A population needs an average of about 2.1 births per woman to sustain itself over time. This number, called the replacement level fertility rate, ensures that each generation produces enough children to replace the previous one. It’s slightly above 2.0 because not every child born will survive to adulthood, and slightly more boys are born than girls.
Why 2.1 and Not Exactly 2.0
Two parents need two children to replace themselves, so 2.0 seems like the obvious number. But two factors push it higher. First, roughly 105 boys are born for every 100 girls, meaning not every birth produces a future mother. Second, a small percentage of children will die before reaching reproductive age. In wealthy countries with low child mortality, these two factors add up to about 0.1 extra births per woman, giving the familiar 2.1 figure.
In countries where more children die young, the replacement rate climbs higher. The average across developing regions is closer to 2.3 births per woman, and in places with the highest child mortality, it can be higher still. Each country’s true replacement number depends on its own survival rates and sex ratio at birth. Populations with heavily skewed sex ratios (more boys born than girls) need even more total births to produce enough women for the next generation. Research in population modeling has shown that when sex ratios and mortality are both unfavorable, the minimum fertility needed to avoid long-term population decline can rise well above 2.1.
Where the World Stands Now
The global fertility rate in 2024 was 2.25 births per woman, down from about 5 in the 1960s and 3.3 in 1990. That global average masks enormous variation. As of 2024, 131 countries and areas (55 percent of all countries) had fertility rates below 2.1, and those countries account for 68 percent of the world’s population. More than two-thirds of humanity now lives in places that are technically below replacement.
South Korea represents the extreme end, with a fertility rate of just 0.75 in 2024, the lowest on the planet. If that trend continues, researchers project the country’s population could shrink from roughly 52 million to about 28 million by the end of the century. Many other East Asian and European nations sit well below replacement, typically between 1.0 and 1.6.
The United Nations projects that global fertility will decline to 2.1 by the late 2040s. At that point, without migration, the world’s population would be on a path toward eventual stabilization.
Why Populations Keep Growing After Fertility Drops
A country’s birth rate can fall below 2.1, yet its population can keep growing for decades. This effect, called population momentum, happens because of age structure. When a country has spent years with high fertility, it builds up a large generation of young people. Even if those young people have smaller families, there are so many of them entering their childbearing years that total births still outnumber deaths for a long time.
The demographer Nathan Keyfitz demonstrated this in the 1970s: even an immediate, overnight drop to replacement fertility in a high-fertility population would still produce decades of continued growth, simply because so many women were already in or approaching their reproductive years. This is why global population is still rising today even as most of the world has fallen below replacement. The large cohorts born during the high-fertility era of the mid-20th century are still alive and, in many developing countries, still having children.
How Migration Fills the Gap
For individual countries below replacement, immigration is the main mechanism that offsets declining births. The scale of the effect is significant. UN projections show that without any migration, Europe’s population would be about 6 percent smaller by 2050 compared to current trends. For North America and Oceania, the gap is even larger: a 13 percent difference.
Europe provides a concrete example of how migration and natural decline interact. Between 2015 and 2050, Europe is projected to experience 57 million more deaths than births. A net inflow of around 32 million migrants over that period would limit the actual population loss to about 25 million. Migration doesn’t fully erase the gap, but it substantially softens the decline.
The Economic Pressure of Low Fertility
When fertility stays below replacement for a long period, the age structure of a population shifts dramatically. Fewer children are born, the working-age population shrinks relative to the elderly, and the dependency ratio rises. This ratio measures how many people of non-working age (children and retirees) each working-age adult must economically support.
A low dependency ratio is favorable: more workers generating tax revenue, funding pensions, and sustaining healthcare systems. When birth rates drop well below replacement, the ratio tips the other way. Fewer workers support a growing number of retirees, straining pension systems, healthcare budgets, and economic growth. Countries like Japan, South Korea, and much of Southern Europe are already experiencing this shift, with some projections showing fewer than two workers for every retiree within a few decades.
Can Government Policies Raise Birth Rates?
Many countries have tried financial incentives to encourage larger families, with mixed results. Australia introduced a one-time birth incentive in 2004 that produced a 12.8 percent increase in fertility. Spain offered 2,500 euros per birth from 2007 to 2010 and saw a 4.7 percent bump. Canada ran quarterly birth payments from 1988 to 1997, ranging from $500 for a first child to $8,000 for a third, and observed increases of 4.4 to 9 percent depending on the program period.
Hungary has been among the more aggressive examples, offering cash benefits for second, third, and fourth births that increased fertility by 26, 32, and 14 percent respectively. These are notable jumps in percentage terms, but context matters. A 26 percent increase on a fertility rate of 1.3 brings it to about 1.6, still well short of 2.1. No country that has fallen significantly below replacement has managed to climb all the way back through policy incentives alone. Financial support helps at the margins, but the broad social and economic forces that drive smaller family sizes, including higher education costs, later marriage, urbanization, and women’s workforce participation, have proven remarkably resistant to reversal.
The 2.1 threshold remains a useful benchmark, but it’s not a cliff edge. Populations don’t collapse the moment fertility dips below it. The real consequences unfold over decades, shaped by how far below replacement fertility falls, how long it stays there, and whether migration, policy, or shifting social norms alter the trajectory.