: Birth rates fall below death rates, leading to an aging and potentially shrinking population—a stage now characterizing many advanced economies like Japan and Italy.
Most modern demographic analysis is framed by the , which describes the historical shift from high birth and death rates to low ones as societies develop.
: The movement of people across borders. While net migration is zero at a global level, it is a critical driver of "fast demography" at the national level, often offsetting natural population declines in developed countries. The Demographic Transition Model (DTM) Demography: The Study of Human Population
: High birth and death rates; population size remains stable but low.
: The actual reproductive performance of a population. Demographers measure this through the Total Fertility Rate (TFR) —the average number of children a woman would have in her lifetime. Currently, the world is nearing the "replacement level" of 2.1, below which a population eventually begins to shrink. : Birth rates fall below death rates, leading
The structure and evolution of any population are determined by three fundamental variables:
: Death rates fall due to better sanitation and medicine, while birth rates remain high, leading to rapid population growth. While net migration is zero at a global
: The incidence of death in a population. Improvements in healthcare and nutrition have led to a significant increase in global life expectancy, which rose by over eight years between 1995 and 2026.