A new study has found that human populations may need a fertility rate of at least 2.7 children per woman — far higher than the long-accepted 2.1 — to reliably avoid long-term extinction.

The research, published April 30 in PLOS One and led by Takuya Okabe of Shizuoka University in Japan, challenges decades of demographic assumptions, Phys.org reported. Using mathematical models, the team examined how variations in fertility, mortality, and the likelihood that some adults never reproduce can significantly increase the risk of population decline — even when a society meets the traditional “replacement level” birth rate.

“Considering stochasticity in fertility and mortality rates, and sex ratios,” co-author Diane Carmeliza N. Cuaresma said, “a fertility rate higher than the standard replacement level is necessary to ensure sustainability of our population.”

The study highlights how smaller populations are especially vulnerable to these random demographic shifts, which can gradually eliminate entire family lineages. A slight increase in female births appears to mitigate this risk, possibly explaining why more girls tend to be born during times of extreme conditions such as war, famine, or upheaval — a phenomenon long noted by evolutionary scientists.

While the researchers do not predict imminent collapse for large developed nations, they caution that most individual family lines, along with their cultural and linguistic heritage, are likely to fade over time unless fertility rates rise.