Few people are as famous for being wrong as Thomas Robert Malthus, the English cleric and early student of population growth. Malthus thought that while population could grow exponentially, food production could grow only linearly. Therefore population growth would always outpace food production, and famine would always then cause population to decline to a sustainable level. In An Essay on the Principle of Population, first published in 1798, he wrote:

We should be led into an error if we were thence to suppose that population and food ever really increase in the same ratio. The one is still a geometrical and the other an arithmetical ratio, that is, one increases by multiplication, and the other by addition. Where there are few people, and a great quantity of fertile land, the power of the earth to afford a yearly increase of food may be compared to a great reservoir of water, supplied by a moderate stream. The faster population increases, the more help will be got to draw off the water, and consequently an increasing quantity will be taken every year. But the sooner, undoubtedly, will the reservoir be exhausted, and the streams only remain. When acre has been added to acre, till all the fertile land is occupied, the yearly increase of food will depend upon the amelioration of the land already in possession; and even this moderate stream will be gradually diminishing. But population, could it be supplied with food, would go on with unexhausted vigour, and the increase of one period would furnish the power of a greater increase the next, and this without any limit.

The dynamics that Malthus described did hold true for most of human history. Technological improvements allowed the number of people that could be supported off the land to increase, but that increase in population absorbed most of the increase in output and average living standards did not rise much–exactly as Malthus predicted. Before the industrial revolution, there is little evidence of any sustained increase in per-capita income over time. Nonetheless, Malthus stopped being right not too long after the publication of his book. The last two centuries or so of modern economic growth show that the production of food, and other goods and services, can in fact grow exponentially for sustained periods thanks to technological progress. (Gregory Clark’s A Farewell to Alms is a very clear exposition of the Malthusian model for prior human economic history, and how the industrial revolution transformed it.)

One of the most interesting ideas in Charles Kenny’s compact and lively new history of humanity’s struggle with infectious disease, The Plague Cycle, is that this common understanding of how Malthus was wrong is itself wrong. Malthus’ mistake was not simply a failure of imagination, the inability to imagine exponential technological progress. Kenny argues that Malthus’ mistake was even more basic: in fact, food production has never operated as a real constraint on human population growth:

For most of the time civilization has existed, pestilence has wiped out far more lives than famine and violence combined—so much so that Malthus’s proposed final limit of land and resources as the check to human numbers has rarely been approached. Disease has usually kept populations below the levels that could have been supported given agricultural technologies at the time.

Kenny credits Ester Boserup, a Danish economist and consultant to the United Nations, for the insight that food production was always more capable of improvement than Malthus assumed, and that the potential for bringing more land into production, and making land more productive, was hardly ever exhausted. In her 1965 book The Conditions of Agricultural Growth, she argued that population growth actually makes higher agricultural output possible through intensified cultivation (for those who, like me, were not aware of Boserup’s work, there’s a useful review at EH.net.) Malthus’ argument that improvements in food output were only linear rather than exponential was therefore wrong before the industrial revolution, as well as after it.

Something clearly operated to keep human population in check before its explosion over the last couple of centuries. But it was generally not starvation. The real problem is that population growth begets population density, and population density, in the absence of sanitation, antiobiotics, and other checks on infection, begets disease and death:

Earth scientist Jed Kaplan and colleagues suggest that less than one-half of the land currently used for food production was used in 1600 and less than one-third in 100 CE. It is true that making more of the land available takes more work—sometimes brutally hard work, and that risks malnutrition. Again, some land couldn’t be cultivated without innovations, including heavy plows and irrigation. Nonetheless, it seems clear that throughout most of history the number of humans on earth fluctuated far below the maximum possible.

Instead, we should probably thank (or blame) the regulatory mechanism of infection for limiting populations. As the number of people grew, population density drove up disease rates. This thinning mechanism was, in most places, probably the most powerful check on the number of people, particularly during the centuries that humans have been farmers. …And when infectious death declined in the nineteenth and twentieth centuries, population, urbanization, intensification, land use, and prosperity all climbed to historically unprecedented levels worldwide.

Although there’s a lot of interesting history in Kenny’s book, I’m not sure he does enough with this insight. To me it seems a fairly important finding that the foundation of modern technological civilization is the ability to control infectious disease. (To be fair to Malthus, he did discuss disease as one of the mechanisms that acted to limit human population growth, but he generally discounted its importance relative to food supply.) Modern economies are all fundamentally dense, urban economies–the US is 82% urban, China 60%–and this population structure cannot be sustained without a set of technologies and practices that manage infectious disease.

When industrialization and urbanization happened without those controls, as they did in early 19th-century Britain, they led to actual declines in living standards and life expectancy. Rampant disease in pre-industrial cities like ancient Rome killed off residents faster than they could reproduce, requiring a continuing inflow of migrants to maintain their population. If our current systems for controlling infectious disease weaken or fail, therefore, we’re in trouble. That may be a remote tail risk, but it seems at least as serious a tail risk as, say, an asteroid crashing into Earth, a possibility that seems to get a lot more popular discussion. Kenny acknowledges the risk, but ever the optimist, quickly brushes it off:

Given how many infections we share with animals, how many animal diseases may be only a few mutations away from infecting humans, and how rapidly viruses and microbes in particular can mutate and then spread in a connected world, new global pandemics will surely continue to hurl themselves at humanity. Ronald Barrett and colleagues from the Department of Anthropology at Emory University in Atlanta have gone as far as to suggest that the emergence and re-emergence of disease threats owing to globalization and antibiotic resistance is a sign that we’re entering a “third epidemiologic transition” comparable to the rise of infection at the dawn of civilization and its fall in the last century and a half. That (hopefully) goes too far, but it certainly suggests the scale of the risk we need to confront.

The Malthusian fear that physical resource constraints could stop economic advance and population growth has been a persistent one in economic thinking over decades (and one to which I must confess making my own contribution). Kenny’s book suggests we should have instead been working to better understand and manage the fundamental biological systems on which our civilization depends.