Over a single century, comparing people born in 1896 with people born in 1996, South Korean women gained more height than any group of women anywhere in the world: about 20.2 centimetres. That is roughly the length of a hardback book, added to the average woman across four or five generations. Over the same stretch, South Korean men grew by about 15.2 centimetres. And before anyone reaches for genetics: this happened far too fast to be about DNA.

We are not clinicians, geneticists, or public-health researchers. What follows is our reading of one large study and a related one, not medical or nutritional advice. The figures here are population averages by year of birth, and patterns across whole populations don’t predict anything about a single person or family.

The numbers come from “A century of trends in adult human height,” published in the open-access journal eLife in 2016 by the NCD Risk Factor Collaboration. It is a substantial piece of work: a reanalysis of 1,472 studies covering more than 18.6 million adults born between 1896 and 1996 across 200 countries.

The tempting wrong read

The intuitive explanation is that Koreans always carried genes for greater height, and something switched them on. It is a natural guess, because for any one person, height really is strongly inherited. If your parents are tall, you probably will be too.

But that fact about individuals is exactly what misleads people about whole populations. The study is direct about it: differences between populations are thought to come mainly from environment, not genes, above all from nutrition and from infections in childhood and the teenage years.

A gene pool does not shift 20 centimetres in a century. Four generations is nowhere near enough time. Whatever happened in Korea happened to the world those children grew up in, not to their DNA.

What the data actually shows

What changed, in the century after 1896, was almost everything about how Korean children were fed, housed, and kept safe from disease. More reliable food, cleaner water, fewer childhood infections, better care. A body that spends less of its early energy fighting illness has more left over to grow.

Height, on this reading, is less a trophy than a receipt. It records the conditions of the years when a person was still growing.

A later study from the same group shows where those conditions matter most. In a 2020 analysis drawing on measurements from 65 million school-aged children and teenagers, the researchers found that “Children in some countries grow healthily to five years, but fall behind in school years.” Ezzati, the senior author, points to nutrition in those school years as the main driver, which is the team’s read rather than a settled fact.

This rests on one study, really two from the same group, so it is a strong clue rather than the last word. But the shape of the finding is hard to explain any other way. When a population’s diet improves and its childhood diseases fall, its children grow taller fast. When they don’t, they don’t.

Why women gained more than men

The part we find most telling: women gained 20.2 centimetres; men gained 15.2. That five-centimetre difference is not noise, and it is not a sign that Korean women somehow caught up biologically.

The average height gap between men and women within a country stayed fairly steady across the whole century, around 11 to 12 centimetres. So men did not stall while women surged. The likelier story is about starting points. Korean women began the century further below their own potential than Korean men did, which left them more room to gain once conditions improved. The larger female gain is perhaps a measure of how far behind they started.

What height is really measuring

Taller does not mean superior, and the study is not a ranking of national worth.

The Dutch, whose men born in 1996 average around 182.5 centimetres and remain the tallest population on the planet, did not win anything. What they, and the Koreans, and the Iranians, whose men gained the most of any male population at about 16.5 centimetres, mostly did was give their children better decades to grow up in.

Seen that way, the century of Korean height is a clear record of childhood conditions, generation by generation. And a record like that can run in reverse. The same study notes that in South Korea the gains started to flatten for men born in the 1980s, that height stalled across much of South Asia, and that it even reversed in parts of sub-Saharan Africa. As Ezzati put it, the study “gives us a picture of the health of nations over the past century, and reveals the average height of some nations may even be shrinking while others continue to grow taller.” A reversal is not a genetic event. It is a signal that the conditions children need have stopped improving, or have gotten worse.

What decides whether the next century’s line stays the same, keeps rising or falls seems to be the ordinary, unglamorous business of how well a society feeds and protects its kids.