“It can't just be nutrition,” Hatton told me by phone. “There must be something else going on.”

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When thinking about why we grow, it can help to visualize the body as a type of machine, as the economist Richard Steckel did:

“This machine expends fuel at rest (basal metabolism) amounting to some 1,200 to 1,400 calories per day (depending upon the size of the person) to breathe, keep warm, circulate the blood and so forth, and in physical effort, fighting infection and physical growth.”

We eat food to keep our inner engines running. Our bodies then use those calories to lengthen our bones and multiply our cells until we reach the end of puberty. But when we’re ravaged by infections or deprived of nutrients, growth takes a back seat to keeping the heart and organs functioning. If we’re struck by too many diseases and deficiencies, we stop short (literally) of the height we might have achieved.

While about 80 percent of height is determined by genes, auxologists (those are height scientists) now believe that nutrition and sanitation determine much of the rest. As the New Yorker’s Burkhard Bilger put it in 2004:

“Height variations within a population are largely genetic, but height variations between populations are mostly environmental, anthropometric history suggests. If Joe is taller than Jack, it’s probably because his parents are taller. But if the average Norwegian is taller than the average Nigerian it’s because Norwegians live healthier lives.”

Your childhood environment can give you (or take away) three or four inches. A lack of nutrient-rich food and clean water explains why stunting is prevalent among children in developing countries. Studies of North Koreans found that those born after the country was divided in two were about two inches shorter than their counterparts in the South.

When Barry Bogin, an anthropologist at Temple University, measured the heights of children from the Maya ethnic group, he found that Maya refugee children growing up in the United States were about four inches taller than Maya children who were still living in their native Guatemala. He chalked up the difference to America’s superior nutrition and healthcare.

***

As Hatton charted Europeans’ remarkable growth spurt, he also found that the increase in height corresponded with a simultaneous drop in infant mortality. But he wasn’t sure what was driving those two metrics.

For a new working paper, he wanted to examine what it was, exactly, about the life circumstances of those 20th-century Europeans that determined whether they became lanky or squat.

Hatton and his colleagues, Roy E. Bailey from the University of Essex and Kris Inwood from the University of Guelph, created a database of 2,236 British soldiers who served in World War I, and then they looked up their birth records. The soldiers were relatively representative of the male population as a whole—about two-thirds of the 1890 British male birth cohort enlisted. It turns out that subtle differences in their heights hinted at their origins:

Those from white-collar backgrounds were taller: This follows the theory that wealth buys better food and living conditions, and thus greater height in adulthood. The men who hailed from the top two social classes stood a half-inch taller, on average.

The more kids there were in a household, the shorter they were: Not only because there was less food to go around, but also because it made it more likely that there were more people in each bedroom. “Crowding can help spread respiratory and gastrointestinal infections,” Hatton said. “People sneezing on each other, that sort of thing.” Each additional sibling cost the men an eighth of an inch, and having more than one person per bedroom shaved off a quarter-inch.

Children of literate mothers were taller: When mothers couldn’t read, they were less likely to know about the importance of a balanced diet or clean cutlery. The researchers measured the percentage of women by region who were only able to sign their marriage certificates with an X, rather than their name. People from areas with a high percentage of illiterate mothers were a quarter-inch shorter.