The Demographic Transition Model
| Stage | Birth rate | Death rate | Natural increase | Pyramid shape | Examples in this app |
|---|---|---|---|---|---|
| 1 | High (35–45‰) | High (30–40‰), unstable | Near zero; population fluctuates with famine and disease | Expansive — very wide base, almost nobody past 60 | “Pre-industrial society (model)”. No real country is in Stage 1 today. |
| 2 | High, still 35–50‰ | Falling fast (medicine, sanitation, food) | Very high, 2–3.5%/yr | Expansive — wide base, each bar much larger than the one above | Niger today; Nigeria; India, Iran, and Mexico in the 1970s |
| 3 | Falling (urbanization, education, contraception) | Low | Moderate, slowing | Base stops widening — the youngest bars are no longer the biggest | Iran in the 1990s; Mexico and Brazil around 2000; Nigeria by 2050 |
| 4 | Low (10–15‰) | Low (7–11‰) | Near zero | Stationary — nearly straight sides up to old age | United States; Canada (growing only through immigration); India today; Brazil and Mexico by 2050 |
| 5 | Very low (under 10‰) | Rising as the population ages | Negative — natural decrease | Constrictive — pinched base, bulge in middle or old age | Japan; Germany; Ukraine; South Korea; China after 2030 |
Vocabulary that shows up on the exam
- Cohort: everyone born in the same span of years. Each bar is a five-year cohort; click one in Compare mode and it stays highlighted as the years advance.
- Dependency ratio: (people under 15 + people 65 and over) ÷ people 15–64. High in Stage 2 because of children, high again in Stage 5 because of elders.
- Demographic dividend: the window in Stage 3 when a huge cohort of young workers arrives and the child share shrinks — low dependency, fast economic growth if there are jobs.
- Echo boom: a bulge that reappears 25–30 years later when a large cohort has its own children. Watch the U.S. or China.
- Demographic momentum: a population keeps growing after fertility drops to replacement because so many people are still in their childbearing years. Watch Niger or Nigeria after 2050: fertility is falling fast, yet the base of the pyramid stays wide for decades.
- Epidemiological transition: the mortality half of the story — deaths shift from infectious disease (Stages 1–2) to chronic, degenerative disease (Stages 4–5).
Where the numbers come from
Every pyramid here is generated by a cohort-component model: each country starts from its approximate 1950 age structure and is projected forward in five-year steps using fertility, life expectancy, and net-migration trends that follow the UN World Population Prospects. Births enter at the bottom, each cohort survives up one bar, and migrants are added by age.
That means the shapes are realistic and internally consistent — a baby boom really does climb the pyramid and produce an echo — but the figures are approximations, not official statistics. Use them to reason about structure and stage, and cite the UN or U.S. Census Bureau International Database for exact numbers.
Stage rules used by this app
- Stage 1 if births ≥ 35‰ and deaths ≥ 25‰ with growth under 1.5%.
- Stage 5 if deaths exceed births (by at least 1‰) and fertility is below 2.1.
- Stage 2 if fertility ≥ 4.5 or births ≥ 30‰.
- Stage 3 if fertility ≥ 2.3 or births ≥ 19‰.
- Otherwise Stage 4.
Real countries sit on a spectrum, so a pyramid can sit near the line between two stages — the rates in each panel show which way it is moving.