The recent announcement of a paper by Motessharrry, Rivas and Kalnay (MRK) on the collapse of complex societies has generated much debate, especially with the publication of an enthusiastic comment by Nafeez Ahmed who defined it a "NASA-funded" study. The term rapidly went viral - no matter how irrelevant the source of funds for the study is - and the discussion soon veered to the kind of clash of absolutes that took place after the publication of the first report to the Club of Rome, "The Limits to Growth" of 1972. For instance, a rather heavy handed criticism of the study can be found in a post by Keith Kloor.
Apart from these rather predictable reactions, what can we actually say of this study? Is it really saying something new, or is it just another "cry wolf" study? Let me try a brief appraisal.
First of all, the MRK study is firmly grounded in system dynamics (even though the authors don't use the term in their paper), the method of modeling created in the 1960s by Jay Forrester. It has also many elements in common with the models used for the original "The Limits to Growth" study of 1972, and the subsequent updates. However, it is a simpler model which makes no attempt to compare the results with historical data. In this sense, it is similar to the "mind sized" models which I have been discussing in a paper of mine on "Sustainability".
Going into the details, we see that the MRK model is a simple, four-stock model. One is the "Natural Resources" stock which is gradually transformed into the stock that the authors call "Wealth" (which in other models is called "capital"). The other stocks are the two classes of the population: "commoners" and "elite". Both take their subsistence from the "wealth" stock, but only the commoners replenish it. The elites, instead, do not produce anything.
The results are not unexpected. Depending on a variety of possible initial assumption, the system may reach a steady state, oscillate, or show a series of peaks and collapse. The figure below, from the paper, is the result that most looks like the "Limits to Growth" "base case" scenario - except for the fact that the population stock collapses in two phases, rather than in a single one.
he MRK model is telling us no more than previous studies did: qualitative models (e.g. Catton's "Overshoot" and Hardin's "The tragedy of commons") and quantitative ones such as "The Limits to Growth". In most cases, this subdivision of the population in two stocks doesn't change very much the results of the model in comparison to single stock ones (also reported in the MRK paper). But in some cases the authors observe rather surprising results, such as this one:
As you see, here society literally commits suicide by having the elites draw so much wealth from the accumulated resources that nothing is left to commoners - who die out. But, as the elites don't produce anything, the wealth stock disappears and the final result is that they also disappear. The elites are so ruthless that they destroy themselves.
Note that in this scenario, the nature stock recovers its initial level: collapse is not the result of lack of resources, but of the inability of society to access them. It is a result that eerily reminds a possible interpretation of the collapse of the Roman Empire. The Romans may have directed so much wealth to non-producers (the army) that producers (slaves) nearly disappeared. Since there was hardly anyone left to cultivate the land, eventually the whole society collapsed.
Of course, this is an interpretation to be taken with some caution. One reason is that in the MRK model commoners cannot become elite and elites cannot become commoners; the two classes are completely separated and impermeable to each other. This is surely an oversimplification: before dying out completely, the elites would at least try to learn to produce something. On the other hand, however, it is true that - for instance - government bureaucrats make very bad peasants.
Possibly a more important shortcoming of the MRK model is that it lacks the critical parameter of persistent pollution - present instead in the "Limits to Growth" model. Pollution, if it were considered, would likely play a fundamental role in this "labor shortage" scenario. It would likely show that the disappearance of the Roman Empire was mainly related, instead, by soil erosion - possibly a more realistic description of the events of that time.
In any case, these considerations illustrate the power of models to stimulate the interpretative capabilities of the human mind. We don't need any formal model to describe the final destiny of economic systems which grow on overexploiting the resources they use. In time, they must go back to a condition compatible to the available (remaining) resources. Most models tell us that this "return" happens as the result of a cycle of overshoot and collapse, which is indeed a characteristic pattern of those socio-economic structures we call "empires".
The MRK model has highlighted a factor that so far has been scarcely explored: how the unequal distribution of wealth affects the trajectory of an economic system in overshoot. This is an important parameter because, in this period, we are seeing an epochal transfer of wealth from the lower classes of society to the elites. Whether this phenomenon will accelerate collapse or slow it down, we cannot say. But surely it is something we need to study and understand.