The age–sex pyramid is a rare chart that tells you as much about the future as about the past. Its shape is a frozen history of fertility, wars and migration across a whole century, and its silhouette already sketches how many workers, pensioners and schoolchildren the country will have twenty years from now.
The pyramid that stopped being a pyramid
Male to the left, female to the right. Bar width is the size of the age group (millions).
The classic “pyramid” — a broad base of children and a narrow apex of the old — is characteristic of a society with high fertility and short lives. But our chart is no longer a triangle: the base has narrowed, the middle has broadened, and the apex has grown heavier. This is the portrait of a society that has been through the demographic transition — from “many born, many die” to “few born, long-lived”.
Growth that has inertia
The transition also shows up in the population trajectory. The population is still growing, but the growth rate is steadily falling.
The solid line is population (M); the dashed line is annual growth (%). Growth falls but stays positive.
Here lies the counterintuitive part. Even when fertility drops to the replacement level, a population keeps growing for decades — thanks to demographic momentum: the large generations born in the high-fertility era are still of childbearing age. Like the ocean in a neighbouring piece, demography is a system with memory: today’s decision shows up a generation later.
Fertility can be lowered in a single generation, but population growth cannot be stopped at once: too many future parents have already been born.
From countryside to city
A second great shift runs in parallel — urbanisation. The urban share of the population rises over decades, and the process is tightly linked to falling fertility: in a city children are economically “dearer”, and parents’ time has more alternatives.
Stacked to 100%: how the urban and rural shares changed.
The scissors of the transition
The essence of the demographic transition shows best in one combined chart: fertility falls, life expectancy rises. The two lines diverge like the blades of a pair of scissors.
Total fertility rate (left axis) and life expectancy (right axis).
What this changes for a reader
Three takeaways. First: the shape of the age pyramid is a forecast, not just a snapshot; it reveals the future burden on pensions and the labour market. Second: population growth has inertia, so there is no such thing as an “instant” demographic decision. Third: fertility, urbanisation and life expectancy are not separate stories but three projections of one process; reading them apart means missing the whole.
