Study: Current Fertility Rates Not Enough to Prevent Population Collapse
A new study published in the open-access journal PLOS One suggests that human populations may need a fertility rate of at least 2.7 children per woman to reliably avoid long-term extinction, significantly higher than the commonly cited replacement level of 2.1. The research was led by Takuya Okabe of Shizuoka University, Japan.
Although a fertility rate of 2.1 is typically viewed as sufficient to maintain population size, this standard estimate does not consider several important factors. These include random fluctuations in the number of children individuals have, mortality rates, sex ratios, and the possibility that some adults may never reproduce. In smaller populations, these stochastic variations can result in the loss of entire family lines.
To explore the long-term consequences of such demographic randomness, the researchers developed mathematical models that simulate population dynamics across generations. Their findings reveal that higher fertility rates are necessary to buffer against these inherent uncertainties and ensure population survival over time.
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.
In stark contrast to the newly suggested sustainability threshold of 2.7 children per woman, global fertility rates have been steadily declining for decades. According to the United Nations, the global average fertility rate in 2024 was approximately 2.3 and is projected to drop below 2.1 by mid-century.
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.”
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