Two universal pathways in demographic transition

Abstract

Demographic transition, characterized by declines in fertility and mortality, is a global phenomenon associated with modernization. While typical patterns of fertility decline are observed in Western countries, their applicability to other regions and the underlying mechanisms remain unclear. By analyzing demographic data from 195 countries over 200 years, this study identifies two universal pathways in the changes in the crude birth rate (i.e., births per 1,000 individuals λ) and life expectancy at birth ({e_0}), characterized by the conservation of either {λe0} or {λ\exp(e_0/18)}. These pathways define two distinct phases governed by different mechanisms. Phase I, characterized by the conservation of {λe_0}, dominated until the mid-20th century, with high child mortality and steady population growth. In contrast, Phase II, conserving {λ\exp(e_0/18)}, has prevailed since 1950, featuring low child mortality and steady GDP per capita growth. A theoretical model considering the trade-off between reproduction and education elucidates the transition between these phases, demonstrating that population size is prioritized in Phase I, while productivity is maximized in Phase II. Modernization processes, such as declining educational costs and increasing social mobility, are identified as key accelerators of the transition to Phase II. The findings suggest that reducing educational costs can foster fertility recovery without compromising educational standards, offering potential policy interventions. This study provides a novel theoretical framework for understanding demographic transition by applying principles from statistical physics to uncover universal macroscopic laws and their underlying mechanisms.