Friday, June 12, 2009

An Essay on the Principle of Population

The Rev. Thomas Malthus’s An Essay on the Principle of Population was one of those runaway intellectual successes that turns its author’s name into an adjective. Alas, as someone observed, classics are like the aristocracy: we learn their titles and pretend acquaintance. It is a book far more invoked than read.

The Penguin edition contains the first edition of the Essay conjoined with A Summary View of the Principle of Population, Malthus’s distillation of his views, plus a particularly useful and intelligent introduction by philosopher Anthony Flew. (I recommend that you read Flew’s introduction, then read the two works by Malthus, and then re-read Flew’s introduction.)

The Essay has three elements to it: Malthus’s statement and explication of his principle of population; critiques of Condorcet and Godwin; and a justification of the works of God to man.

The last has the standard difficulties of that genre, if rather more intelligently expressed than is often the case. The second is striking for reminding us how much the same arguments are continually revisited over the decades.

Malthus is very much an empiricist and a person of the sceptical Enlightenment—things, and particularly people, are as they are: the point is to understand such and deal with that, for otherwise lies disaster. Condorcet and Godwin are very much of the radical Enlightenment—humans are perfectible, so society is perfectible, so radical change can produce unparalleled human felicity. In the light of history—with one major caveat—Malthus clearly has the better of the argument here.
The major caveat is an error in Malthus’s economics. He does not think trade and manufactures adds to a country’s wealth and particularly not to the betterment of the labourers. The argument being that such work is unhealthy and the production of basic sustenance (particularly food) is what is crucial, so trade and manufactures divert resources from such greater than any benefit they provide.

Clearly, Malthus was wrong. A Malthusian economy as he conceived it is one where the basic economic ratio is the amount of land under cultivation compared to the total population. For most of human history, capital was a minor factor. But hardly always a negligible player: contemplation of the history of Venice, for example, might have led Malthus to reconsider his view on this matter. (He dismisses the Dutch case on the grounds that food is expensive in the Netherlands: which it was. But nor did they have famines.)

The reason why Malthus was wrong—the reason why he did not agree with Adam Smith on this matter but did agree with the Physiocrats, though he thought they were right for the wrong reasons—was because of his reasoning on population, which led him to take food to be a fundamental and pervasive constraint not systematically amenable to human action: the part of his essay that turned his name into an adjective. And also made him one of the founding thinkers of demographics.

To read Malthus’s Essay is to be in the intellectual presence of a genuinely original thinker. His discussion of the connection between high death rates, early marriages and high birth rates is deeply intelligent and perceptive. He is also, as the Summary with its discussion of population statistics makes clear, very much an empirical thinker.

He has had enormous intellectual influence: most profoundly by giving both Charles Darwin and Alfred Wallace—quite independently—the mechanism that they applied to evolutionary theory to produce the theory of natural selection. In Charles Darwin’s own words from The Origin of Species:
… the Struggle for Existence amongst all organic beings throughout the world, which inevitably follows from their high geometrical powers of increase … This is the doctrine of Malthus, applied to the whole of the animal and vegetable kingdoms. As many more individuals of each species are born than can possibly survive; and, as a consequence, there is frequently recurring struggle for existence, it follows that any being, if it vary however slightly in any manner profitable to itself, under the complex and sometimes varying conditions of life, will have a better chance of surviving, and will thus be naturally selected. From the strong principle of inheritance, any selected variety will tend to propagate its new and modified form.
Or Alfred Wallace from My Life:
One day something brought to my recollection Malthus’ Principle of Population, which I had read about twelve years before. I thought of his clear exposition of the ‘positive checks’ to increase … which keep down the population … It then occurred to me that these causes or their equivalents are continually acting in the case of animals also; and, as animals usually breed much more rapidly than does mankind, the destructions every year from these causes must be enormous in order to keep down the numbers of each species, since they evidently do not increases regularly from year to year, as otherwise the world would long ago have become densely crowded with those that breed most quickly … Why do some die and some live? And the answer was clearly, that one the whole, the best fitted live … Then it flashed upon me that this self-acting process would necessarily improve the race, because in every generation the inferior would inevitably be killed off and the superior would remain – that is, the fittest would survive … The more I thought it over the more I became convinced that I had at length found the long-sought-for law of nature that solved the problem of the origin of species.
One of the striking things about various passing comments in Malthus’s Essay, is how much evolutionary theory was “in the air” well before Darwin and Wallace published their original papers: papers whose originality was not the theory of evolution but in identifying a mechanism by which evolution happened.

His theory
Malthus’s reasoning on population was straightforward. Flew provides a nice explication of Malthus’s conceptual framework using Malthus's own words.
Premise (1) :In taking a view of animated nature, we cannot fail to be struck with a prodigious power of increase in plants and animals Indeed …all animals, according to the known laws by which they are produced, must have a capacity of increasing in geometric progression.
Premise (2): … the means of subsistence, under circumstances most favourable to human industry, could not possibly be made to increase faster than in an arithmetic ratio.
From (1) and (2) comes:
Conclusion (3): By that law of nature which makes food necessary to the life of man, the effects of these two unequal power must be kept equal. This implies a strong and constantly operating check on population from the difficulty of subsistence. This difficulty must fall somewhere; and must necessarily be severely felt by a large portion of mankind.
This conclusion was somewhat adjusted by Malthus over time. So, in the Summary it becomes:
Conclusion (3.1) it follows necessarily that the average rate of the actual increase of population over the greatest part of the globe, obeying the same laws as the increase of food, must be totally of a different character from the rate at which it would increase if unchecked. The great question, then, which remains to be considered, is the manner in which this constant and necessary check upon population practically operates. (Emphasis in original.)
In his Second Essay, Malthus discusses by what means population is kept within the bounds of subsistence. Despite what later invokers were to claim, Malthus was not advancing an overpopulation thesis. On the contrary, if there is constant and necessary check upon population, it follows that population will never exceed the means of subsistence for any length of time.

In the original Essay, Malthus does not discuss the question of the actually operating means that kept population down. He instead moves on to consider what types of checks exist. These are two: preventive (keep births down) and positive, by which he means ‘active’, (death). This is the exhaustive scheme of Malthus-as-social scientist. But there is also Malthus-as-policy wonk. In the latter form, he has a three-part scheme—virtue, vice and misery. Virtue is sexual restraint (specifically, delaying or avoiding marriage: Malthus takes marriage as being not properly a venue for sexual restraint). Vice is such things as contraception, abortion and homosexuality. Misery is war, pestilence, starvation and other forms of death.

Malthus clearly gives a major role for human volition in his final theory: both good volition (virtue) and bad volition (vice). And, in his discussions about concern for subsistence, he hovers somewhat near coming up with a version of niche theory. But his presumption that married folk will just produce babies roughly (inversely) proportional to the age at which they get married gets in the way. Newtonian mechanics is perhaps somewhat too much in his mind as a model.

To Malthus, in a country suffering land constraint (such as China), lots of babies imply high mortality. An alternative possibility is that high mortality implies lots of babies.

About that volition
Let us suppose that parents don’t generally have babies willy-nilly. Let us suppose they have some idea of how many children they can support. If there is a high infant mortality rate, then you need more babies to reach the designated number of children, since you must expect to lose some children on the way through.

If infant mortality rate drops dramatically, then—after a suitable pause for it to become clear infant mortality rates have dropped—one would expect the fertility rate to drop. This is known as the demographic transition and it is precisely what we do observe.

It is not, however, a Malthusian process where mortality is based on food limitations but one where fertility is a response to conditions.

And if parents have a sense of the costs of a child—if you like, a sense of the “niche” size for a child—and the costs increase—i.e., the “niche size” expands—then one would also expect the fertility rate to drop. Which we also observe: most famously the poor breeding more than the rich. Given the children of the poor have a lower survival rate and cost less than the children of the rich, hardly surprising.

Unless one was a Malthusian, in which case the rich would have more resources and so therefore have more babies (a point William Godwin made against Malthus at the time). Clearly this is also not a Malthusian process.

What both the over-time and across-society forms of the demographic transition are, in fact, is an intensification of the offspring strategy of which homo sapiens are the most extreme form—invest less in number of offspring and more in their care: originally, because our cognitive development takes so long. Now, we add on education time. But that is just further cognitive development: a cultural add-on to the biological process that enables culture.

Obviously, Malthus greatly underestimated the capacity of technology to increase food production. His economic theory and his population theory erroneously reinforced each other.

But, as the point about parents making decisions about having babies (or, at least, raising children) indicates, there were more basic problems with Malthus’s theory.

To start with, given food is also made of populations, the notion that populations that are food increase arithmetically while populations that are not food increase geometrically is not coherent, as William Hazlitt pointed out at the time. (This is a difficulty Darwin easily avoided by including sunshine, soil, weather, etc as constraints.)

There are two further, related, problems. The first is a point made by Paul Colinvaux in The Fates of Nations. Human children have always required considerable investment by their parents to raise. If our ancestors could not make reasonably good judgments about how many children to raise, we would have died out. Of course, this is a point from the theory of natural selection that was not available to Malthus or his contemporary critics but has since become available as a result of the use of Malthus’s theory.

There is a certain intellectual irony, not to say paradox, in this.

To say parents make deliberate choices about having children is not to argue against providing contraception. First, more precise control of fertility choices is surely good: especially for poor folk. Particularly as there are some grim ways of not raising too many children at once (abandonment and infanticide). Second, the big thing about the pill is not that it is a contraceptive device, but that it is a female-controlled contraceptive device. The notion that poor folk “just have babies” is condescending nonsense. (I prefer the flattering unifying rationality of economics to the condescending multiplying delusions of other social schemas.)

The second problem was identified at the time by the economist Nassau Senior and the philosopher Archbishop Whately. Whately pointed out that there were two different senses of tendency. There is a difference between tendency in the sense of:
the existence of a cause which, if operating unimpeded, would produce the result
the existence of such a state of things that the result may be expected to take place (emphasis in original).
Parents clearly can produce far too many offspring for resources to support but may well not do so because they judge that resources available are not sufficient to sustain the required investment. In such circumstances, the tendency in the first sense fails to become one in the second.

Now, parents can make judgments that turn out to be wrong. Circumstances may change, perhaps drastically. But, clearly, Malthus’s theory seriously missed the mark, and the failure to distinguish between two different senses of tendency was central to so missing.

That there turned out to be fundamental problems with Malthus’s theory does not detract from it being a major intellectual achievement. We owe both the theory of natural selection and modern demographics to it. He was very much on to something, just not quite what he thought he was on to.

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