USA) → narrowing pentagon (Japan).
• Three big population "issues" show up in pyramids: ageing populations, falling birth rates, and migration imbalances.
• The dependency ratio is a formula that tells you how many "non-workers" each 100 workers must support.
• Japan is the key case study: shrinking population, ultra-low fertility rate, and the world's highest life expectancy.

1. What Exactly Is a Population Pyramid?

Picture two bar charts glued back-to-back around a vertical axis. On the left, bars stretch out for males; on the right, bars stretch out for females. Each horizontal bar represents an age group (usually in 5-year bands: 0–4, 5–9, 10–14... up to 100+), and the length of the bar shows how many people are in that age group.

Why does this matter so much to a government? Because you can't plan anything — how many schools to build, how many hospital beds to fund, how many pension cheques to write — without knowing how many people are actually in each age group. A pyramid answers that at a glance, which is why geographers and governments rely on it constantly.

The Four Groups You Must Be Able to Spot

• Young dependents: ages 0–14 (roughly). Too young to work, dependent on others for support.
• Economically active population: ages 15–64 (roughly). The "working population" — they earn money, pay taxes, and support the dependents.
• Old dependents: ages 65+. Retired, often relying on pensions, healthcare, and family/state support.
• Dependency ratio: a calculated number showing how "burdened" the working population is by dependents (more on this below).

2. The Four Classic Pyramid Shapes (and What They Mean)

3. The Dependency Ratio

for every worker, how many people are they effectively supporting?

4. Implications of Population Structure