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# 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.

Updated Nov 5, 2026

## Key points

- Stage 1: high births and deaths, stable population. Stage 2: deaths fall, rapid growth.
- Stage 3: births fall and growth slows. Stage 4: both low; the population ages.
- Expansive pyramid (wide base) = young population; constrictive (base narrower than the middle) = falling births.
- Dependency ratio = (under 15 + 65 and over) / population aged 15 to 64 × 100.
- Ageing index = 65 and over / under 15 × 100.
- WHO projects that the world's share of people over 60 will rise from 12% in 2015 to 22% in 2050.

## 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).

## Memory aid

First fewer graves, then fewer cradles: that is how the pyramid turns from a triangle into a column.

## 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.

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## Connections

## Related 4

- [Epidemiologic transition](https://durgical.com/en/epidemiology/epidemiologic-transition/)
- [Life expectancy](https://durgical.com/en/epidemiology/life-expectancy/)
- [Mortality rates: crude and specific](https://durgical.com/en/epidemiology/mortality-rates-crude-and-specific/)
- [Infant, neonatal and perinatal mortality](https://durgical.com/en/epidemiology/infant-neonatal-and-perinatal-mortality/)

[← All of epidemiology](https://durgical.com/en/epidemiology/)

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](https://www.who.int/news-room/fact-sheets/detail/ageing-and-health) (World Health Organization, 2026); [Principles of Epidemiology | Lesson 3 - Section 4](https://archive.cdc.gov/www_cdc_gov/csels/dsepd/ss1978/lesson3/section4.html) (Centers for Disease Control and Prevention, 2012).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/epidemiology/demographic-transition-and-the-population-pyramid/