How Many Kids Per Woman Are Enough to Prevent Human Extinction?

Human populations require an average fertility rate of at least 2.7 children per woman to avoid eventual extinction over time, a threshold significantly higher than the widely accepted replacement level of 2.1, according to a new study published in the open-access journal PLOS One by Takuya Okabe of Shizuoka University, Japan, and colleagues (1^✔ ✔Trusted Source
Threshold fertility for the avoidance of extinction under critical conditions

Go to source). 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.

Impact of Birth Ratios and Population Size

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.

Fertility Targets for Sustainability

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."

Reference:

1. Threshold fertility for the avoidance of extinction under critical conditions - (https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0322174)

Source-Eurekalert