Joseph Kopecky

And every generation or so — catastrophe. In 1773 a failed harvest pushed deaths to 53 per 1,000, double the birth rate, in a single year. In 1809 war and typhus did it again. This was the old rhythm of human numbers: slow growth, then a knockback. Repeat for millennia.

Every spike in that red line is a year I shrink. I do not care for the spikes.

Then, around 1810, the deaths line stops spiking and starts sliding. Smallpox vaccination. Potatoes. Peace. Eventually soap, sewers, and doctors who wash their hands. But look at births: they don't budge for another seventy years. That shaded wedge — more arrivals than departures, every single year — is the escape. Sweden's population triples in a century.

Births eventually follow deaths down. Once children stop dying, families stop needing six of them. By the 1930s the two lines are reunited at the bottom, low and quiet, where they've stayed. That whole arc — high and wild to low and calm — is the demographic transition. Every country on Earth is somewhere along it.

But why did births fall? Economists are still arguing, which tells you it's a good question. The classic suspects:

• The quantity–quality tradeoff: once education started paying, parents chose fewer children and invested more in each.
• Falling child mortality itself: when children stopped dying, insurance-sized families lost their point.
• And culture. Exhibit A is France, where families began limiting births around 1760, a century before anyone else and well before death rates fell. Recent work ties that shift to the secularization of French society.

For the full argument, this modern survey is a fine rabbit hole to dive down.

Two hundred years of data and the answer is still “it's complicated.” Economists love this.

Zoom out. Sweden walked the transition over two centuries; humanity is walking it now, all at once, each country at its own pace. The world needed roughly 300,000 years to reach its first billion people, in 1805. The second billion took 122 years. The eighth took 12.

The UN's best guess is that the escape ends the way it began — with the line going flat — at around 10.3 billion people in the 2080s. Everything else on this page lives inside this curve: the waves that pulse through it, the island it emptied, the countries sprinting through it, and the last region still mid-leap.

The next four facts are hiding in this curve. Let's go find them.

Then the line does something the demographic transition is not supposed to do. It turns around and climbs — for eighteen straight years — reaching 3.8 children per woman at the 1958 peak, a level America hadn't seen since before the First World War. Then it resumes its fall as if nothing had happened. That red window is the baby boom: not a pause in the long decline, but a genuine, generation-long reversal of it.

And it was not an American quirk. Canada boomed even harder; Australia and France bounced hard too; Britain and Sweden — both already below replacement in the mid-1930s — turned upward for a generation. Across most of the Western world, the mid-century did the same "impossible" thing at the same time. Each of those bumps set a wave rolling through a population. Here's the American one.

Time to see what a surge like that does to the shape of a country. This is a population pyramid — each bar a five-year cohort, youngest at the bottom — and only three forces can ever change its shape. The first is fertility, and its work is finished the day a cohort is born: it sets the size of the bar entering at the bottom. The highlighted bar is 1960’s newest arrivals, the class of 1956–60, enormous next to the thin Depression-era cohorts stacked above it.

The second force is mortality, and it decides how much of the bar survives the climb. Watch one hundred newborns live out their lives twice: once under the death rates of 1900, once under today’s. Under 1900’s rules a fifth are gone before their fifth birthday, and only 39 of the 100 reach 65. Under 2023’s rules, 83 do. Modern pyramids keep their width for sixty years not because more people enter, but because almost nobody leaves early anymore.

The third force is migration, the wildcard. Fertility only adds at the bottom; mortality only subtracts on the way up. Migration respects neither rule. Watch it swell and squeeze the pyramid wherever the movers happen to be: usually the working ages, but truly any bar, in either direction. Hold that thought: one fact from now, we’ll visit a country where this force overwhelmed the other two for a full century.

Now, the wave itself. Between 1946 and 1964, American maternity wards delivered seventy-six million children: two straight decades of the crowded nurseries the 1930s had said weren’t coming. There they are in red: the baby boomers, stacked into the bottom of the pyramid, every one of them under twenty.

Keep your eye on the red band. It's going to shove me out of shape for a century.

Populations age the way people do: one year per year. By 1985 the wave has rolled into the workforce: an enormous, young, increasingly educated labor supply arriving all at once. Economists call the growth that follows a demographic dividend, and it helped power the late-century boom.

By 2010 the boomers fill the peak earning-and-saving years. More workers per dependent than the country had ever seen, and a flood of retirement savings chasing assets. A tailwind for growth, for markets, for budgets — and dangerously easy to mistake for normal weather.

Then the wave crests past 65. In 1960 the United States had more than five working-age adults for every person of retirement age; by 2030 it will have fewer than three (watch the counter above the chart). Pensions, healthcare, labor markets, even interest rates: this single moving band is quietly reshaping all of them. (This is the corner of the pyramid where my research lives.)

Step back and consider how strange the era you just scrolled through was. For seventy-odd years, the wave held off the arithmetic of the transition: while the boomers filled the working ages, the rich world enjoyed pyramid-style worker-to-retiree ratios inside what was already becoming a pillar. Cheap pensions, swelling labor forces, booming asset demand: a one-time demographic subsidy, easy to mistake for permanent. It's now being withdrawn, cohort by cohort. If my research has one message, it's this: the postwar decades weren't demographically normal. They were the wave.

Now, why does that matter so much? Because the modern welfare state's arithmetic was set by this history. Rewind to 1935. When Social Security passed, the math looked heavenly: more than nine working-age adults per person past 65, and in the program's early years, forty-plus covered workers paying in per beneficiary drawing out. But notice: that was not the boom. There hadn't been one yet. The ratio was high because yesterday's enormous generations still filled the working ages while high mortality kept the top of the pyramid thin. And fertility, as you just saw, was collapsing at that very moment. The founding math was a snapshot of a shape that was already disappearing.

Then the wave arrived, and the arithmetic won a reprieve. For decades the boom poured fresh workers into the bottom of the system. So when Medicare and Medicaid passed in 1965, five and a half workers still stood behind every retiree, and pay-as-you-go looked as sound as ever. There they are in red, stacked at the base, about to pour into the very workforce that would fund it all. The wave didn't create these programs' logic — it subsidized it, masking for two generations where the transition was always going to take that number.

The number quietly doing the work in all of this is the old-age dependency ratio: adults of working age (blue, 20–64) per person 65 and over (red). Social Security was born when it stood above nine; Medicare, near five and a half. Transfer systems — from today's workers to today's retirees — run beautifully while that number stays big; the assumption is built into their arithmetic. And you already know where it's heading: under three by 2030. The programs aren't broken. They were born on a vanishing shape, granted a two-generation reprieve by the wave — and the reprieve is ending.

And then the wave passes. But look at what it leaves behind: not a pyramid — a pillar. The boom was a one-time event, and once fertility settled low, the wide base never came back. The wave changed the shape of the country permanently on its way through.

Between 1845 and 1852, blight destroyed the potato crop that fed the island's poor. At least one million people died of hunger and disease, and by 1855 some two million more had boarded ships — for Liverpool, New York, Boston, anywhere. In a single decade the island lost around a quarter of its people. On this chart, catastrophe is not a spike. It's a cliff.

The famine ended; the leaving didn't. For the better part of a century, emigration outran the cradle, and census after census the line fell, then scraped along the bottom from the 1920s to the 1960s. While the world's population tripled through the greatest growth era in human history, Ireland halved. Population change isn't only about births and deaths. People move — and whole national histories can be written by the movers.

Where did they go? In the famine decade alone, America took roughly a million and a half. Some 340,000 sailed for Canada on the infamous "coffin ships" to Quebec. Some 300,000 settled in Britain — far more crossed, but nobody was counting the boats to Liverpool and Glasgow. And somewhere between seventy and a hundred thousand made it all the way to Australia, most only after gold turned up in Victoria in 1851. The boats set a pattern that held for a century: once a family had someone in Boston, the next departure was already half-decided.

Which brings us to the accounting that governs every population on Earth: four flows. Births and arrivals fill the tub; deaths and departures drain it. Most countries' stories are dominated by the first pair. Ireland is the rare country where the second pair ran the show for a century. Now it runs in reverse: today Ireland is one of the fastest-growing countries in Europe, and most of that growth walks off a plane.

Every population everywhere is just these four taps. Remember them — they run the whole page.

And here is the reversal that would have astonished the famine generation: at the 2022 census, one resident in five was born somewhere else. England and Wales lead (history runs on long tides), then Poland, India, Romania, Brazil. The planes now land where the boats once left.

And still — after independence, EU membership, the Celtic Tiger, and two decades of record immigration — the island holds about seven million people. The red line is 1841. A hundred and eighty years of history, and Ireland has yet to climb back over it.

Then came the fastest development run in economic history and, with it, the fastest fertility fall ever recorded. By 2025 the wide base is gone. In 2023 South Korea recorded 0.72 births per woman: the lowest national fertility figure ever measured, anywhere, ever.

Run it forward and the pyramid flips upside down. By 2100 the UN projects South Korea's population at roughly half of today's, with the biggest bars sitting above seventy. An entire society flipped on its head: begun within living memory, finished within the lifetime of a child born today.

You try standing like this for eighty years.

The whole story fits in one line: six births per woman to fewer than one, in two generations. South Korea crossed the replacement threshold in the early 1980s and simply… kept going. No war, no famine, no mandate — just development, urbanization, education, and brutal costs of housing and childrearing, all pressing the same direction.

So is Korea an outlier? Honestly? Yes. Nobody else has printed 0.72. But it is not alone; it's just furthest down a road nearly everyone is on. Italy and Spain crossed replacement in the late 1970s and early '80s, China and Thailand in the early 1990s, Iran by 1999. Today roughly two-thirds of humanity lives in a country below replacement fertility.

Still picturing the "booming" countries as the exception? Look closer. Vietnam crossed in 1998, Brazil in 2002, Mexico in 2016. And India, the most populous country on Earth, the one everyone imagines growing forever, slipped below replacement in 2020. Indonesia and Bangladesh sit a rounding error above the line. The growth these countries are famous for is momentum from births that already happened. The engine underneath has already turned over. Which leaves exactly one region still running hot…

Now run it to 2100. The layers stop moving — all but one. Asia peaks and turns down; Europe shrinks outright; the Americas flatten. Only the red band keeps widening: Africa, from 230 million people in 1950 to a projected 3.8 billion by 2100. From one person in eleven to more than one in three.

Here's the number worth memorizing: between now and 2100 the world is projected to add about 2 billion people — and Africa alone adds 2.3 billion. That is not a typo. Africa accounts for more than one hundred percent of future world population growth, because everyone else, combined, starts to shrink.

The race in one picture: the rest of the world peaks in the 2050s and declines; Africa keeps climbing right through the century. The future of the global labor force, of growth, of migration — and yes, of pensions — runs through this chart.

The last wide base on Earth. Pyramids don't stay pyramids — ask Korea.

Here we are today. The base has stopped widening (the world's median age has climbed to about thirty) and the shape is visibly straightening. Every story you just scrolled through is in this picture: the boomer bulge working upward, Korea's missing base, Africa's still-wide foundation.

So where does it settle? The UN's medium path says the curve flattens near 10.3 billion in the 2080s and drifts down: humanity's pyramid finishing its slow turn into a pillar.

But be honest about the error bars. If fertility recovers a little, the high path keeps climbing past 12 billion. If it keeps falling the way Korea, Italy, and now most of the world suggest it might, the peak arrives decades sooner — and the pillar narrows, generation by generation, into an urn. The demographic transition only happens once, and nobody has seen the ending. That's what makes it worth studying.

Wake me in 2100. One of these lines will be right.