The structure and evolution of any population are determined by three fundamental variables:
: High birth and death rates; population size remains stable but low. Demography: The Study of Human Population
: Death rates fall due to better sanitation and medicine, while birth rates remain high, leading to rapid population growth. The structure and evolution of any population are
: 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. while birth rates remain high