3.2.4.4 - Natural population change and the demographic dividend
Learn to interpret vital rates and age–sex composition, apply demographic-transition ideas in contrasting settings, and explain when a larger working-age share can produce a demographic dividend.
Vital rates and natural change
Natural population change is births minus deaths; migration is a separate component of overall change.
Crude birth and death rates
Annual births or deaths per 1,000 total population. They are crude because the denominator includes all ages, not just those exposed to the same age-specific risk.
Subtract the death rate from the birth rate when both use the same population and year. Convert a rate per 1,000 into a percentage by dividing by ten.
For an original illustrative population, birth rate 24 and death rate 8 per 1,000 produce natural increase of 16 per 1,000, or approximately 1.6% annually. In a population of 2 million, this is about 32,000 additional people before migration. Rates using mid-year populations and rounding give approximations; do not apply a five-year rate as if it were annual.
Total fertility rate estimates the number of children per woman under the period's age-specific fertility rates. It differs from the crude birth rate: a youthful population can produce many births even after fertility falls. Infant mortality is deaths before age one per 1,000 live births, not per 1,000 total residents. Life expectancy summarises age-specific mortality. An older population can have a high crude death rate despite good healthcare because more residents are at high-mortality ages.
Replacement fertility is the level at which generations replace themselves under specified mortality and migration assumptions; about 2.1 is a common low-mortality approximation. Falling below it does not guarantee immediate decline because age structure and migration matter.
The demographic transition model
The demographic transition model describes changes in birth and death rates associated with changing living conditions; it is a generalisation rather than a compulsory national timetable.
| Stage | Typical rates | Natural change and mechanisms |
|---|---|---|
| 1 | High, fluctuating births and deaths | Little sustained growth; mortality vulnerable to disease and food crises |
| 2 | Deaths fall; births remain high | Rapid increase as sanitation, food and medicine improve survival |
| 3 | Births fall; deaths remain relatively low | Growth slows as reproductive choice, education and family costs change |
| 4 | Low births and deaths | Small natural increase or balance, with an older structure |
| Proposed 5 | Births below deaths | Natural decrease, often with low fertility and ageing |
Death-rate decline can precede fertility decline because lifesaving services spread before family expectations or reproductive access change. In stage 3, child survival, education and income security can reduce desired family size, while contraception enables people to act on preferences. These are interacting explanations, not a claim that everyone changes behaviour when income reaches one threshold.
The model largely generalises historical transitions and omits migration from its birth/death curves. War, epidemics and unequal development can interrupt them. Some countries experience fertility decline without following the same industrial history; subnational communities can occupy different demographic conditions. A declining population is therefore not proof that its whole society has passed through an identical five-stage path.
Age–sex composition and population momentum
Age–sex composition shows how past births, deaths and migration shape the people available to have children, work and need services now.
A population pyramid normally places age groups on the vertical axis and male/female numbers or percentages on opposite horizontal sides. A broad base suggests many recent births; a narrow base relative to working ages can indicate falling births. A large older share reflects survival and earlier cohort sizes. A notch may reflect fewer births, deaths or migration; a shape alone cannot identify its cause. Check whether axes show counts or percentages and use comparable age bands.
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