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Why did humans grow four inches in 100 years? It wasn’t just diet

It is a commonplace for children to be taller than their parents, but four generations ago this wasn’t the case. A recent study of soldiers around the age of 20 who enlisted in the army during World War…

Modern man. Stupidmommy, CC BY-NC

It is a commonplace for children to be taller than their parents, but four generations ago this wasn’t the case. A recent study of soldiers around the age of 20 who enlisted in the army during World War I revealed an average height of five feet six inches (168cm). Today the average for young men is five feet ten inches (178cm).

A gain of four inches seems a lot. But it is not unique to Britain; similar gains have been found in a range of western countries even though the timing differs. Over a slightly earlier period the Danes and the Spanish grew by about five inches while the French, the Italians and the Swedes grew by around four inches.

Shooting up: increase in heights over 100 years in centimetres. Tim Hatton

Yet in the longer span of history this is quite unprecedented. Economic historians have uncovered evidence on heights all the way back to the middle ages in order to chart what they call the biological standard of living. Across the generations there were ups and downs in height, but there is nothing like the four-inch gain of the past century.

If adult height reflects nutrition during childhood then we have a sensitive indicator of living standards. The 20th century saw dramatic improvements in diet. The quantity of food intake increased and its quality improved, as incomes rose faster than ever before. But in some countries economic historians have found that during the early stages of industrialisation average heights fell at the same time as income per capita increased. The most debated case is the US in the three decades before the civil war – this has become known as the “antebellum puzzle”. So income and height don’t always move in lockstep. But, as in earlier times, the correlation between trends in height and income is far from perfect.

Something else is at work: exposure to infection. Repeated infection during infancy and childhood slows growth as nutrition intake declines or is used by the body to fight disease. Predominant among these illnesses are respiratory infections, notably pneumonia and bronchitis, and gastro-intestinal infections, especially diarrhoea and dysentery.

The key factor here is the urban environment. Sanitary reforms improved the quality of water supply and the disposal and treatment of sewage. In urban districts horses disappeared from the streets and pigs from backyards. Equally important was the reduction in overcrowding and improvement in the quality of housing, as the slums were gradually cleared.

Little and large on the battlefield. Drake Goodman, CC BY-NC-SA

How do we know this? Our World War I servicemen hold some important clues. Those that grew up in localities with high infant mortality (a clear marker of the disease environment) tended to be shorter as adults. And we can confirm that the disease environment was worse the more households were overcrowded and the more industrial the district. In the heaviest industrial areas the servicemen were shorter by an inch. This effect would have faded as heavy industry declined and what remained became less toxic.

By tracing their families in the 1901 census we can also see the household circumstances of the servicemen when they were growing up. This reveals that those from middle class families were taller and those with more siblings were shorter. The latter reflects a trade-off between the quantity of children and their average quality in terms of health. This too was a source of height gain, as average family size fell from five in Victorian times to just two by the 1930s.

Did medical improvements also play a part? There is less hard evidence for this but access to better medical treatments, and to medical services for children, expanded only modestly until after World War II. Yet basic knowledge about nutrition and hygiene did percolate down the hierarchy of class and income. One hint of this – given that higher socioeconomic classes tend to have more education – is that in areas where parents (particularly mothers) were more educated the servicemen were taller.

Over the 20th century dramatic advances in education combined with smaller families meant significant improvements in the nurturing of children. These influences are hard to identify precisely, but they probably contributed more to increases in height, health and longevity than was previously believed.


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7 Comments sorted by

  1. John Doyle

    architect

    What this is also implying is that our prehistoric ancestors, who were also tall, had lives of better health and not so different from today. Whereas the usual conclusion that switching to a grain based diet let to shorter stature is also in error. It was the living in communities like villages and towns that did the damage.

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    1. Florence Leroy

      logged in via Facebook

      In reply to John Doyle

      Yes, and I also read somewhere, actually probably in the book "The year 1000" by Robert Lacey and Danny Danziger, that people in Britain at that time (just 100 years ago) had a very similar size to ours at the moment (as well as superbe sets of teeth), contrary to what one might have thought.

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  2. Peter Greenhouse

    Consultant in Sexual Health

    An additional explanation could be the increasing ability to travel away from the town or village of origin & consequently wider genetic mixing.
    As an example of the reverse effect I would cite examples of relatively isolated communities with a long history of interbreeding, such as the King's Lynn area in Norfolk, where mean heights of male & female adults remain significantly shorter than the UK average.
    On the infection front, a major contibutor is probably the (almost complete) disappearance of congenital syphilis since the early 1950s thanks to routine antenatal screening & widespread penicillin treatment - given that population prevalence was between 10-20% in the first quarter of the 20th century.

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  3. terry lockwood

    maths/media/music/drama teacher

    And I always thought that my extra height came from my father having a habit of reaching for the highest fruit in the trees in the years before I was conceived.

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  4. Comment removed by moderator.

  5. Darren Hoffmann

    logged in via Facebook

    Sounds like gradualism to me. If this were always happening then we would be way taller. My bet is that the body gets larger and has a larger brain in proportion, creating "room" for a more complex brain.

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  6. Paul Blood

    logged in via email @gmail.com

    Tim, Is there a mechanistic link between your findings of reduced infection and increased growth in children, and giving antibiotics to farm animals in order to increase their growth? Paul.

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