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The original theory of Thomas Malthus is neatly summarized by an oft-quoted statement from the Essay on the Principle of Population: “Population, when unchecked, increases in a geometrical ratio. Subsistence increases only in an arithmetical ratio” (1798, 14). Population is assumed to grow exponentially, but the growth of a society's means of subsistence is assumed to be only linear. If this is so, exponential growth of population unavoidably outpaces the linear increase of subsistence. Alas, population levels are constrained by food supply as people need enough food. Tragically, food intake per capita shrinks as population grows faster than subsistence. Linear growth in food supply cannot make up for the skyrocketing needs of the exponentially growing population. At some point, population growth runs against the limit imposed by minimum food intake per capita.
In a society characterized by social inequality, the poorest of the poor will be the first to feel the looming food scarcity. As population levels rise and food per capita decreases, the food available to the poor will fall below the minimum intake that is necessary for their subsistence. Redistribution can keep the poor fed for a while, but this will not prevent more and more people from becoming destitute due to the inexorable fall of food per capita. In the end, the system is likely to be readjusted by brutal mechanisms such as famine, war, and pandemics.
Logically speaking, another solution would be to limit population growth to “arithmetical ratio” in line with the linear growth of food production. In practical terms, this would mean birth control. To Malthus, who was an Anglican country curate and a moralist, family planning and any kind of sex without the aim of reproduction came under the category of sinful behavior. He therefore advocated voluntary forms of “moral restraint”, but at the same time believed that curtailing the reproductive instinct of the masses was simply not realistic.
During his lifetime, Malthus modified his theory several times: first in the two-volume version of the Essay (1803) and then in various further editions (Winch 1987). These modifications need not detain us here, as they left the basic theory in place. Nor is there any need to dwell on the finer points of the theory or its policy implications, which were important during the nineteenth-century debate about the poor laws. For our present purposes, we are only interested in the logical structure of the theory and its applicability to issues of resource management.
The enduring appeal of the theory is mostly due to its plausible assumptions and axiomatic elegance. It is indeed plausible to assume that population grows by an annual rate multiplied by current numbers—much like the stock on a bank account grows by the iterative application of an interest rate. The result of compound interest, or of children and children's children following the reproductive behavior of their forefathers, is exponential growth. Similarly, it appears plausible to assume linear growth for a population's means of subsistence because agricultural innovation and other improvements in food production tend to happen in an incremental fashion, suggesting linear progress rather than a self-reinforcing mechanism. This appears much more plausible than to assume that improvements in food production are like a compound interest rate applied over a stock.
There is an important element missing from the account, or rather implicit in it: namely the notion of overshoot. Overshoot means that a system can temporarily exceed its long-term limits. Malthus assumed that this was indeed possible. Otherwise, why did he assume that population levels would be readjusted through “vice and misery”—shorthand for famine, war, pandemics, and sinful behavior— rather than simply being limited by minimum food intake per capita? In fact, “vice and misery” are unavoidable only insofar as population can temporarily exceed subsistence. Plain commonsense has it that this can easily happen. Population levels may exceed agricultural yields during years of good harvest, but the famine bound to occur in a later year of bad harvest will then be even more catastrophic. Malthus
Fig. 4.1 Classical Malthusianism
assumed that such misery was likely to be accompanied by war and pandemics, as well as objectionable forms of non-reproductive sex, or “vice”.
To illustrate the axiomatic elegance of the theory, consider Fig. 4.1.
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