The population explosion was not caused first by people having more children. It began because fewer people died—especially fewer infants and children—while families continued, for a time, to have many births.
Demographers describe the broad sequence as the demographic transition. In simplified form, a society moves from high mortality and high fertility to low mortality and low fertility. Death rates often fall first, while birth rates respond more slowly. During that gap, births greatly exceed deaths and population rises.
The transition is a model, not a universal timetable. European mortality and fertility declined over generations. In many lower-income countries after the Second World War, public-health measures and imported medical technologies reduced mortality much faster. Similar stages therefore produced very different speeds of growth.
Why mortality falls first
Clean water, sewage systems, vaccination, antibiotics, better nutrition, and safer care around pregnancy and birth all reduce premature death. The UN Inter-agency Group for Child Mortality Estimation documents the dramatic fall in mortality before age five since 1990. Each avoided death is first a human good. Together, those gains transform the age structure: more children survive, cohorts become larger, and the number of future parents rises.
Fertility commonly adjusts later because children are embedded in a household economy and a way of life. Where mortality is high, parents may have more births knowing some children will not survive. Children may contribute labor and provide security in old age. As survival becomes reliable, education expands, cities grow, and the cost of raising children changes, parents generally choose fewer births.
Women’s education and control over timing matter greatly. Urbanization can make children more costly within the household. Formal schooling extends dependence. Pensions can reduce reliance on adult children. Contraception prevents births that would otherwise occur, abortion prevents pregnancies from ending in live births, and later marriage shortens the years available for childbearing. Moving women’s prime reproductive years into education and paid work changes timing and raises the cost of having several children. These mechanisms are obvious even though their relative weight differs by country and period.
The transition does not stop at two
Early versions of the model could be read as if fertility would settle near replacement once mortality became low. Experience has not been so tidy. Many affluent societies passed through replacement and continued downward. Middle-income countries have often followed faster than Europe did, sometimes reaching very low fertility before they became comparably wealthy.
The classic transition explains the great burst of growth: mortality fell before fertility. It does not explain why births remain low for decades, why similar economies diverge, or why some places fall below one child per woman. Those are questions for later chapters.
For now, it supplies the basic chronology. Growth accelerates because survival improves. It slows after births adjust. It may turn negative if low fertility persists and migration does not offset the gap between births and deaths. Before mapping where that process stands, we need to be precise about what fertility measures do—and do not—tell us.
Citations
- United Nations Inter-agency Group for Child Mortality Estimation, Levels & Trends in Child Mortality 2024.
- United Nations, Department of Economic and Social Affairs, Population Division, World Population Prospects 2024: Methodology Report.
- United Nations Population Fund, “Demographic dividend”.