Two hundred years ago, Malthus' principle of population was widelyknown and its importance with regard to economic theory seemed, although controversial among laymen, generally accepted among economists. During the nineteenth century, it constituted the theoretical foundation not only of the science of political economy, but also of the emerging sciences of sociology and bi-ology. However, over the course of the centuries, since new generations were not confronted with the same everyday problems the classical economists were facing, its popularity declined sharply as it was rst increasingly misinterpreted and nally considered to have been falsi ed.
The controversial and famous argument Malthus had brought up was to conjecture that the principle of population would in reality inevitably induce population, rising exponentially (ge-ometrically), to permanently catch up to any higher level of production, which rose merely linearly (arithmetically), for Malthus (1798) had written in his original essay that
natural inequality of the two powers of population and of production in the earth, and that great law of our nature which must constantly keep their eects equal, form the great di culty that to me appears insurmountable in the way to the perfectibility of society. All other arguments are of slight and subordinate consideration in comparison of this. I see no way by which man can escape from the weight of this law which pervades all animated nature.16
13 See Thompson (1930), who observed the fertility transition as part of the demographic transition.
14 See Komlos (2003).
15 See Livi-Bacci (2012).
16 Malthus (1798), chapter I.
Against this statement, an important criticism regarding the great di culty that appears un-surmountable was legitimately raised. Even today, in spite of the illustrated early stagnation of GDP per capita and the recorded evidence in many animal species, the notion of the inevitability of a regime of stagnation does not attract too many economists for the following two reasons:
Firstly, they observe globally increasing GDP per capita. Secondly, they observe at the same time decreasing population growth.
However, in his later editions Malthus became more optimistic and clearly admitted that his original version of an unrestricted principle of population inevitably resulting in a permanent regime of stagnation was misleading. Having travelled within large parts of Europe, gathering impressions and population data, he arrived at the insight that it was possible to embank the exponential power of population, attenuating his former conclusions in his later editions (1803-1826) by employing more frequently the term tendency. Accordingly, the principle of population was to be interpreted as a permanently operating, abstract tendency (using the word tendency to express a propensity toward an increase in numbers) employed as a reference point for theoretical considerations. The actual historical era of stagnation, in contrast, had to be viewed as a readily testable empirical fact, employed as a practical benchmark on real observations. Hence, the principle of population had to be accepted as a xed law, like the law of gravity, whereas the mechanism of a Malthusian trap and the resulting regime of stagnation could be circumvented under proper conditions. Consequently, as it would sometimes not reveal itself at rst glance, the operation of the principle of population alone does not require every population to exhibit exponential growth in reality at all times, as is sometimes asserted, but rather re ects a latent
pressure steadily operating toward an increase of numbers. Among others, McCulloch (1863) sustained the universality of the principle, maintaining that humanity had indeed been facing the principle of population at any point in history:
The principle, whose operation under favourable circumstances has thus developed itself, is, in the language of geometers, a constant quantity. The same power that has doubled the population of Kentucky, Illinois, and New South Wales in ve-and-twenty or thirty years, exists everywhere, and is equally energetic in England, France, and Holland.17
Notwithstanding Malthus' reconsideration, the majority of modern economists seems to stay intellectually trapped in his rst essay on population, inclined to put the tendency of the principle of population on the same level with a selfevident fact, the era of stagnation.18 Senior had already perceived a widespread ignorance regarding Malthus' renewed formulation and he realized that it would become hard to eradicate the original, more popular, more insistent but wrong version:
On the other side are those who maintain that population has a tendency (using the word tendency to express likelihood or probability) to increase beyond the means of subsistence;
or, in other words, that, whatever be the existing means of subsistence, population is likely
17 McCulloch (1863), part I, chapter VIII.
18 As has been remarked, even the most recent attempts to resuscitate the classical Malthusian view seem to refer to a perception of history in which population would permanently and inevitably outgrow production as a self-evident fact and not as a tendency.
fully to come up to them, and even to struggle to pass beyond them, and is kept back principally by the vice and misery which that struggle must produce.19
Obviously, when confronting a person with this older Malthusian statement, he will most proba-bly reasonaproba-bly reject it and be inclined to consider this viewpoint as empirically falsi ed. If he, moreover, regards this argument as being the central one of the Malthusian theory of population, he will also erroneously be inclined to reject the principle of population. Consequently, when Mill and McCulloch employed phrases like
that there is a constant tendency in all animated life to increase beyond the nourishment prepared for it, no one can possibly doubt20,
Senior felt obliged to comment on their wording and clari ed that
we believe that they [Mr. Mill and Mr. McCulloch] have used it without being misled by it themselves, and, perhaps on that very account, without perceiving its tendency to mislead others. But that those whose acquaintance with Political Economy is super cial (and they form the great mass of even the educated classes) have been misled by the form in which the doctrine of population has been expressed appears to us undeniable. When such persons are told that it is the tendency of the human race to increase faster than food. to people a country fully up to the means of subsistence, they infer that what has a tendency to happen is to be expected. Because additional population may bring poverty, they suppose that it necessarily will do so [. . . ] [Such a doctrine] furnishes an easy escape from the trouble or expense implied by every project of improvement. What use would it be, they ask, to promote an extensive emigration? the whole vacuum would be immediately lled up by the necessary increase of population. [. . . ] It is because we believe these misconceptions to be extensively prevalent that we have ventured to detain our readers by this long discussion. A discussion which some may think a mere dispute about the more convenient use of a word, and others an attempt to prove a self-evident fact.21
In summary, following Malthus' later editions, the principle of population will in this work be treated as a xed law that continues to operate during the era of development (although potentially outweighed by some other eect), whereas his original statement of a Malthusian trap is considered as falsi ed after the breakout from stagnation. Consequently, since the end of the regime of stagnation does not mean the end of the principle of population, the positive eect of productivity growth on population must further be modeled as a part of the here advanced uni ed growth mechanism.
19 Senior (1836), p. 146.
20 Mill (1848), book I, chapter VII.
21 Senior (1836), p. 149.