A population structure is a statistical and visual representation of a population classified by age and sex. Population pyramids are the primary tool for analysing structure, and they reveal a great deal about a country’s demographic history, current characteristics and likely future.
A population pyramid (age-sex pyramid) is a back-to-back bar chart showing the number or percentage of males (left) and females (right) in each age cohort (typically 5-year groups: 0–4, 5–9, … 75+).
Reading a pyramid:
- Wide base → high proportion of children → high birth rate
- Narrow base → low birth rate
- Concave sides → high death rate reducing each successive cohort
- Bulge in middle → large cohort (e.g., baby boom)
- Wide top → high life expectancy, ageing population
- Irregular shapes → historical events (war, famine, migration)
| Shape | Description | Country Examples | Stage of DTM |
|---|---|---|---|
| Expansive (triangular) | Very wide base, rapidly narrowing; high birth rate, high death rate | Niger, DR Congo, Mali | Stage 1–2 |
| Transitional | Wide base but narrowing; birth rate falling faster than death rate | Kenya, Philippines, India | Stage 2–3 |
| Constrictive (barrel/urn) | Narrow base, bulging middle; low birth rate, low death rate | Germany, Japan, Australia | Stage 4–5 |
| Stationary (rectangular) | Near-equal width across age groups; very low birth and death rates | Sweden (historically); rare in pure form | Stage 4 |
At a point in time, a pyramid reveals:
- The current birth rate (width of base cohorts)
- The current death rate (rate of narrowing with age)
- Life expectancy (proportion of elderly)
- Sex ratio at various ages (male:female balance)
- Dependency ratio — proportion of population in economically dependent age groups (0–14 and 65+) relative to the working-age population (15–64)
$$\text{Total Dependency Ratio} = \frac{\text{Population aged 0–14 + 65+}}{\text{Population aged 15–64}} \times 100$$
Over time, comparing pyramids from different years shows:
- Whether fertility is rising or falling
- The impact of historical events (visible as narrow cohorts for birth-year cohorts during famines/wars, e.g., China’s missing cohorts from the famine of 1959–1961)
- Population ageing (base narrowing, top widening)
- Migration impacts (young adults over-represented if immigration; under-represented if emigration)
Nigeria (2023): Classic expansive pyramid. Base age cohort (0–4) is extremely wide; proportion over 65 is <3%. TFR ~5.4. Reflects Stage 2 of DTM with high birth rates and declining death rates.
Japan (2023): Deeply constrictive/inverted pyramid. Narrow base (very low TFR ~1.2); bulge at ages 60–75 (baby boom cohort born 1947–1960); wide top. Over 28% of population aged 65+. Severe economic challenges from shrinking workforce and pension pressures.
Australia (2023): Broadly constrictive with mild bulge at 30–55 (baby boom). TFR ~1.6. Immigration maintains a slightly wider working-age group than purely domestic fertility would produce.
China (2023): Irregular constrictive pyramid with visible effects of: (1) the baby boom in the 1960s, (2) the one-child policy (1980–2015) severely narrowing cohorts born after 1980, (3) rapid ageing. TFR ~1.0 in 2023 — below even Japan’s historical lows.
A high youth dependency ratio (many children) strains education and healthcare budgets in developing countries. A high aged dependency ratio (many elderly) creates pension and healthcare pressures in developed countries.
KEY TAKEAWAY: Population pyramids are diagnostic tools — they reveal birth rates, death rates, life expectancy, ageing, and the impacts of historical events. The shape of a pyramid directly corresponds to a country’s stage in the Demographic Transition Model.
EXAM TIP: In a pyramid question, always: (1) describe the overall shape, (2) identify specific features (wide base, narrow top, bulge), (3) state what each feature tells you about birth rate, death rate, or life expectancy, (4) link to DTM stage or a specific historical cause.
COMMON MISTAKE: Confusing the dependency ratio formula. Remember: the denominator is the working-age population (15–64), not the total population. A higher ratio means more dependents per worker — not more workers per dependent.
