Human Demographics

You’ll be able to

Read an age-structure diagram to predict a country’s future growth
Trace a population through the four stages of the demographic transition
Use the rule of 70 to calculate a population’s doubling time

Age-structure diagrams

An age-structure diagram (population pyramid) is a back-to-back bar graph of the number of males and females in each age group. Its shape forecasts growth. A broad-based pyramid — many children, few elderly — signals rapid future growth, because a large young cohort will soon reach reproductive age. A column shape with roughly equal age groups signals a stable population, and a narrowing base (fewer children than adults) signals a declining population. Fast-growing nations are typically less-developed; stable or shrinking ones are typically more-developed.

Total fertility rate and replacement

The total fertility rate (TFR) is the average number of children a woman has in her lifetime. Replacement-level fertility is about 2.1 — two children to replace the parents, plus a fraction to offset childhood mortality. A TFR above 2.1 means a growing population; below 2.1 means an eventually shrinking one (barring immigration). TFR falls when women gain education, when family planning and contraception are available, when child mortality drops, and as societies urbanize and industrialize.

The demographic transition

As countries develop, they tend to pass through the demographic transition in four stages. Stage 1 (pre-industrial): high birth and high death rates — slow growth. Stage 2 (transitional): death rates fall thanks to better food, sanitation, and medicine while birth rates stay high — the fastest growth. Stage 3 (industrial): birth rates fall as families choose fewer children — growth slows. Stage 4 (post-industrial): low birth and low death rates — stable or slightly declining population. The transition explains why the fastest-growing populations are those in the middle of developing.

Rule of 70 (doubling time)

doubling time (years) ≈ 70 / annual growth rate (%)

Enter the growth rate as a plain percent number (a 2% rate → 70/2 = 35 years). Only works for steady exponential growth, and the growth rate must be positive.

Worked example

A country’s population is growing at a steady 3.5% per year. Roughly how long will it take for the population to double?

1.Recall the rule of 70: doubling time ≈ 70 / growth rate (as a percent).
2.Insert the growth rate as the number 3.5 (not 0.035): doubling time ≈ 70 / 3.5.
3.Divide: 70 / 3.5 = 20.
4.So at a sustained 3.5% growth rate, the population doubles in about 20 years.

Answer: About 20 years. Small differences in growth rate matter enormously: at 1% the doubling time would be 70 years, but at 3.5% it is only 20.

Checkpoint

A nation’s population grows at a constant 2% per year. Using the rule of 70, what is its approximate doubling time?

Watch out

When using the rule of 70, plug in the growth rate as a whole percent number, not a decimal. A 2% rate goes in as 2 (giving 35 years), not 0.02. Using the decimal produces a nonsensical answer of 3,500 years.

Checkpoint

A country has an age-structure diagram with a very wide base of young children tapering to a narrow top. What does this predict about its population, and which demographic-transition stage does it fit?

On the exam

Read population pyramids by their base: wide base → growing (young population), straight sides → stable, pinched base → shrinking (aging population). Pair this with the demographic transition — Stage 2 shows the widest base and the fastest growth.

Answer the 2 checkpoints as you read.