The human population may need at least 2.7 children per woman to avoid long-term extinction, according to a new study.

It is widely accepted that the required replacement level is 2.1 children per woman to maintain a stable population. This figure is based on statistical models which expect large, steady populations with low child mortality and equal sex ratios. However, this does not account for random variation in fertility rates, termed demographic stochasticity, which can significantly impact small populations. Researchers at Shizuoka University, Japan, developed mathematical models to include these factors and concluded that a replacement rate of at least 2.7 is required for long-term population stability.

'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,' said Diane Cuaresma from Shizuoka University and first author of the paper published in the journal PLOS One.

Most developed countries currently fall far below both this suggested threshold and the conventional figure. England and Wales reported a total fertility rate of 1.44 in 2024 (see BioNews 1263), the USA recently reported a figure of 1.67 (see BioNews 1286). Mathematically, these trends point toward the eventual extinction of these populations if current patterns persist.

'Extinction is not an immediate issue owing to the large population size in these [developed] countries,' the study authors wrote. 'However, the present results have a profound implication from an individual perspective: The family lineages of almost all individuals are destined to go extinct, whereas very few exceptions may survive for many generations.'

The study authors also highlighted that humans exhibit a potential evolutionary adaptation in times of crisis, such as war or famine – a temporary shift toward a female-biased birth ratio. This may serve to reduce the risk of extinction by increasing the number of potential future child bearers, and support population resilience.

In response to declining birth rates, governments in developed countries have considered strategies to encourage people to have children. PET, the Progress Educational Trust, has recently hosted two events exploring whether fertility services might offer part of the solution to reversing declining birth trends.

Speakers included Professor Bart Fauser, scientific director of the International Federation of Fertility Societies and emeritus professor of reproductive medicine at Utrecht University, the Netherlands. In a previous BioNews article (see BioNews 1227) Professor Fauser and Dr Edgar Mocanu, consultant gynaecologist, said: 'Decades of one-sided policies have contributed to this decline. The focus has not been on family building but on family planning, such as contraception and abortion, to curb population growth, a policy driven mostly by the fear of overcrowding, mass migration, and environmental damage.'

Potential solutions proposed to support population growth in countries with low fertility rates include more accessible and affordable childcare, improved parental leave, education on infertility prevention, and better access to affordable fertility treatments.