3.8 Human Population Dynamics

Historical population growth

Human population grew slowly for most of history, then rose sharply after the Industrial Revolution due to improved agriculture, sanitation, and medicine. This shift from high birth/death rates to low ones is described by the demographic transition model, which links population growth to a country's stage of economic development.

The demographic transition model (DTM)

The DTM has four stages: (1) Pre-industrial—high birth & death rates, slow growth; (2) Transitional—death rates fall (better healthcare/sanitation), births stay high, rapid growth; (3) Industrial—birth rates fall as living standards rise, growth slows; (4) Post-industrial—low birth & death rates, stable/declining population. Not all countries follow it identically.

Measuring population change

Key metrics include total fertility rate (TFR), infant mortality rate, life expectancy, and the age-structure diagram (population pyramid), which shows age/sex distribution and predicts future growth. A pyramid with a wide base indicates a young, fast-growing population; a rectangular shape indicates slow/stable growth.

Key terms

Demographic transition model (DTM)

A model describing how birth and death rates change as a country moves through four stages of economic development, affecting population growth.

Stage 2 of the DTM

Transitional stage: death rates fall due to improved healthcare/sanitation, but birth rates remain high, causing rapid population growth.

Total fertility rate (TFR)

The average number of children a woman is expected to have during her reproductive years.

Age-structure diagram (population pyramid)

A graph showing the distribution of ages and sexes in a population; shape indicates whether population is growing, stable, or declining.

Infant mortality rate

The number of deaths of infants under age one per 1,000 live births in a given year; reflects healthcare quality and development level.

Try an AP-style question

A country has a crude birth rate of 20 per 1,000 and a crude death rate of 8 per 1,000 (no migration). What is its growth rate?

1. 2.8% per year
2. 1.2% per year
3. 12% per year
4. 0.12% per year