Over the course of a century, the average Iranian man grew about 16.5 centimetres taller. Not a few select men, a whole population. British men, over the same stretch, gained around 11 centimetres. American men gained about 6. Iran outpaced them all.

We are not clinicians, nutritionists, or public-health researchers, and nothing here is health advice. This is one reader working through a large observational study. The patterns below are population-level averages, not statements about any individual person or family.

What the study actually measured

The finding comes from a 2016 paper called “A century of trends in adult human height,” published in the journal eLife by the NCD Risk Factor Collaboration, with researchers led from Imperial College London and working in collaboration with the World Health Organization.

The scale makes it worth taking seriously. The team reanalysed 1,472 population-based studies covering more than 18.6 million people, and used them to estimate average adult height for people born between 1896 and 1996 across 200 countries. 

Out of all of it, Iranian men showed the largest gain of any male population: 16.5 centimetres, with a 95% credible interval of 13.3 to 19.7 centimetres. The female record went to South Korean women, who gained 20.2 centimetres.

Why Iran, and why height at all

The study measures the change, not the cause, so anything here is inference. But the direction researchers point in is consistent. The Imperial release put it plainly: “children and teenagers who are better nourished and live in better environments tend to be taller”, and a mother’s health and nutrition during pregnancy may matter too. Height is a signal, not a verdict.

My read is that this is what makes the Iran number interesting rather than just impressive. It isn’t really a story about genes changing over a century. As co-lead author Mariachiara Di Cesare explained, genetic change does not happen that quickly, so a century-scale shift points to environmental conditions.

What can shift is childhood: whether a kid gets enough of the right food, whether serious infections stunt growth, whether clean water and health care are available. Height can record some of those conditions decades later.

Majid Ezzati, the Imperial professor who led the research, framed the whole exercise as a kind of health portrait. He said 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.” That “may even be shrinking” is the part worth dwelling on.

The half of the story nobody quotes

Growth is the easy headline. The reversals are harder to sit with. The same paper found that in several sub-Saharan African countries, including Niger, Rwanda, Sierra Leone, and Uganda, adult height decreased or stopped increasing for cohorts born after the early 1960s, falling by as much as around 5 centimetres from its peak in some countries. In those places, later generations ended up shorter than people born decades before them.

The reason isn’t settled by the study itself. But researchers pointed to childhood nutrition and environment. Mariachiara Di Cesare drew a contrast between two different failures. Asked why some populations lag, she put it this way: “In one case, it’s poverty and no access to food with the right nutrients. In the other, it’s quantity but not nutrients plus inequalities.” One is having too little. The other is having plenty of the wrong things. Di Cesare frames this as an explanation, not something the height data alone proves.

What height can and can’t tell you

It’s tempting to read the tall countries as winners and the shrinking ones as a tragedy, and leave it there. That framing misses what the number actually is. Height isn’t the point. It can act as a stand-in for something we can’t easily measure across a hundred years: the accumulated conditions of childhood in a place. 

There’s also a limit on what any single figure means. A tall population isn’t automatically a healthy one in every respect, and the study didn’t claim it was. Ezzati’s own conclusion was a call to “address children and adolescents’ environment and nutrition on a global scale” and give children the best possible start in life. 

A later NCD-RisC study made the concern more specific. It found that in many countries children had healthy-range height and weight at age five but then fell behind during their school years, suggesting that nutrition and living conditions during school age matter too.

What I keep coming back to is how patient this kind of data is. Nobody set out to record Iran’s twentieth century in centimetres. It accumulated across surveys and records until the trend became visible. One population got taller; another, for a while, got shorter. The tape measure doesn’t editorialize. It just reports.