The Irreversible Trap of Modernization: Fertility, Decline, and Civilizational Fate

This paper proposes and systematically proves the Irreversibility of Modernization Hypothesis, which resolves the core paradox that has long plagued traditional population economics: why economic recessions raised fertility in agrarian societies but further reduce it in modern societies. We construct the Modernization Irreversibility Index (MII)—comprising individual freedom, rigid living costs, time opportunity cost, and value shift—and prove it is a unidirectionally increasing variable immune to economic cycles. Incorporating MII into a dynamic general equilibrium model, we demonstrate that once a civilization completes modernization, its fertility will permanently fall below replacement level and converge to an endogenous steady state of sustained negative population growth and permanent economic recession. Empirical tests using 1960–2024 panel data from 180 countries show that MII has the strongest explanatory power for fertility (R²=0.82) after controlling for income, education, gender equality, and urbanization. All standard policy interventions can only delay fertility decline in the short run but cannot reverse it. Our findings indicate that low fertility and civilizational decline are endogenous fates of modernization, not problems of particular countries or institutions.