Population decline, not even 2.1 children per woman is enough to save us from extinction

Too few to save a population: until now, 2.1 children per woman was the minimum requirement to ensure survival, the so-called 'standard replacement rate', but updated mathematical models indicate that this is not the case at all and shift the threshold to 2.7 children per woman. Human populations therefore need a much higher fertility rate for it to be sufficient to avoid extinction in the long term, note the authors of the research published in the journal Plos One, and coordinated from Japan, with the Shizuoka University group led by Takuya Okabe.

What makes the difference are above all the random factors that for the first time have been included in the mathematical models: differences in the number of children women actually have, mortality rates, sex ratios, and the probability that some adults do not have children are the factors that have brought down the values hitherto considered safe for survival. The new calculations thus indicate that "a fertility rate above the standard replacement level is necessary to ensure the sustainability of our population", as Diane Carmeliza N. Cuaresma, one of the authors of the research, notes. It is especially in small populations that these random differences make their impact felt and are strong enough to wipe out entire family lineages and, with them, their language and cultural traditions. In contrast, the process would occur more slowly in large developed populations, where most family lineages would eventually die out anyway.

No individual countries were considered in the research, but as far as Italy is concerned, the fertility rate of 1.18 children per woman recently published by Istat and relative to 2024 is decidedly very low even compared to the standard replacement rate of 2.1 children per woman. In general, the authors of the research call for a rethinking of conventional fertility targets. In particular, due to random fluctuations in the number of births, they indicate that a fertility rate of at least 2.7 children per woman is needed to have the best chance of avoiding extinction, especially in small populations.

A lifesaver, they observe, is however the birth of more females than males: this difference, the researchers note, helps reduce the risk of extinction, allowing more lineages to survive over time. In this regard, the authors of the research refer to an evolutionary phenomenon observed historically, namely that in particularly difficult times due to wars or famine, more females than males tend to be born. The results of the research are not only relevant to human populations, but could be useful in redefining the fertility rates needed to safeguard endangered species.