1.3 Population structure: Interpret population pyramids.

1.3 Population Structure – Interpreting Population Pyramids

1. What is a Population Pyramid?

A population pyramid is a graphical representation of a country’s or region’s age‑sex structure. The horizontal axis shows the proportion of the population (percentage or absolute number) split into males (left) and females (right). The vertical axis lists five‑year age groups, from the youngest at the bottom to the oldest at the top.

2. Key Features to Observe

• Shape of the pyramid – indicates the stage of the Demographic Transition Model (DTM) and whether the population is youthful, stationary or ageing.
• Width of each bar – size of a particular age‑sex cohort.
• Symmetry between the sexes – large differences may signal migration, gender‑specific mortality or cultural practices.
• Peaks and troughs – reveal historical events such as baby booms, wars, epidemics or policy changes.

3. The Demographic Transition Model (DTM)

The DTM links economic development with changes in birth and death rates, producing characteristic pyramid shapes.

Evaluation (AO3): The DTM provides a useful framework for linking development with population change, but it assumes a linear path, overlooks cultural factors, and does not fully account for migration or policy interventions.

4. Core Demographic Rates (Definitions & Calculations)

Worked‑out example: A country has 22 births and 8 deaths per 1 000 population.

• CBR = 22 / 1 000
• CDR = 8 / 1 000
• NIR = 22 – 8 = 14 / 1 000 (or 1.4 % per year)

5. Population Momentum

Definition (one sentence): Population momentum is the tendency of a population to continue growing after fertility has fallen to replacement level because of a relatively large proportion of people in their child‑bearing years.

Numeric illustration: Assume a country with a TFR of 1.8 (below replacement) but a very broad base: 30 % of the population is aged 0‑14 and 25 % is aged 15‑29. Even with the low TFR, the large number of women entering reproductive age will add roughly 1.2 million people over the next 20 years, keeping the total population on an upward trajectory.

6. Dependency Ratio

Formula: \[ \text{Dependency Ratio} = \frac{\text{(Population 0‑14 + Population 65+)} }{\text{Population 15‑64}} \times 100 \]

Worked example: In a country of 50 million:

• 0‑14 = 12 million
• 15‑64 = 30 million
• 65+ = 8 million

Dependency Ratio = ((12 + 8) ÷ 30) × 100 = (20 ÷ 30) × 100 ≈ 67 %.

Interpretation: 67 % means there are roughly 67 dependents for every 100 people of working age, indicating a relatively high pressure on the labour force to provide education, health and pension services.

7. Causes of Youthful vs. Ageing Structures

• Fertility – High fertility creates a wide base (e.g., Niger, TFR ≈ 7.0). Low fertility narrows the base (e.g., Japan, TFR ≈ 1.3).
• Mortality – High infant/child mortality reduces younger cohorts; low mortality expands them and lengthens older cohorts.
• Migration – In‑migration of young workers widens middle‑age bars (e.g., Gulf states). Out‑migration of young adults thins those bars (e.g., Eastern Europe).

8. International Migration – Types & Push‑Pull Factors

• Voluntary labour migration – Pull: higher wages, better living standards; Push: unemployment, low wages at home.
• Family reunification migration – Pull: joining relatives; Push: political or economic instability that separates families.
• Forced migration (refugees, asylum seekers) – Pull: safety, international protection; Push: war, persecution, natural disaster.
• Seasonal/temporary migration – Pull: seasonal work (e.g., agriculture, tourism); Push: lack of year‑round employment.

These flows leave characteristic signatures on pyramids: tall, narrow male bars in the 20‑34 age groups for male‑dominant labour migration; spikes in child and female cohorts for refugee influxes; thin 15‑29 bars where young adults emigrate.

9. Typical Pyramid Shapes and Their Implications

1. Expansive (young) pyramid – Broad base, steep sides, narrow top.
   Implications: High birth & death rates, high youth dependency, pressure on primary education and child health. Typical of DTM Stages 1‑2.
2. Stationary (rectangular) pyramid – Relatively uniform width across most age groups, slight taper at the top.
   Implications: Low birth & death rates, stable growth, moderate dependency. Typical of Stage 3.
3. Constrictive (aging) pyramid – Narrow base, wide middle, large elderly cohort.
   Implications: Low birth rates, high life expectancy, high old‑age dependency, potential labour shortages. Typical of Stages 4‑5.

10. Impacts of Youthful and Ageing Populations

11. Pro‑ and Anti‑Natalist Policies – Evaluation

12. Reading Specific Indicators in a Pyramid

13. Worked Example – Real Country Pyramids (2024)

13.1 Nigeria – Youthful Structure

Source: UN World Population Prospects 2024 (percent of total population).

Interpretation

• Very wide base → high fertility (≈5.4 children / woman).
• Steep decline after age 30 → relatively low life expectancy (≈61 years).
• Male and female bars are almost equal → limited gender‑biased migration.
• Shape = expansive → DTM Stage 2.
• Implications: high youth dependency, pressure on primary education, potential demographic dividend if jobs are created.

13.2 Japan – Ageing Structure

Source: UN World Population Prospects 2024.

Interpretation

• Narrow base → very low fertility (≈1.4 children / woman).
• Broad middle and large top → high life expectancy (≈84 years) and a growing elderly cohort.
• Female bars wider in older groups → greater female longevity.
• Shape = constrictive → DTM Stage 5.
• Implications: high old‑age dependency, pressure on health‑care and pensions, potential labour shortages mitigated by automation or immigration.