Our current fertility rate isn't enough for human survival

Humanity has spent the last century wrestling with population growth. For decades, global efforts focused on reducing fertility rates to curb overpopulation. Family planning, access to contraception, and changing social norms contributed to a steep decline in births.

The global total fertility rate (TFR) dropped from 5.3 in the 1960s to 2.3 by 2023. Today, however, we face a paradox. Instead of a crowded future, scientists are now warning of potential collapse – not from too many people, but too few.

A recent study, published in the journal PLOS One, shifts the narrative again. Led by Takuya Okabe of Shizuoka University and colleagues, the research argues that the widely accepted replacement rate of 2.1 children per woman is too low to guarantee long-term survival.

The analysis suggests a more realistic number is 2.7. This higher threshold takes into account the unpredictable forces that shape real-world populations – forces that statistics alone often overlook.

Why 2.1 isn’t enough anymore

The replacement fertility rate of 2.1 assumes a stable world. It expects low child mortality, equal numbers of boys and girls, and large, steady populations. These assumptions work in clean models, but reality is messier.

People don’t always have two children. Some have none. Some die before reproducing. And in small populations, such random variation – called demographic stochasticity – can wipe out entire family lines.

Okabe and his team developed mathematical models to test what happens over generations when these fluctuations are included. They simulated populations using a branching process, a tool commonly used in population ecology.

Each woman could have any number of children, following a Poisson distribution, and not all offspring would survive. These models revealed that unless the average number of children reaches 2.7, the risk of extinction remains high over time.

In short, the study calls for a reevaluation of what sustainability truly means. The average isn’t enough if too many fall below it.

Low fertility rate may erase family lines

The consequences aren’t only theoretical. Even in large countries with millions of people, most family lineages will eventually disappear if the fertility rate remains below 2.7.

This has profound implications for identity, heritage, and cultural memory. Your surname, your traditions, your family stories – without enough descendants, all of it vanishes with time.

“Considering stochasticity in fertility and mortality rates, and sex ratios,” said Diane Carmeliza N. Cuaresma. “A fertility rate higher than the standard replacement level is necessary to ensure sustainability of our population.”

Extinction here doesn’t mean the end of humanity in one dramatic event. Rather, it means the quiet fading away of individual histories, surnames, and ancestral lines. It’s an erosion of human diversity, invisible until it’s too late to reverse. This applies not only to families, but to the world’s languages and cultures.

At least 40% of the 6,700 spoken languages are predicted to vanish in the next century. Each lost language takes with it stories, songs, and spiritual beliefs that no data set can replace.

Fertility rate is dropping across the world

While global population numbers still rise, the future looks different depending on where you live. Two-thirds of the world’s people now reside in countries with fertility rates below the 2.1 mark.

In Japan, the rate is 1.3. In South Korea, it has dropped as low as 0.87. Even in the United States, it’s only 1.66. These numbers suggest slow decline, but the reality may be steeper.

Japan’s population, for example, is projected to shrink by 31% every generation if current trends continue. This decline is compounded by aging, as fewer young people are born to support older ones.

Policies in countries like France and Singapore attempt to raise birth rates through incentives, but they’ve seen only modest results.

Modern life comes with trade-offs. Education and careers delay parenthood. Housing and childcare costs are high. In some cultures, single-child families have become the norm. These social realities are harder to change than the numbers suggest.

Female-biased births reduce extinction risk

The study offers a compelling insight: more girls can help prevent collapse. This isn’t about social engineering but about biology’s quiet adjustments. A female-biased sex ratio at birth increases the pool of future mothers. This gives populations a better chance of sustaining themselves.

Stressful environments often trigger such shifts. Wars, famines, and environmental disasters have led to more girls being born.

In humans, psychological stress, poor nutrition, pollution, and even natural disasters have all been linked to a rise in female births. The same pattern appears in mammals like cattle, pigs, rats, and mice.

These adaptive shifts may be nature’s way of giving species a survival edge when facing extreme pressure. The presence of more females, even slightly, improves the odds that some lineages will persist through hard times.

Low fertility rate often leads to collapse

Okabe’s team simulated thousands of generations to see what happens when fertility remains below the 2.7 mark. Most populations collapsed within 20 generations.

Even at a fertility rate of 2.1, extinction was all but certain. A handful of simulations survived longer – but those were exceptions, not the rule.

These findings show that survival, while not impossible, becomes highly improbable when fertility drops too far. As fertility approaches 2.7, survival chances improve. But anything lower becomes a gamble against mathematical odds.

In this model, randomness plays a central role. A few unlucky generations can eliminate an entire lineage, especially if there are no surviving males or females to continue it. That risk grows when fewer children are born on average. It’s not enough to hit 2.1 once. Consistency across generations is key.

What this means for future planning

Though the study focuses on human populations, its lessons apply broadly. Conservationists often aim to boost fertility among endangered species. But many plans fail to consider randomness, sex ratios, and mortality.

As this study shows, achieving a simple average isn’t sufficient for long-term stability. Species with small populations, especially those isolated or fragmented, face similar extinction dynamics.

A slightly skewed sex ratio or unexpected drop in births can push them toward collapse. Adjusting targets to reflect these deeper risks could strengthen conservation strategies.

For humans, the study urges policymakers to rethink population goals. It challenges the notion that economic or environmental concerns justify perpetual decline.

Sustainability isn’t just about shrinking footprints. It’s also about keeping enough people – and diversity – to carry forward what makes humanity rich and resilient.

We must rethink population goals

The debate over population often swings between panic over too many and fear of too few.

This research brings nuance to that conversation. It shows that the path to survival isn’t just about numbers – it’s about understanding the complex web of chance, biology, and behavior.

As fertility rates fall and the world changes, we must ask what kind of future we want. Not just one where humans survive, but one where traditions, languages, and lineages live on. A fertility rate of 2.1 may no longer be enough to keep that future alive.

The study is published in the journal PLOS One.

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