Families need 3 children each to avoid extinction, study warns

A new study published April 30 in PLOS ONE finds that families need to have an average of 2.7 children to avoid extinction — far higher than the long-accepted replacement rate of 2.1. The research suggests that under current fertility trends, most family lineages in developed countries may vanish within a few generations.

Conducted by researchers from Japan and the Philippines, the study used a “branching process” model to simulate the fate of family lines over generations. The researchers found that when fertility rates fall below 2.7, the vast majority of lineages eventually die out, even in ideal conditions with equal birth sex ratios and low mortality.

Traditional replacement models, based on a rate of 2.1, do not account for demographic randomness — namely, that some individuals will not have children. This variability increases the likelihood of a family line ending, even in large populations. The authors describe this overlooked factor as a critical flaw in how fertility sustainability has long been understood.

The findings carry particular weight in East Asia, where fertility rates are already among the world’s lowest. South Korea recorded a rate of just 0.69 in 2024, while Japan’s was 1.20. These levels fall not only below the revised threshold but also far beneath the outdated 2.1 standard.

A separate study in 2019 found that Chinese, Japanese and Korean families in the U.S. tend to have children later in life and primarily within marriage, contributing to persistently low fertility rates.

According to the study, one overlooked factor that can raise the chances of a family’s survival is a higher ratio of female births, which has been observed in humans and other mammals under environmental or psychological stress. While not a solution, this tendency may be nature’s way of preserving lineages during hardship.

The broader implication is that many family names and cultural heritages could disappear — even as overall population numbers remain high. The extinction of a family line, the researchers argue, is not just a rare occurrence but a predictable mathematical outcome when fertility falls below critical thresholds.