3.1.2 Human Population Size & Composition
A Population Pyramid is a specialized horizontal bar graph that illustrates the distribution of various age groups in a population, typically split by gender (males on the left, females on the right).
The Vertical Axis represents age cohorts (usually in 5-year increments), while the Horizontal Axis represents the total number or percentage of the population in that cohort.
The Dependency Ratio is a critical metric derived from these structures, measuring the pressure on the productive part of the population to support the non-productive parts.
Dependency Ratio Formula:
| Feature | Expansive Pyramid | Constrictive Pyramid | Stationary Pyramid |
|---|---|---|---|
| Shape | Broad base, tapering top | Narrow base, wider middle | Rectangular/Columnar |
| Birth Rate | Very High | Low | Balanced with Death Rate |
| Life Expectancy | Low | High | High |
| Growth Trend | Rapid Increase | Potential Decline | Stable/Zero Growth |
Young Dependents are typically defined as those aged 0-14, while Old Dependents are those aged 65 and over.
The Economically Active group (ages 15-64) provides the tax base and labor force required to support both dependent groups.
Analyze the Base: A wide base always indicates a high birth rate, regardless of the total population size. If the base is significantly wider than the middle, expect high future growth.
Identify Bulges: Look for asymmetrical 'bulges' in the pyramid. A bulge in the middle-age range often indicates a period of high immigration or a historical 'baby boom'.
Check the Slope: A steep, concave slope on the sides of the pyramid suggests high mortality rates across all age groups, often seen in developing regions.
Verify Units: When calculating the dependency ratio, ensure you multiply by 100 to express the result as a percentage of the working population.
Size vs. Growth Rate: Students often confuse a large population with a high growth rate. A country can have hundreds of millions of people but a growth rate of zero or less.
Dependency Ratio Misinterpretation: A high dependency ratio does not always mean a 'poor' country; it can be caused by a high number of elderly people (aging population) or a high number of children (youth bulge).
Ignoring Migration: When asked about population change, do not focus solely on births and deaths. Migration can drastically alter the composition of the working-age group in a very short time.