2.1 kids per woman might not be enough for population survival

Human populations need at least 2.7 children per woman – a much higher fertility rate than previously believed – to reliably avoid long-term extinction, according to a new study published April 30, 2025 in the open-access journal PLOS One by Takuya Okabe of Shizuoka University, Japan, and colleagues.

While a fertility rate of 2.1 children per woman is often considered the replacement level needed to sustain a population, this figure doesn’t account for random differences in how many children people have – as well as mortality rates, sex ratios, and the probability that some adults never have children. In small populations, these chance variations can wipe out entire family lineages. In the new study, researchers used mathematical models to examine how this demographic variability affects the survival of populations over many generations.

The study found that, due to random fluctuations in birth numbers, a fertility rate of at least 2.7 children per woman is needed to reliably avoid eventual extinction – especially in small populations. However, a female-biased birth ratio, with more females than males born, reduces the extinction risk, helping more lineages survive over time. This insight may help explain a long-observed evolutionary phenomenon: under severe conditions – such as war, famine, or environmental disruption – more females tend to be born than males. It also suggests that, while extinction isn’t imminent in large developed populations, most family lineages will eventually fade out. 

The authors conclude that true population sustainability – as well as the sustainability of languages, cultural traditions, and diverse family lineages – requires rethinking conventional fertility targets. The findings also have implications for conservation efforts of endangered species in which target fertility rates are set, they point out.

Diane Carmeliza N. Cuaresma adds, "Considering stochasticity in fertility and mortality rates, and sex ratios, a fertility rate higher than the standard replacement level is necessary to ensure sustainability of our population." 

In your coverage, please use this URL to provide access to the freely available article in PLOS One: https://plos.io/4lu0M6h

Citation: Cuaresma DCN, Ito H, Arima H, Yoshimura J, Morita S, Okabe T (2025) Threshold fertility for the avoidance of extinction under critical conditions. PLoS ONE 20(4): e0322174. https://doi.org/10.1371/journal.pone.0322174

Author countries: Japan, Philippines

Funding: Japan Society for the Promotion of Science (JSPS) KAKENHI grant nos. 23KK0210 and 21H01575 (HI), 21K21115 (HA), 21K03387 (SM), and 21K12047 (TO) The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.