Below the line.

There is a single number that decides whether a population grows, holds steady, or eventually disappears. Demographers call it the replacement rate. In most places it sits at roughly 2.1 births per woman over a lifetime — two to replace the parents, and a small fraction more to cover children who do not survive to adulthood. Stay above it and the next generation is at least as large as this one. Fall below it, and births quietly start running short of deaths.

For most of human history this was never a worry. The worry ran the other way: too many mouths, not enough food. As recently as 1950, every region on Earth was comfortably above replacement, and the global average was around five children per woman. That world is gone. The global figure has roughly halved, to about 2.2 today, and it is still falling. Roughly seven in ten people now live in a country that has dropped below the line.

The regions did not cross the line together. Northern America slipped below replacement first, around 1972. Europe followed a few years later. Latin America and the Caribbean held on until 2014; Asia crossed in 2019. The map that once divided the world into "developed, low birth rate" and "developing, high birth rate" has been overtaken — middle-income giants are now below the line too.

Look at where the major economies actually sit today, and the picture stops being abstract.

South Korea has become the headline case — a fertility rate of 0.75 means each generation is on track to be roughly a third the size of the one before it. But it is not alone, and it is not extreme so much as early. China has fallen to about 1.0. Japan, Italy and Spain hover near 1.1 to 1.2. Even the United States, long the outlier among rich nations, has settled near 1.6 and has not been above replacement since 2007. India, the world's most populous country, slipped below the line for the first time in 2024.

A single generation did most of the falling

The drop did not creep down over centuries. In country after country, fertility collapsed inside the span of one or two generations — fast enough that grandparents and grandchildren grew up in demographically different worlds. The steepest falls happened in East Asia, where rates went from among the world's highest to among its lowest in about fifty years.

Three things stand out. First, the convergence: countries that began the period miles apart — South Korea at six children per woman, the United States at under four — have funnelled toward the same narrow band below the line. Second, the speed in Asia: South Korea, China and India each shed three or four children per woman in roughly the time it takes one cohort to grow up. Third, the holdout: Nigeria, and Sub-Saharan Africa broadly, is still high and falling slowly. The future of global population growth is concentrating into a shrinking set of regions.

A pattern this consistent, across countries with little else in common, points to shared causes. They are not mysterious. They are mostly the byproducts of things almost everyone agrees are good.

Modern life keeps subtracting births

No single lever explains the fall. It is the accumulation of several shifts that each push in the same direction — and reinforce one another. Here are the four that do most of the work.

Driver 01Education and literacy

The most reliable predictor of a country's fertility is how long its girls stay in school. As female literacy and schooling rise, average family size falls — almost without exception, across every culture and income level. Education delays the age at which women begin having children, widens the range of lives they can imagine beyond motherhood, and tends to travel alongside better access to contraception and family planning. A girl who finishes secondary school has, on average, dramatically fewer children than one who does not. This is the engine underneath nearly every other driver.

Driver 02Women in the workforce

When women can earn, the cost of stepping away to raise children rises — not just in lost wages, but in lost seniority, skills and independence. Where workplaces and partners adapt, families and careers coexist. Where they do not, women face what researchers studying Japan call a "dual burden": expected to compete at work as equals while still carrying most of the childrearing at home. Faced with that bind, many delay marriage and children, or forgo them. It is no coincidence that some of the lowest fertility rates appear in fast-modernised economies where workplace demands raced ahead of changes at home.

Driver 03The delay to childbearing

Across the rich world, the age at which women have their first child has climbed steadily for half a century. In Japan it rose from the mid-twenties in the 1970s to about 31 today; in South Korea it is now around 33, among the highest anywhere. Later starts mean fewer years in which to have children, and they compress the window for a second or third. Marriage — still closely tied to childbearing in much of East Asia — is being postponed alongside it. The chart below shows the drift.

Driver 04Infertility — the biological tail of delay

Delay carries a quiet cost. Human fecundity — the biological capacity to conceive — declines with age, gradually through the thirties and more sharply after. A couple that starts trying for children at 35 is, on average, less likely to succeed quickly, and more likely to want fewer children than they would have had they started earlier. Assisted reproduction helps at the margins but cannot fully reverse the arithmetic of timing. So the same delay that subtracts births directly also subtracts them indirectly, by running into the limits of biology. The result is a gap, found in survey after survey, between the number of children people say they want and the number they actually have.

Around these four sit the familiar supporting pressures: the rising cost of housing and raising a child, the spread of cities where space is tight and children are expensive, economic insecurity that makes long commitments feel risky, and the simple fact that reliable contraception finally lets people have the family size they choose rather than the one biology hands them. None of these is a scandal. Most are things societies worked hard to achieve. That is precisely what makes the trend so hard to reverse.

Birth rates carry momentum. A country can change the mood overnight and still not change the outcome for decades, because the women who would have the children are already born — or already not born.

The line was never just a statistic. It is the threshold between a society that renews itself and one that, slowly and politely, does not.

Most of the world is now on the wrong side of it.