A new study of ancient and modern societies shows that human populations need at least 2.7 children per woman – a much higher fertility rate than previously believed – to reliably avoid long-term extinction. 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. Why Declining Fertility Rates are a Ticking Time Bomb In the new study, published April 30, 2025 in the open-access journal PLOS One
