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