A new study published in PLOS ONE in 2025 has raised alarm among demographers and policymakers by arguing that the previously accepted replacement fertility rate of 2.1 children per woman is no longer sufficient to ensure long-term population stability. According to researchers, a fertility rate of at least 2.7 children per woman may be necessary—particularly in developed nations—to reliably prevent population extinction over time.
The study is based on stochastic demographic modeling that simulates population changes under various real-world conditions, including fluctuations in birth rates, random demographic shocks, migration stagnation, and shifts in reproductive age. These simulations revealed that only populations maintaining a birthrate above 2.7 were consistently able to avoid multi-generational decline and extinction scenarios.
The urgency of the study’s findings is underscored by recent data from multiple countries. In 2025, South Korea reported a record-low fertility rate of 0.72, the lowest globally. Japan followed with 1.23, while Italy and Spain posted rates of 1.20 and 1.19 respectively, according to Eurostat. The United Kingdom’s fertility rate dropped to 1.44—its lowest since record-keeping began in 1938—and the United States continues to hover at 1.62, below the replacement level for the fifteenth consecutive year. These figures, far below even the previous benchmark of 2.1, now look even more unsustainable under the 2.7 threshold proposed by the study.
Researchers also noted that a female-biased birth ratio could slightly reduce extinction risk, as reproductive continuity is more likely in populations with a higher proportion of females. Nonetheless, birth rates across high-income countries have continued to fall despite aggressive government interventions. France, Sweden, and Hungary have implemented generous parental leave programs and child allowances; Singapore and Israel have expanded access to state-funded fertility treatments; and China has introduced subsidies for housing and childcare in a bid to slow its population decline. Yet, according to a 2025 OECD report, such measures often affect only the timing of births, not the total number of children individuals choose to have.
Economists and sociologists point to a broad range of interrelated factors driving down fertility: high costs of housing and childcare, greater career prioritization, changing social norms, access to higher education, and widespread use of contraception. In wealthier nations, cultural values increasingly support delayed or child-free lifestyles, particularly among younger generations.
The implications of these demographic shifts are significant. The United Nations Population Division reports that more than 40% of the global population now lives in countries with fertility rates below the replacement level. Shrinking workforces, strained pension systems, and intergenerational inequity are expected to worsen as the demographic structure skews older. If the new research is correct, maintaining the old target of 2.1 children per woman will not be enough. Unless future fertility rises significantly or is offset by high levels of sustained immigration, many societies may face irreversible demographic contraction in the decades ahead.
