Jesús Fernández-Villaverde, a Professor of Economics at the University of Pennsylvania and a Research Associate at the National Bureau of Economic Research (NBER), recently outlined the complex "unpleasant arithmetic" behind why modern advanced societies struggle to achieve the 2.1 replacement fertility rate. His analysis highlights the demographic realities that make population self-replacement a significant challenge, even when many families have two or more children.
The economist explained that a population of 100 women in an advanced economy requires 210 children to replace itself, accounting for various biological and societal factors. This figure considers the natural sex ratio at birth, where approximately 105 boys are born for every 100 girls, and the slight mortality rates before women reach their fertile age. > "Absent sex-selective practices, roughly 105 boys are born for every 100 girls," Fernández-Villaverde stated in his tweet, noting that about 98% of girls typically survive to reproductive age in advanced economies.
A significant hurdle to reaching replacement levels is involuntary childlessness, which Fernández-Villaverde estimates affects at least 10% of women, a conservative figure compared to medical literature suggesting 13% or more. His model illustrates that even if 75% of women have two or more children, as is common, the population would still fall short of replacement. He argues that > "to reach 210 births, you need a substantial share of women to have three or more children."
However, modern societal structures often make having three or more children a "costly proposition" for most families. Factors such as rising costs of living, childcare expenses, economic uncertainty, and women's increased participation in education and careers contribute to smaller family sizes. Data from the OECD shows the average total fertility rate across its member countries was 1.59 in 2022, with some nations like South Korea (0.78) and Spain (1.19) significantly below even the 1.8 rate mentioned in Fernández-Villaverde's example.
Fernández-Villaverde also dismissed the notion of any "self-regulating mechanism" that would naturally push societies back to a 2.1 fertility rate. He critiqued the market-clearing analogy often used by economists, asserting that > "scarcity feedback does not work the same way." The sustained sub-replacement fertility rates observed globally, including a U.S. rate of 1.66 in 2022, underscore the profound demographic shifts occurring.
The long-term economic implications of sub-replacement fertility are significant, including an aging population, a shrinking workforce, and increased strain on social security and healthcare systems. These demographic trends pose substantial challenges for governments seeking to maintain economic growth and social stability in the coming decades.
