2.5 Population Growth and Decline Over Time and Space

Reading a population pyramid

A population pyramid graphs age and sex structure. A wide base with sharply narrowing top shows high birth/death rates and rapid growth (common in Stage 2 countries). A column-like shape shows slow or no growth (Stage 4/5). A pyramid pinched at the base indicates declining population. Pyramids reveal the dependency ratio — the working-age population relative to dependents.

Crude rates and natural increase

Crude Birth Rate (CBR) and Crude Death Rate (CDR) measure births/deaths per 1,000 people yearly. Rate of Natural Increase (RNI) = (CBR − CDR)/10, expressed as a percentage. High RNI means rapid growth; negative RNI means natural decrease. These rates, combined with migration, determine overall population change and are central to the Demographic Transition Model (DTM).

Momentum and doubling time

Population momentum occurs when a large cohort of young people continues to grow the population even after fertility drops, since many are entering reproductive age. Doubling time estimates how long it takes a population to double at its current growth rate, using the Rule of 70 (70 ÷ growth rate %). Countries with young pyramids have strong momentum and short doubling times.

Key terms

Population Pyramid

A graph showing age and sex distribution of a population, revealing growth trends.

Crude Birth Rate (CBR)

Number of live births per 1,000 people per year.

Rate of Natural Increase (RNI)

(CBR − CDR)/10, expressed as a percentage; measures population growth from births and deaths only.

Population Momentum

Continued population growth caused by a large young cohort moving into reproductive age, even as fertility declines.

Doubling Time (Rule of 70)

70 ÷ growth rate % = approximate number of years for a population to double.

Try an AP-style question

What is the rate of natural increase?

1. 2%
2. 20%
3. 0.2%
4. 3%