Women born in Spain in 1896 were estimated to average about 151.1 centimetres tall. Those born in 1996 averaged about 163.4 centimetres — a gain of 12.3 centimetres across a century of birth cohorts.

Picture two Spanish women side by side. One was born in 1896, the other in 1996. On average, the younger one would be taller by about the width of a hand, roughly 12 centimetres. \

The underlying estimates put the difference at 12.29 cm: 151.11 cm for Spanish women born in 1896 and 163.40 cm for those born in 1996. Neither is a real person, of course. They are the two ends of a line drawn through a century of birth years, and the gap between them is one record of how living conditions during growth changed over that century.

One thing to get straight first. A single average height figure describes a population, not one body. And the year attached to it is the year that generation was born, not the year anyone was measured. The dataset defines these as mean adult heights by year of birth. A woman in the youngest, 1996 cohort reached 18 in 2014, which is why the data series effectively ends there. So the two numbers, 151.11 and 163.40 centimetres, are best read as population estimates for the start and end of a long curve, not readings off a tape measure.

Where the numbers come from

The figures come from a large study by the NCD Risk Factor Collaboration, published in eLife in 2016. The researchers reanalysed 1,472 population-based studies containing measured heights from more than 18.6 million people to estimate mean adult height for people born between 1896 and 1996 in 200 countries.

That is a huge dataset. It is also a modelled one. The researchers did not have a perfectly continuous national survey running in every country for a century. Instead, they brought together population-based measurement studies and used a Bayesian hierarchical model to estimate how average height changed by birth cohort.

The exact Spanish values are openly available in the processed NCD-RisC dataset. Spanish women born in 1896 are estimated at 151.11 cm and those born in 1996 at 163.40 cm, a gain of 12.29 cm. Our World in Data identifies the source as the NCD Risk Factor Collaboration’s 2016 height study.

We are not clinicians, nutritionists, or epidemiologists. What follows is our reading of one large observational study, not health advice. Population averages describe generations, not any one person, and a number that describes a country tells you nothing about how tall you or your children should be.

Why populations grow taller

The interesting thing about a rising average height is that differences between populations are not thought to be mainly genetic. The study stresses that although height is one of the most heritable human traits, differences between populations are believed to be related to “non-genetic, environmental factors”.

Among those factors, fetal growth, nutrition, and infections during childhood and adolescence are particularly important determinants of eventual adult height. When those conditions improve across generations, more children can come closer to the height their biology allows.

The data shows how much room there was to climb, and how unevenly it played out. The biggest gain over the century went to South Korean women, who gained an estimated 20.2 cm. Iranian men gained 16.5 cm. At the other end, many countries in sub-Saharan Africa and South Asia saw little gain. Spain’s 12.29 cm increase for women puts it well above the global middle, though still far below South Korea’s extraordinary change.

Majid Ezzati, who led the research at Imperial College London, treated the dataset as a health record rather than a curiosity. 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.” He also drew a policy point from it, arguing that the findings show “we urgently need to address children and adolescents’ environment and nutrition on a global scale, and ensure we’re giving the world’s children the best possible start in life.”

Why Spain kept climbing when others stopped

Not every wealthy country kept rising. Some stopped decades ago.

Researchers reported that the United States was among the first high-income countries to plateau, followed by similar patterns in the UK, Finland and Japan, while Spain was among the countries where height was still increasing.

Earlier research using European conscript data likewise concluded that height had plateaued in Scandinavia, the Netherlands and Italy while continuing to increase in Spain, Portugal and Belgium.

Spain’s rise also appears to have continued relatively late. A later study of Spanish birth cohorts found a marked upward trend in mean height through much of the second half of the twentieth century. For women in that dataset the rise began later than for men, and for both sexes it stagnated or even declined among the 1990s birth cohorts. The authors cautiously concluded that Spain may have reached a stable mean height by the 1990s cohorts.

That makes the broad story plausible: Spain’s large twentieth-century gain continued later than in some northern European countries, before apparently flattening toward the end of the period. But it is safer not to claim that one particular improvement — food, sanitation or healthcare — individually caused the Spanish pattern. The height literature treats stature as the combined outcome of nutrition, disease exposure and the wider childhood environment.

What did not happen was the world’s populations simply converging on one height. Mary De Silva of the Wellcome Trust, which helped fund the study, put the emphasis there. For her, the most striking finding was “that despite the huge increases in height seen in some countries, there is still a considerable gap between the shortest and tallest countries.”

The tallest people in the dataset were Dutch men born in 1996, at an estimated 182.5 cm. At the opposite end of the century-long comparison were Guatemalan women born in 1896, at 140.3 cm. Most strikingly, the study found that the gap between the world’s tallest and shortest female populations remained about 20 cm over the century.

What a height figure actually records

A national average height is more than a fact about how tall people are. The researchers describe adult height as a kind of “mirror” of the social and environmental conditions affecting nutrition, health and prosperity during growth. They argue that height trends can therefore help reveal the effects of childhood and adolescent nutrition and environment across generations.

The 12.29 cm between the estimated average Spanish woman born in 1896 and the one born in 1996 is not a measurement of two bodies. It is the difference between two population estimates separated by a century of birth cohorts. The younger generation grew up under very different nutritional, economic and disease conditions from the older one. Average height is one of the biological traces those changing childhood environments left behind.