Demographic transition and the population pyramid
- Concept
Populations move from many births and many deaths to few of both. The age pyramid changes shape and the population grows older.
What it is
The demographic transition describes how a population moves from high birth and death rates to low birth and death rates. Mortality falls first (especially in infants) and the population grows fast; then births fall and growth slows. The population pyramid shows the result: a wide base in young populations, a bell or urn shape in ageing ones.
Worked example
- A made-up municipality today has 2,400 people under 15, 6,000 aged 15 to 64 and 600 aged 65 or over.
- Total dependency ratio: (2,400 + 600) / 6,000 × 100 = 50 dependants per 100 people of working age.
- Youth: 2,400 / 6,000 × 100 = 40; old age: 600 / 6,000 × 100 = 10; ageing index: 600 / 2,400 × 100 = 25.
- Thirty years later, with fewer births and better survival: 1,800 under 15, 6,600 aged 15 to 64 and 1,650 aged 65 or over.
- Total dependency: 3,450 / 6,600 × 100 = 52.3; youth 27.3 and old age 25.0 (rounded to one decimal).
- Ageing index: 1,650 / 1,800 × 100 = 91.7 older people per 100 under 15.
- Reading: total dependency barely changes, but it shifts from children to older adults: the services needed change.
Stages of the transition and shape of the pyramid
| Stage | Births | Deaths | Pyramid shape |
|---|---|---|---|
| 1 | High | High | Wide base and narrow top |
| 2 | High | Falling | Very wide base: expansive |
| 3 | Falling | Low | Narrowing base |
| 4 | Low | Low | Column or urn: constrictive, with more older adults |
Markers
- Pyramid
- Dependency ratio
- Ageing index
- Birth rate
In practice
- Ageing raises the demand for chronic disease care, long-term care and rehabilitation.
- Much of Latin America is moving into the late stages of the transition, with falling births and an ageing population.
Common pitfalls
- Reading the dependency ratio as an exact economic figure: it uses age only, not whether people work.
- Assuming the transition is the same across a whole country: rural and urban regions can be at different stages.
- Confusing the demographic transition (births and deaths) with the epidemiologic transition (causes of disease and death).
Test yourself
Think of the answer, then tap the question to see it.
What falls first in the classic demographic transition, births or deaths?
Deaths, especially infant deaths; births fall later.
How is the dependency ratio calculated?
(People under 15 + people 65 and over) / people aged 15 to 64 × 100.
What shape is the pyramid of a young population with high births?
Expansive: a wide base that narrows quickly towards the top.
There are 1,200 people aged 65 or over and 2,000 under 15. What is the ageing index?
1,200 / 2,000 × 100 = 60 older people per 100 under 15.
Connections
Educational content on epidemiological and statistical methods; the examples use made-up numbers to teach the calculation. Not a clinical or legal guide: current Colombian rules are in the official sources.
Sources: Ageing and health (World Health Organization, 2026); Principles of Epidemiology | Lesson 3 - Section 4 (Centers for Disease Control and Prevention, 2012).
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Representative model, for educational purposes only.