: 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.
: 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)
: Death rates fall due to better sanitation and medicine, while birth rates remain high, leading to rapid population growth. Demography: The Study of Human Population
: Both rates are low; the population stabilizes.
: Birth rates begin to fall as society urbanizes and education (especially for women) increases. : The actual reproductive performance of a population
The structure and evolution of any population are determined by three fundamental variables:
: High birth and death rates; population size remains stable but low. : The movement of people across borders
: 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.