Tuesday, June 28, 2011

To eat or not to eat, that is the question: was Malthus right?

The subject of the most recent BBC 4 radio program, In Our Time, is Thomas Malthus and his ideas on population.  These ideas were very influential to Charles Darwin as he developed his theory of evolution and natural selection.

Thomas Malthus
Malthus (1766 - 1834) was an English preacher and scholar.  His most influential contribution was his book written in response to the ideas of utopianists like William Godwin, an anarchist who believed in the 'perfectibility of society', that society's potential was essentially limitless -- women would eventually be equal to men, aristocracy would disappear along with the monarchy, indeed, any form of government.  Once the evils of society were gone, men would no longer be evil, as it was social pressures that made them so.  Godwin was one of many Europeans who were arguing in this way, which was quite threatening to monarchies (and look what happened in France!!). (Godwin was, incidentally, married to Mary Wollstonecraft and the father of Mary Godwin Shelley, who wrote Frankenstein.)

Malthus argued, in contrast, that it was a law of nature that society could never improve because when times were good, population would rise only to be checked by famine, war and disease.  As he wrote in his famous book, An Essay on the Principle of Population, 
Assuming then my postulate as granted, I say, that the power of population is indefinitely greater than the power in the earth to produce subsistence for man.
Population, when unchecked, increases in a geometrical ratio. Subsistence increases only in an arithmetical ratio. A slight acquaintance with numbers will shew the immensity of the first power in comparison of the second.
By that law of our nature which makes food necessary to the life of man, the effects of these two unequal powers 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.
William Godwin
There was no getting ahead, and poverty was inevitable and constant. But Malthus's argument was not a generic one.  It was an explicit ad hominem attack on Godwin's ideas, and his powers of reasoning.  For example (one of many), in chapter 10 of the Essay, which is devoted to a critique of Godwin, Malthus says,
In reading Mr Godwin's ingenious and able work on Political Justice, it is impossible not to be struck with the spirit and energy of his style, the force and precision of some of his reasonings, the ardent tone of his thoughts, and particularly with that impressive earnestness of manner which gives an air of truth to the whole. At the same time, it must be confessed that he has not proceeded in his inquiries with the caution that sound philosophy seems to require. His conclusions are often unwarranted by his premises. He fails sometimes in removing the objections which he himself brings forward. He relies too much on general and abstract propositions which will not admit of application. And his conjectures certainly far outstrip the modesty of nature.
Godwin rose to the bait, much against his better judgement, and wrote a rebuttal to Malthus's treatise.  He didn't buy the idea that population rose geometrically, and he demanded proof.

And Godwin was right.  Indeed, as discussed by the experts on In Our Time, Malthus basically made up the idea of geometrically increasing population vs arithmetically increasing food supplies out of whole cloth.  The idea was that population doubles every generation because the number of offspring next generation is proportional to the number of parents in this generation: each parent more than reproduces him/herself. But agricultural progress on fixed amounts of acreage only increase gradually--even if by some amount per year, it's not as fast as population growth (Malthus said). But the argument was bolstered by little more than vague data and lots of hand-waving.

Yet from then, through his inspiration of Darwin and Wallace as a justification of natural selection, it is still believed.  Even in Malthus' time, population was not in excess of resources.

This shows how close examination can reveal things that casual acceptance misses: we ourselves, as evolutionary biologists, have read, re-read, and marked-up Malthus, and read Darwin's and Wallace's comments about Malthus, yet were so blinded that we did not question the evidence for the differential growth assertions.

Overpopulation is a reality, but many specialists debate what, when, where, and how that is the case, and not everyone today accepts Malthus' basic tenets.  Indeed, pro-business interests say that industry can feed everyone and it's just politics that prevents the distribution.

Darwin and others extended--at their own recognizance and not really taking from Malthus--the idea that the struggle was not just the population against the environment, but among the population because of the environmental constraint.  Again, this is a whole-cloth extension, basically.  Indeed, Wallace was more about populations (species) against environment than about mano a mano competition.

Only if it was a true Law of Nature, and not something that just happened now and then, would overpopulation force natural selection.  It is manifestly and obviously true that in almost all circumstances the vast, vast majority of indivduals are not on the very edge of survival, scrawny, scrambling for the last scrap so they can screw one more time and perpetuate their genotypes.  There are competitive elements in Nature, to be sure, but they are much less law-like and ubiquitous, and there are other ways than just selection for evolution to occur.

The canonization of Malthus, who was largely defending the privileged monarchy against Utopian idealists, shows how an idea, easy, simplistic and appealing even if badly flawed even at the time, can come to dominate and drive even an important area of scientific theory inhabited by people as intelligent and well-trained as there are.  It isn't the first time in the history of science that poorly supported theory has survived (Galen, phlebotomy, celestial spheres, Genesis). It's more the rule than the exception. So why then are we so convinced by this one?

This does not imply that population can't or doesn't often press up against carrying capacity, nor that this cannot sometimes induce competition.  But it does imply that such is not a fundamental law of life, and it forces us--if we recognize the truth of it--to think more subtly about the many forms of differential reproduction that as far as we know must be possible for the diversity and adapted nature of life.

And it should force us to be a little more humble about our own wise insights and theories.....


Bjørn Østman said...

Two comments:

I find it somewhat vexing that you would mention in a scientific discourse what business interests think about the matter. I mean, you really think they care much about evidence in that line of work?

Aren't we seeing exactly what Malthus predicted in Niger right now? And if we forget humans, the fact that there is a carrying capacity at all, doesn't that suggest Malthus was correct (in, at least, that populations grow to live on the edge)?

Ken Weiss said...

I wouldn't say I care what businesses think except that they say that modern agribusiness could feed everyone sustainably. I don't think that's true, because population will just grow if there's more food, and the inequity of distribution which is what business often points to is not something one can really see disappearing.

There may not be a hard carrying capacity in the classical Malthusian sense, but clearly populations can outgrow their resources. Is it a fundamental 'law' or just something that is often if not usually true, and that may have an influence on competition (but natural selection can occur without population pressure in the Malthus sense).

As to places like Niger, I would agree with what you say, and it is not persuasive that this is 'just' a distribution problem. And, of course, there is the longer-run Malthusian problem of soil exhaustion etc. Business says that technology can solve all problems, but of course that's self-interested and Utopian thinking hasn't born fruit in the past.