post on genetics and population height differences..(omar)

from 3QD.

SCIENCE SHEDS LIGHT ON POPULATION HISTORY AND LIVING STANDARDS

by Omar Ali

In a sense, all modern historiography includes the attempt to find objective facts rather than relying on folklore and opinion. To varying extents, a scientific mindset is part of the intellectual toolkit of all modern people and while no person can be entirely rational and no judgment is as perfectly evidence-based as the idealized models would imply, there is a trend towards greater objectivity and a willingness (at least in principle) to change one’s mind if new facts come to light. There is an assumption among liberals (I self-identify as liberal and spend most of my time with others who do the same) that modern liberals are more “science-minded” than conservatives (the so-called “fact-based community”). Whether this is really true has been challenged  but I will assume that liberals DO prefer a scientific approach to history and will touch on two examples where science brings objective information to bear upon history.  One is genetics, which has transformed our knowledge of the origins and relationships of different human populations. The other is height and what average height can tell us about different populations.

First, to genetics; a few days ago,blogger Razib Khan wrote a blog post about the population genetics of India and what those genetics can tell us about the origins and composition of the people of India. If you have not read that post, you should definitely do so; it is a superb and user friendly (and not overly detailed) example of how recent advances in genetics are radically transforming our view of human populations and their recent and distant history.  In some cases, the facts being uncovered are not entirely new or surprising, but in all cases, they provide a level of scientific certainty to debates that previously lacked such certitude.  Read another one of his posts (and other related articles) for examples of more detailed and finer scale analysis of the genetic data. These posts focus on India, but similar information (and in some cases, much more detailed information) is available about other populations and all of it is worth reading.

I am not going to spend more time on genetics, since I think Razib and his friends cover this area better than I ever could and I will be happy if you go to those links and start exploring on your own. But genetics is not the only way in which scientific knowledge can impact our view of history.

The other “objective datum” I want to touch on in some detail is height. Human height is strongly influenced by heredity (tall parents tend to have tall children) and the heritability is estimated to be around 80%. That does not mean 80% of your height is inherited and 20% is “environmental”. It means that 80% of the variation we see around us is explained by heredity and only 20% by environmental factors. In other words, if I am taller than you, that is mostly because I have “taller genes” and relatively little of the difference comes from the way I was raised and what I was fed and so on. It is immediately obvious that if the environment is very hostile and variable (e.g. if many people are facing serious starvation) then a lot of environmental variation will enter the picture but if the environment is more uniformly favorable, then every person will reach his or her genetic potential and most of the difference we see will be hereditary. So this heritability is not fixed, it changes according to the prevailing environmental conditions.

Now, if we look at populations rather than individuals, they too may differ because of inherited differences (in this case, “racial” differences in the genetic background) or because of environmental factors (e.g., one population being relatively better fed). If we look at various populations around the world, we find they are not all the same height. For example, the Dutch are taller than the Japanese. Does this mean that the Dutch are genetically taller or does it mean that something in their environment is different and makes them taller (or makes the Japanese shorter)? A few decades ago, our answer would have been unequiovocal; some races are taller than others. Europeans are taller than east Asians or Indians and so on. But as we get better and better data about changes over time, this straightforward judgment becomes significantly murkier. Somewhat contrary to expectations, it seems that much (though probably not all) of the difference in mean height between different populations is environmental, not genetic. This does not mean that some races are not outliers. There probably are some taller races and some shorter races. But the secular trends of the last 200 years provide very strong evidence that differences that seem obvious and immense at first look can disappear or change very significantly as the environment changes. There may yet be racial differences, but the difference may be much smaller than we imagine and we cannot be too sure of them until we have eliminated the environmental factors for several generations, not just one generation.

Height is a relatively easy measure and fairly good estimates of the height of even ancient populations can be made if sufficient number of ancient adult bones are available for study. But such a trove of bones is not always found, so most of the reliable data about the average height of populations comes from more recent times. In Europe, the modernization of armies occurred earlier than in Asia and Africa (a fact that has more than tangential relationship to the wave of European colonization that occurred around the same time) and combined with greater bureaucratic  and social sophistication, this permitted good records of the heights of conscripts to become available from the 18^th century onwards. As a result, we have fairly good estimates of the average heights of European males from around 1750 onwards. Data for females are harder to find, but by the 19^thcentury those too start to become available.  As with male height data, data for females also lag in Asian and African countries.  I will focus on male height today, but trends are similar (though not always equivalent) in females.

What these height records reveal is that the average height of populations is not stable. There are clear changes in average height over time and such trends are labeled “secular trends”.  “Secular changes” is acutally a better term, since “secular trends” implies a one way change, but in practice the term “secular trend” is frequently used where “secular change” would be more accurate. If we look at the secular tend in heights over time, we find that all European populations (without exception) have seen an  increase in height in the last 2-300 years. And this increase is not insignificant. In 1800, the average height of English males was 167 cm (5 feet and 5.7 inches) . At the same time, the average height of German, French and Dutch males was 164 cm (5 feet and 4.5 inches). And the height of the average Norwegian male in around 1750 was only 160 cm (only 5 feet 3inches).  There was also a rural-urban divide in England, with the rural population being taller. At the same time, children in urban slums in relatively tall England were a full 20 cm shorter than their upper class counterparts.  In other words, the upper classes very literally looked down upon the lower classes. By 1900, British average heights had increased to 169 cm, but by that point the Dutch had overtaken them and their average height was a 170 cm. Meanwhile, the Swedes were the tallest of them all, with an average height of 172.5 cm. At this time, the urban population in England also caught up with their rural counterparts.  By 1950, the Dutch were already 178 cm tall (on average) and today they are the tallest population in the world at 181 (or 183) cm, with the Swedes and the Norwegians running close behind. In the same period, the trend in the US has also been upwards, but relative positions have switched. In 1860, the average American White was 174 cm tall (inches taller than their European cousins), but still shorter than the Sioux Indians. By the 20^th century, they had left the Sioux behind, but failed to keep up with their European brethren and are now over an inch shorter than the Dutch and the Scandinavians. 

But all these are racially similar societies, what about other races? Within Europe, Southern Europeans were shorter than Northern Europeans a hundred years ago, but have had a bigger height gain than their northern cousins and have significantly closed the gap (and many are still growing while Northern Europeans may be plateuing, so the catch-up phase may not be over yet).  Asian populations are generally shorter than Europeans, but have seen bigger gains in height in the last 50 years than the Europeans; in short, they too are narrowing the gap. The average Chinese is about 168 cm, the average Japanese 171 and the average Indian only 165 cm. These averages also hide local variation. For example the Sikhs (at 170.35 cm) are taller than Muslims (165cm) and Hindus (164.5) in India, while wealthier people (of all religions) are taller than poorer ones and (somewhat surprisingly) urban dwellers are slightly taller than their rural brethren. When these relatively short populations migrate to more developed nations, they tend to become taller and continue to become taller over the next 2-3 generations. 

So what does all this mean? Some conclusions are obvious: Better standards of living translate into increased mean height, almost all across the world (but apparently, not in Tahiti, where there was no change in mean height from 1902 to 1970, perhaps because the Polynesians already enjoyed near optimum nutrition?).  These trends have barely begun in some places (India lags behind China, for example) but can be seen in almost every population. And they do not seem to plateau within one generation even if the environment changes abruptly (as in migration to the United States).  These trends provide an objective measure of public health and nutrition (thus, contrary to what Fox News may claim, Americans do not enjoy the same average living standard as Northern  Europeans). Like most objective data, they are open to interpretation, and they do not always satisfy the biases of any particular ideology. Thus, while socialists may be delighted to see the “American paradox” (that the US population has fallen behind the Northern Europeans even while becoming heavier in terms of weight and enjoying a similar or even higher per capita income)  since it indicates that some degree of state socialism is good for public health, they may not be equally comfortable with the fact that in general it is capitalism that seems to have led to the greatest improvement in living standards in human history. North Korea, for example, has managed to fall 11 cm behind South Korea in average height even as the South Koreans have been unabashed capitalist-roaders. It is also apparent that lay intuition about living standards and nutrition does not necessarily match scientific facts. This is because of several factors:

1.  PRENATAL (before birth) growth is the most rapid growth phase in the human life cycle (in absolute terms; we grow from 0 to 50 cm in 9 months. At no stage in life do we ever grow as much in 9 months). Gains and losses in this period are compensated to some degree in later life, but not entirely. Prenatal influences matter a lot and are not necessarily made up by later improved environmental factors. And it is not just mother’s nutrition, but mom’s physical size which is a limitation. Small uterus= small baby. And so on. Also keep in mind that the egg that is growing into a baby started its life when MOM was still a fetus inher mother’s belly. Human ova (female germ cells) start their life when the female is still in the womb and the same cell matures and mates with a sperm to start a new life. Grandma can have a direct influence on the egg cell that grows into her grandchild.
2. Postnatally, the most rapid growth is in the first two years. Human children reach about HALF of their adult height by age 2 (a little more than half for girls, a little less than half for boys). A lot depends on what happens in these first two years. Moving to America when you are 3 may already be too late to make a big difference in height.
3. Nutritional influences are much more complex than just obvious starvation. Protein, for example, is a huge factor. Populations that drink a lot of milk are taller than those that do not. Populations that increase their milk intake increase their average height. Populations that eat more meat tend to be taller. Finally, the pre-agricultural hominin diet, while it may have been healthier in terms of the risk of some modern maladies like heart disease, was not ideal for growth. The introduction of cereals around 10,000 BC definitely increased our caloric supply and while Malthusian conditions limited the benefit to individuals, it is worth keeping in mind that it is not easy to get 2400 calories from scrounging for roots and berries in the forest. The introduction of dairy between 5-8000 BC dramatically improved our access to reliable, high quality protein and along with the availability of the meat of dairy animals, has been a huge boon to humans. Of course, the meat does not have to be in dairy farms;  it is likely Sioux Indians were taller than better-off Whites in the late 19^th century because they had more meat per capita, mostly from Buffalo.
4. Rice eating populations tend to be smaller. The prevailing opinion is that this is due to relative lack of high quality protein. But scientifically, we have not yet excluded other possibilities; for example, that there may be factors in rice that directly stunt growth.
5. Iron deficiency, iodine deficiency, vitamin deficiencies, all add up, even in people who look well fed. Upper class Indians may have abundant food, but may still be deficient in protein and iron due to religious restrictions or cultural habits.
6. Disease plays a big role, especially childhood infections and parasites. Modern people tend to forget how disease ridden childhood was (and continues to be) in pre-modern people.

Finally, it does not mean that some populations are not inherently shorter than others. Some may indeed be shorter. But the point is that we do not know for sure yet for most large human populations. The Chinese, for example, are still shorter than Europeans but their secular trend (increase in mean height over time) has been more impressive than the Europeans in the last 20 years and the gap is still narrowing. They may yet plateau below the European level, but we don’t know where that plateau is. It may be (and probably is) that East Asians and perhaps South Indians (who have a greater ASI component, see genetics links above) really are genetically shorter than Eurpeans, but the same is not likely to be true of “ancestral north Indians” and the difference between Indian and European heights may be much smaller than the current 13 cm gap (5 inches) if Indians get more calories, more protein, more iron and less disease for a couple of generations. We do already know that Greeks and Turks are not much shorter than North Europeans and since they have not plateau-ed  yet, we cannot say for sure that they won’t catch up completely. 

Keep in mind that there are small populations that seem truly genetically short (so-called pygmies) but even in those populations, the genetic basis of their short stature is only partly known or is still unknown and we don’t know what their height will stabilize at once nutrition and other environmental factors are changed. There are also some populations at the extremes of climate who may have evolved somewhat different body proportions (taller and longer-limbed Masai, shorter-limbed and stockier Eskimos) but in the vast Eurasian landmass, we may see differences in height narrow considerably in the generations to come. And of course, the new global elite is increasingly multi-racial and their descendants are no longer easy to classify into past racial classifications. Still, the mixing of genes among different large populations is a very slow process, so racial differences are not going to be eliminated in the near future. Thus, whatever portion of the population difference in height is truly genetic, it is not likely to be eliminated by biological intermixing within the next few generations. Environment, on the other hand, can change more rapidly and environmental differences in height will likely narrow; and while they do so, they will be a useful measure of living standards.

 

Comments

[Ruchira]

Very interesting analysis. Despite excellent nutrition and relatively tall parents by Indian standards (5'11" and 5'5"), my sister and I beat the altitudinal odds. I guess a pair of short grandmas are to blame:-)

[gaddeswarup]

This may be of interest too http://www.nytimes.com/2011/04/27/books/robert-w-fogel-investigates-human-evolution.html?_r=1&src=tptw&pagewanted=all
It seems that there will be much discussion on these topics.

[Anna]

Interesting. Ruchira's grandma comment raises a point, which is that the genetic contribution may be from elsewhere than directly from the parents. A friend, the moderately short (maybe 5'1") daughter of a very tall Polish-American man and extraordinarily petite (maybe 4'9") Japanese-American woman, married a very tall (over 6') Anglo-American man, but has an entirely healthy and well-fed 3 year old daughter who nonetheless barely measures in the first percentile on American growth charts. I always wonder if the child's grandmother wasn't a similar size at the same age.

The growth of babies in utero and the first trimester that Omar references is an extraordinary thing. Holding my whopper of a 9 month old-- 90th percentile for height at his check-up last week (60th or so for weight)-- I frequently marvel that he fit inside my body less than a year ago. Like most kids, he'd doubled his weight well within the first three months of birth that pediatricians sometimes refer to as the "fourth" trimester, and he's now 50 percent taller than he was at birth. Given that rapid rate of growth, it's easy to imagine how the uncertain availability of food that my grandparents faced in their firs years of life might account for any difference between their ultimate height (though my grandfather, although profoundly poor as a child and sick until his early death, was nonetheless 6' tall) and my son's.

[Anna]

To clarify-- obviously, the genetic contribution is from the parents, in the literal sense...I meant might have been expressed elsewhere than by the parents.

[Sujatha]

For you, Anna- you will be someday in my shoes :-)
My son is still about a couple of inches short of the 6 foot mark, though his hair probably gives him an extra inch, it doesn't really count at the doctor's office.
My daughter, at age 10, is now barely up to my chin. Everyone now exclaims at her height,telling her "Oh my, you've grown!", something that I can't see for seeing her daily.Despite her being in the solidly 25th percentile range for height, she is likely going to be an inch or two taller than me (I'm about 5'4").

[Anna]

A lovely post, Sujatha. And yes, at least my son may be in your son's shoes-- he was the same length as your son, at birth, and is already up to 18-24 month old sizes in both pants and shoes.