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Wednesday, July 30, 2003

Malthus, Demography and the Great Transition

Brad has been museing on Malthus, demography and economic growth, and has stirred up a bit of controversy in the process:

Begin with the shape of the demographic transition since 1820, with population growth rates plotted as a function of levels of guestimates of levels of real GDP per capita (measured in Maddison's 1990 "International Dollars") for the world's various regions for six irregular (but sensible) subperiods. The figure shows how population growth rises rapidly as societies progress and grow their annual per-capita incomes from a Malthusian near-subsistence level of $400 (1990 International Dollars) per person per year up to $1,100 or so. Then population growth levels off--typically at 1.75% per year or so--as fertility restriction becomes widespread. Once societies pass $4,000 per capita a year or so, the demographic transition proper sets in, and population growth rates start to decline markedly.
Source: Semi Daily Journal

This interpretation is backed up with a very nice looking scatterplot, which makes the correlation look almost undeniable. In broad sweep Brad is obviously right here, there is an obvious association between societies with annual incomes in the range $400 -$600 pre 1800 and low population growth. There is then an undeniable 'take off' post 1800 as improvements in diet, medical care, public health etc gradually reduce infant mortality, and population starts to rise. What is not so clear is what is cause and what is effect here. This is a topic that has been argued over for years, and will doubtless continue to be argued over. One thing is, however, clear, what Malthus calls the 'preventive check' was far more widespread (and socially configured) than many contemporary economists seem to accept. Part of the problem, is I feel, a methodological one. Most contemporary economists start from the individual, and the individual decision making process and work outwards. Even in our highly individualistic modern societies this procedure is questionable, but in more traditional, and family or clan-structured societies, it seems highly illegitimate. In some sense you have to start from the social, and what reproduction there is a socially regulated reproduction. Hence the importance of ecological-system arguments. It is this that leads Brad into his most controversial and contested argument:

What held back population growth? What keeps the numbers of the human race from growing at more than 0.2% even under the most favorable pre-industrial conditions? The conclusion seems inescapable: desperate poverty. For the overwhelming bulk of recorded history, population growth rates have been kept low by poverty so dire that women's fat reserves are so low that ovulation is a hit-or-miss affair, and by poverty so dire that nutritional deficits are so great as to seriously compromise immune systems' abilities to deal with the endemic disease pool.

Here I feel I cannot do better than 'recover' a couple of the posters from the comments section:

Brad's conclusion that « desperate poverty » and chronic malnutrition held back the growth rate of the human population up to say 1800 CE becomes doubtful if you try to extend it beyond the dawn of agriculture (≈ 10 000 BP). Demographic growth rates in the long hunter-gatherer era must have varied quite a lot, first constrained by competition in Africa and expanding rapidly when skilled hunters moved in on unexploited Eurasian and American fauna, but overall they must have been modest. Still, the modern and archaeological evidence is that hunter-gatherers are pretty healthy. Their population is too low to support endemic infectious disease, and is controlled by other factors than chronic, Malthusian malnutrition: predation, accidents, intraspecies violence, infanticide, catastrophes and late weaning. I’m not sure about parasites.

Recently discovered bones of very early African humans show big frames; they had to be strong because the diet included hippos. Imagine hunting hippos with stone-tipped weapons, poor-quality ropes and nets, and shallow pit traps. If the humans won there was plenty of food to share; but many must have died trying.

I like the baby-food theory: hunter-gatherer mothers must breast-feed infants and wean them late on to a chewy mixed diet, farmer mothers can substitute some gruelly pap early and get pregnant again. The population booms, the farmers drive out the hunter-gatherers by force of numbers; but are now locked into a Malthusian trap, where their health deteriorates by loss of diversity in diet and greater susceptibility to diseases. (In fairness I should state that I tried this theory on the eminent food historian Felipe Fernandez-Armesto, who says it doesn't fit the known facts about adoption of farming.)

The Malthusian constraint is a good theory for the Era of Agriculture. It doesn't fit either the preceding Hunter-Gatherer Era or the Knowledge Era we live in.
James Wimberly

Brad DeLong writes, "[...] population growth rates have been kept low by poverty so dire that women's fat reserves are so low that ovulation is a hit-or-miss affair"

Elliot Oti responded, "Sounds unlikely to me. The increase in global population, at least in the last 200 years, has almost invariably been due to declining death rates (especially in infant and child mortality), rather than increasing birth rates."

Yes, exactly. The reason human population swelled in the 20th century was that people stopped dying like flies, not that they started breeding like rabbits. It was reductions in infant and child mortality that caused the 20th century "population explosion," not increases in birth rates.

Dr. DeLong's theory simply isn't supported by facts. Women had MORE children--more ovulations resulting in pregnancies--prior to 1820 than they have today...at least in the U.S. and other developed countries. It's just that an unbelievably high percentage of the children that were conceived prior to the 1820s never made it to the age that they could reproduce.

For example, I think it was Oliver Wendell Holmes who observed that, as of the start of the 20th century, a doctor's care was as likely to harm as it was to help. And even as late as the U.S. Civil War, sanitary practices in miliary hospitals were shockingly poor. Only 1 in 4 (!) Civil War patients could be expected to live after receiving treatment in military hospitals.

These answers (ie Brad's conclusions, Edward) are NOT even supported by the preponderance of available data, let alone an "inescapable conclusion." Dr. DeLong's "inescapable conclusions" are contradicted by:

1) Population growth was not caused by having more babies, but by having more children who children who subsequently reached reproductive age (as could be determined from birth records from the early 19th century, versus birth records for the middle 20th century), and

2) Population growth was caused by knowledge of the causes of death and disease (e.g. knowledge of microbiological foundations of improper sanitation, and communicable diseases like malaria). This knowledge became available to even the poorest people in the 20th century. For example, the populations of India and China exploded in the middle of the 20th century. But the per-capita wealth of those two countries in that period probably wasn't substantially greater than the per-capita wealth of the colonists and early 18th century U.S. citizens. What was available in India and China, that was not available to the colonists, was knowledge of the causes and preventative measures for diseases (both diseases caused by improper sanitation, and diseases like malaria, diptheria, and whooping cough).
Mark Bahner

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