Average height has increased substantially in the 20th century — roughly 10 cm (4 inches) across many countries in just three generations — and the primary driver wasn't genetics but improvements in childhood nutrition, disease reduction, and living conditions, which means height variation between populations tells us more about historical public health than about genetic differences
The previous articles on this site covered height and health, height and sport physics, child growth charts, morning vs evening height variation, and height conversion errors in medical records. This article addresses secular height trends — how average heights have changed over time, what the drivers of these changes reveal, and what height variation between contemporary populations actually reflects.
The secular trend in height: how fast humans grew taller
The increase in average height over the 20th century (called the "secular trend" in anthropology) is one of the most dramatic and well-documented examples of rapid environmental change affecting a physical trait:
Netherlands: average male height in the 1870s was approximately 165 cm. By the 2000s, it had reached approximately 183 cm — an 18 cm increase in roughly 130 years. Dutch men are now the tallest in the world on average.
Japan: average male height in the early 20th century was approximately 158 cm; by the late 20th century, approximately 171 cm — a 13 cm increase over about 70 years.
UK: average male height increased from approximately 167 cm in 1880 to approximately 177 cm in the 2000s — 10 cm over 120 years.
The speed of change is the key point: human genetics don't change meaningfully over 3-4 generations. The height increase was driven entirely by environmental improvements — because genetic changes require many generations to spread through a population.
What drove height increase: the evidence
Three primary factors have been identified across the research:
Nutrition improvements: better availability of protein (dairy, meat), micronutrients (zinc, iodine, vitamins A and D), and calories during childhood. Childhood is the critical period — undernutrition during the growth years permanently reduces adult height even if nutrition improves in adulthood.
Disease reduction: infections during childhood divert nutrients from growth to immune response. Reductions in diarrhoeal disease, respiratory infections, and childhood illnesses freed more energy and nutrients for growth. The introduction of clean water and sanitation in the late 19th and early 20th centuries correlates strongly with height increases in many countries.
Reduced physical labour during childhood: children who work physically demanding labour from a young age don't divert energy to the same extent as non-working children, but reduced childhood stress generally.
The relative contributions: studies on identical twins raised in different environments show that at the population level, approximately 60-80% of adult height variation is genetic — but this figure applies within a population with similar environmental conditions. Between populations with very different nutrition and health histories, environmental factors explain much more of the height difference.
Height and socioeconomic status: the persistent gradient
Within modern populations, taller average heights are consistently associated with higher socioeconomic status (SES):
UK studies: men in the highest quintile of socioeconomic status are on average approximately 3-4 cm taller than men in the lowest quintile.
The causal mechanism: higher SES is associated with better nutrition, fewer childhood infections, better healthcare access, and better living conditions during childhood — all the factors that drive height increase at the population level. The SES-height relationship is driven by the same environmental factors, operating within a single population across socioeconomic groups.
The chicken-and-egg question: does height affect SES (through labour market discrimination favouring taller candidates, as some research suggests) or does SES affect height (through better childhood conditions)? The evidence suggests both operate simultaneously.
The global height plateau: why heights may be levelling off
The secular trend in height has slowed or plateaued in most high-income countries since the 1980s-1990s:
Dutch men's average height has been approximately stable at 182-183 cm since roughly 1990. UK, Swedish, and most Western European populations show similar plateaus.
The plateau interpretation: researchers believe this represents populations approaching their genetic potential — meaning nutrition and health conditions have improved sufficiently that most growth is now determined by genetic rather than environmental factors. A well-nourished, healthy population reaches its genetic potential; further environmental improvements have diminishing returns on height.
Continued growth in middle-income countries: populations in South Korea, China, Iran, and other countries that have experienced rapid economic development in recent decades continue to show height increases — they haven't yet reached their genetic potential due to earlier nutritional constraints.
Why contemporary population height differences aren't primarily genetic
The height differences between populations today — South Asian adults averaging shorter than Northern European adults, for example — are frequently misattributed to genetic differences. The evidence suggests this is largely wrong for most major differences:
South Koreans vs North Koreans: genetically virtually identical populations; South Korean men average approximately 174 cm, North Korean men approximately 165 cm — a 9 cm gap that opened over 50 years of dramatically different nutrition and health conditions.
The Japanese example: Japanese Americans are significantly taller than Japanese people living in Japan in the early-to-mid 20th century — same genes, different nutrition.
Within-population height increases: the secular trend affects the same genetic populations, showing that genetics didn't change but height did.
The genetic determinism caution: when researchers calculate that height is "80% heritable," this is within a specific population with relatively similar environmental conditions. Between populations with very different histories of nutrition and health, the environmental contribution to height differences is much larger than within-population heritability suggests.
How to use the Height Converter on sadiqbd.com
- For cross-reference: UK and US health documentation uses feet and inches; European medical records use centimetres — convert between them when comparing health data across contexts
- For growth chart percentile interpretation: WHO and CDC growth charts use centimetres; if measurements are in feet/inches, convert before looking up percentiles
- For historical height comparison: historical height data is frequently in imperial units (feet, inches) from British and American records — convert to centimetres for comparison against modern population statistics
Frequently Asked Questions
If height is 60-80% heritable, how can environmental factors explain so much of the population-level increases? Heritability is context-dependent — it describes variance within a specific environment, not potential across all environments. Within a well-nourished modern population, most height variation between individuals is explained by genetic differences — because everyone is getting adequate nutrition, so nutrition differences between individuals are small. But the same genes in a malnourished environment produce shorter people. The heritability figure tells you why person A is taller than person B within a given context; it doesn't tell you why the average person in 2000 is taller than the average person in 1900. These are different questions, and confusing them is a common error in popular discussions of height and genetics.
Is the Height Converter free? Yes — completely free, no sign-up required.
Try the Height Converter free at sadiqbd.com — convert between feet and inches and centimetres instantly.
