Starting sometime in the mid-19th century, children taller than their parents became features of the newly affluent worldwide and of post-industrialization countries in particular though even there, average heights started peaking sometime around 1980. While potential for maximum height appears to be genetically determined, environmental conditions during growth years determine how much of this potential is achieved. Recent average human height increases relate to
- Post-19th century fertility rate declines (fewer siblings so greater resources/child, also lower child-to-child microbial transmission rates).
- Infant mortality rate reductions.
- Unprecedented improvements in drinking water supplies (filtration and chlorination), hygiene and sanitation (and linked unprecedented reduction in human microbial transmission).
- Greatly improved access to nutrition, ventilated housing and public health systems.
All these combined to yield steady 20th century average height increases in countries that industrialized earlier. However, by ~ 1980, data suggest some populations may have reached peak heights, i.e., reached human height genetic limits.
General Features of Human Height: Genetics Plus Early Life Conditions Largely Determine Adult Height
Human growth spurts, called peak growth velocity, occur largely in infancy (1st 2 years of life) and during puberty (1), though such suppositions aren’t universal. For e.g., Pume girls in Venezuela reach the bulk of their adult height by age 10 and don’t really experience a stereotypical adolescent growth spurt (2).
The British pediatrician James Mourilyan Tanner, who developed the Tanner scale, long influenced theories of height. According to him, adult height was a proxy for childhood nutrition, i.e., as child nutrition improves, adult height increases. Childhood nutrition certainly has a major effect on adult height.
However, while average heights continued increasing from generation to generation in industrializing countries, mainly in Europe, 20th century European history was punctuated by sudden and precipitous declines in living standards during wars and their aftermath. This is particularly true during and after WWII. And yet even there, in the aftermath of such widespread devastation, average human heights increased steadily (1). How to explain such data?
Even if a child experiences famine in infancy, as long as it can access adequate nutrition during adolescence, ‘catch-up growth‘ (3) can apparently allay permanent stunting. A salient example is the Dutch city, Rotterdam. In 1944 to 1945, as German troops retreated, Rotterdam was deprived of food imports for a number of months leading to a terrible famine (3). However, since this acute food shortage was temporary, catch-up growth mitigated permanent stunting among Rotterdam children trapped in that war zone.
Average Human Height Appears To Vary Inversely With Microbial Transmission Rates
Early life microbial exposure reduced dramatically during the 20th century in countries that industrialized earlier. Albertine S. Beard and Martin J. Blaser argue this unprecedented reduction contributed to overall height increases that occurred in these countries in this time period (See figure below from 4).
- Post-industrialization advent of modern plumbing, sanitation and ventilation fundamentally changed human habitations including homes in the late 19th to early 20th centuries.
- With the invention of the internal combustion engine, automobiles displaced horses, which further reduced exposure to animals and the microbes they carry.
- Vast swaths of the population moved from farming to other occupations.
Such changes dramatically reduced overall human exposure to microbes. After all, pre-Industrialization modes of living entailed greater microbial contact while older hygiene, sanitation and ventilation practices ensured heavy human-to-human microbial transmission. But that’s not all. The next major human-microbe disruptor was just around the corner, namely, antibiotics. Here industrial livestock production offers additional insight into human growth patterns. The 20th century saw a rapid switch from pre-industrial pastoral mode of livestock raising to industrial production.
- In the late 1940s, scientists at Lederle Laboratories accidentally discovered that sub-therapeutic doses of antibiotics accelerated chicken growth (5). Called Antimicrobial Growth Promotion (AGP, Antibiotic use in livestock), it had the same effect on pigs (6) and other livestock. No matter we still don’t understand how it works, its economic advantage (rapid growth rates, less feed costs, less time to market) ensured it became standard in global industrial livestock production in short order.
- Akin to its effect on livestock, obviously, some proportion of average human height increase since the 1950s is linked to antibiotic exposure. And this is not just about pathogens. Rather, this is about antibiotic effects on human microbiota, i.e., on the micro-organisms that are an indigenous part of the human body.
Alterations of a human’s microbiota composition in infancy may fundamentally alter metabolism and change how the growing body allocates energy and resources for growth and development. While increasing height may find mention on the positive column of the ledger, early life microbiota disruptions also likely play a decisive role in conditions ranging from allergies, autoimmunity to Neurodevelopmental disorder like Autism spectrum to obesity, all net costs earning a place on the negative column.
Average Heights Seem To Have Peaked In Some Countries That Industrialized Earlier, i.e., Reached Genetic Maximum
Notwithstanding trends of average height increases from 19th through 20th centuries, data from more recent decades suggest certain countries may have reached peak heights,
- Average American male heights for example have even started declining in recent decades (see figure below from 7).
- Even accounting for sufficient access to nutrition, height gain differences are tremendous even among earliest industrialized countries. For e.g., according to Stulp et al (8), average American male height increased by only ~6 cm over the last 150 years while it increased by ~20 cm among Dutch men (average height ~185 cm in 2004 compared to ~165 cm in 1860), who are now the tallest in the world (9). The US has a higher per capita income, spends more on health care and has similar daily caloric availability. So what gives? The Dutch are more egalitarian while social and healthcare access inequalities are much higher in the US. Thus ‘there are diminishing returns on height with increasing wealth‘ (10) while in a more egalitarian society like the Netherlands greater increases in height among the poor can counterbalance height stagnation among the wealthy (10). Impact of presumably shorter immigrants on the US dataset OTOH is tempered by the fact that their children continue to grow up to be much taller (10).
- However, latest signs suggest even Dutch men may have reached peak height (11).
- Indeed height gains started earlier in many Northern European countries (Austria, Belgium, Denmark, Finland, Ireland, Sweden) where they appear to have stabilized by 1980 while gains continue to be higher in Southern European countries (Greece, Italy, Portugal, Spain) (12). For e.g., average Finnish men’s heights born in the early 1950s and 1970s, were 177.8 cm and 178.7 cm, respectively, i.e., <1 cm increase, while those for Spanish men were 171.3 cm and 176.1 cm, i.e., ~5 cm increase for the same time period (12).
- Average heights tend to be high even in relatively poor African countries like Chad (average female height of 164 cm for those born in 1980) and Mali (average female height of 162 cm for those born in 1960) (13). This suggests not affluence alone but rather a complex interplay underpinning life-history trade-offs between genetics and environmental factors such as nutrition among many others lead to height increases.
- The longest running multigenerational study in biomedical research, the Framingham Heart Study provides one example of such trade-offs in the context of heights. In one study on this group of American women, Byars et al found Natural selection trends that drive their heights downwards (14).
Obviously, genetics plays a role in height heritability
- Twin studies show height Heritability is ~0.8 (15, 16). Heritability index of either 0 or 1 indicates none or all, respectively, of the trait’s variability among study subjects is due to genetic factors. Heritability index of 0.8 for height suggests it’s a trait with high heritability.
- Genome-wide association study (GWAS) have found height-associated or height-determining genes that overlap across ethnicities (17, 18, 19, 20, 21).
- There are also population-specific genetic differences that could partly explain differences between countries. For e.g., why Koreans tend to be shorter on average compared to Europeans (22).
However, gene sequences alone don’t completely predict height heritability.
Epigenetics also plays a role in height heritability
- For e.g., epigenetic inactivation of several height-associated genes (BMP2, BMP6, CABLES1, DLEU7, GNAS, GNASAS, HHIP, MOS, PLAGL1) leads to height decrease, an outcome similar to their physical loss, i.e., to classic Mendelian genetics (23, 24).
- Epigenetic defects are implicated in several human disease syndromes. Some, such as Angelman syndrome, Beckwith–Wiedemann syndrome, Prader–Willi syndrome, Rett syndrome, Silver–Russell syndrome, also have hereditary growth anomalies, suggesting height involves heritable epigenetic processes (23).
Height also exhibits Sexual dimorphism
- Across observed adult human populations, men are taller on average than women (25).
- Studies suggest women preferring taller men while men preferring shorter women is generalizable across cultures (26, 27).
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