INTRODUCTION

Over the past seven decades, fertility decline has become one of the most significant demographic transformations worldwide. The global total fertility rate (TFR)—defined as the average number of children a woman would bear if current age-specific fertility rates prevailed throughout her reproductive life—declined from approximately five births per woman in the early 1960s to around 2.3 births per woman in the early 2020s (United Nations, 2022; Ritchie, Spooner & Roser, 2023). Although the pace and timing of fertility decline have differed across regions, the overall direction has been remarkably consistent. Most high-income countries reached replacement-level fertility (approximately 2.1 births per woman) by the late twentieth century and have remained below this threshold since. Many middle-income countries followed during the late twentieth and early twenty-first centuries, while even historically high-fertility regions such as Sub-Saharan Africa have begun experiencing sustained fertility reductions (United Nations, 2022).

These global trends are commonly interpreted through the framework of demographic transition theory, which describes the shift from regimes of high mortality and high fertility to regimes characterized by low mortality and low fertility as societies undergo economic, social, and institutional transformation (Notestein, 1945). In the classical four-stage formulation of the theory, mortality declines first due to improvements in health, sanitation, and nutrition, followed by a gradual decline in fertility as family size preferences adjust to new economic and social conditions. The final stage is typically associated with demographic stabilization, in which low fertility and low mortality produce slow or near-zero population growth. However, contemporary demographic research increasingly suggests that many advanced economies have moved beyond this equilibrium into a phase of persistent sub-replacement fertility accompanied by population ageing and natural decrease, sometimes described as a “fifth stage” of demographic transition (Lesthaeghe, 2010; United Nations, 2022).

Empirical cross-country comparisons further illustrate how countries are distributed across different stages of this transition. One way to observe these differences is by examining the joint distribution of crude birth and death rates. Countries with high birth rates relative to death rates typically experience rapid population growth and are characteristic of earlier stages of demographic transition. In contrast, countries with low birth rates and relatively higher death rates—often reflecting ageing populations—are more likely to experience slow growth or natural population decline. As illustrated in Figure 2, most high-income countries and an increasing number of middle-income countries cluster in the low-fertility, low-mortality region of the distribution, reflecting advanced stages of demographic transition. Meanwhile, several countries in Sub-Saharan Africa remain characterized by high fertility and relatively high mortality, indicating earlier stages of the transition.

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