Why are Americans no longer the tallest people in the world? One theory: inequality.

Childhood poverty can influence a person’s entire life, regardless of whether that person climbs up the economic ladder.

And recent research underscores how poverty can impact people on a permanent biological level.

One important — and perhaps overlooked — way childhood poverty can shape our bodies is in our height. We only grow until age 18. If we get poor nutrition during critical stages of development during that time, we literally won’t grow to our fullest potential.

Height is “a measure of our health over an important part of our lives — during childhood and adolescence,” Mariachiara Di Cesare, a public health researcher with Imperial College London, tells me. Research has shown height is linked to our lifelong well-being. It’s associated with greater economic attainment and greater health.

Researchers like Di Cesare are turning to huge data sets on average height across the globe to better understand disparities in childhood nutrition. And unlike other indicators of health influenced by environment — like blood pressure or cellular stress — height data is relatively easy to obtain.

Recently Di Cesare, along with several hundred researchers around the world, compiled data from surveys in 200 countries (in a some cases, they mined military records when no such studies existed). The data set, recently published in the journal Elife, spans a century and contains data derived from 18.6 million people born between 1896 and 1996.

While it reveals that men in the Netherlands and women in Latvia are the tallest in the world, it also shows some shocking trends: Some countries have seen astounding gains in average height, while people in other, poorer countries may be, on average, shrinking.

A century’s worth of height data shows a lot of progress — and some troubling trends

Most countries made tremendous gains in height in the past 100 years. “Iranian men born in 1996 were around 17 cm taller than those born in 1896, and South Korean women were 20 cm taller,” the study reports. The authors interpret these gains as being indicative of increasing childhood access to good nutrition and health care. (Scroll to the bottom of this post to see how the average height of men and women changed across the globe over 100 years.)

But despite progress across the globe, there is still a great inequality in height. The countries with the tallest citizens are still similarly taller than the shortest countries from 100 years ago. It underscores a point we already understand well: that economic inequality is a persistent problem across the globe.

Countries in sub-Saharan African show an acutely disturbing trend: People there are actually growing shorter, decreasing around 5 centimeters (about 2 inches) over the past 40 years. “That is clearly a sign of the condition in which people have grown and have access to quality food and to help reduce the risk of getting infectious disease,” Di Cesare says.

eLife

That the data spans 100 years (1896 to 1996) allows researchers to see how countries are progressing relative to one another. In the early part of the 20th century, Americans were some of the tallest in the world. Now we rank 37th for men and 42nd for women.

“There are multiple factors playing here,” Di Cesare says of the drop in the US ranking. “But one that is probably important is ... inequality in nutrition. There’s probably a part of the population that is in good health and a part that is not. And then those people lag behind.”

(This trend is also seen in mortality data: America has some of the longest life expectancies in the world, and some of the shortest, depending on the county. The US ranks 39th and 40th for life expectancy for males and females — which mirrors the height data.)

The height data adds evidence to the idea that all the little stressors of growing up poor set a lower target for our health and wellness as we age. (People who grew up poor show greater levels of inflammation and stress response in tissue later in life. There’s also some research that finds that poverty in childhood leaves people more susceptible to colds later in life.)

Di Cesare stresses that this is an ongoing project. Ideally, the database would also include finer-scale data, like on cities.

“Global estimates are interesting,” she says. “But then, as we say, in a country there can be many different countries. Everything that is more regional or more spatial that is linked to smaller areas is of interest. Purely from the public health view it’s key to understand how there are differences within a country. You can identify where you need to act and intervene as soon as possible. “

But overall, Di Cesare sees the height data as proof that we humans can improve out environments to improve our health. “Many times we think it is all genetics, and it is not — otherwise, we wouldn’t have observed such big changes in the population in a century.” Which is really just a short amount of time.

And perhaps the data on height can help determine where to intervene.

Changes in average male height 1896 to 1996

(Click here for a larger version)

Changes in average female height 1896 to 1996

(Click here for a larger version)

CORRECTION: This post originally stated the study included prisoner data. It did not. “It would be wrong to use those kind of data because prisoners are a selected group of the population and their height can't be extrapolated to the general population,” Di Cesare clarifies in an email.

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