Population collapse and chance

by Edward Pegler on 14 July, 2018

What do Alexander the Great’s march on Asia, the Arab Conquests and overly hard exams have in common? The answer is the increased chance of getting lucky when the numbers are low.

So you’re sitting in an exam and the questions seem exceptionally hard compared to all those practice papers you were doing. How should you feel? Frustrated? Fair enough. After all this seems unfair. Comforted? Up to a point. If the paper is generally hard then the exam board is going to have to scale up everybody’s mark. Uncertain? Well, yes. Because you could get a worse mark than you really deserved…

… but on the other hand you might just get lucky.

Low numbers and luck

Why? Let’s take an extreme case. The exam has 100 marks in it. In a perfect world people doing the exam could get 101 different marks (0 to 100). If you divide that into five categories (crap, poor, average, good and great), that’s about 20 marks for each category. If you were lucky and scored 4 extra marks for getting one rather complex question right, due to having a dad who knew about such things, you could be called good instead of just average. So what?

But what if the exam was so hard that the best mark in the exam was only 9, people having scored 10 different marks (0 to 9)? Now the exam board has to use this range for its categories, which means that there’s only 2 marks for each category. In this case, if you managed to get lucky and score 4 extra marks on some complex question, due to having a dad who knew about such things, you could be called great instead of just average.

(Of course it could go the other way. And remember that the same rules apply to everybody. Indeed this effectively means that such an exam is invalidated as a way of checking people’s ability.)

So what’s this got to do with ancient history, or even prehistory?

Great Men, Great Machines, Great Philosophies?

The reason is simple. People often look for a genius or a fundamental religious or technological cause for some astonishing event which changes the future. We all know of Alexander the Great’s extraordinary military prowess or of the religious zeal of the Arabs in the 7th century. We think that we are equally aware of the astonishing power of horse riding or of the lethal effects of the arquebus.

But are people really so infallible? And in what way is this religion so much more oomphy than that one? And surely some of the locals might take to using horses or those guns on the attackers. In fact, in the short term people do often achieve advances through military genius, technology or whatever, but the long term effects are rarely, if ever, that great.


There is one notable exception to this, introduced to most of us by William McNeill in his memorable book ‘Plagues and Peoples’. He certainly wasn’t the first to say it, but the idea that the Americas were won for the Conquistadores and other European settlers due to wiping out natives by European disease has taken a powerful hold on our imagination. No technology, religion or genius is now invoked in this conquest.

Such causes may well also explain the Bronze Age expansions of peoples from the steppe, as argued tentatively by David Reich. However, I don’t think that people would argue for the introduction of disease to the Mediterranean and Middle East by the Arabs, or to central Asia by Alexander’s army (many of whom probably came from central Asia).

Low populations and luck

However, these radical moments in history become much easier to explain when the population of a region shrinks to a half or less of its original size, due to some general epidemic which affects everyone. For in these situations chance can become much more significant.

A clear example of this seems to be the widespread epidemic known as the Justinian Plague, which started in the mid sixth century and continued to reduce the population of the Mediterranean and neighbouring regions for the following 150 years. Events which followed this include the great Slavic Migrations and the Arab Conquest. These were world-changing.

However, a similar case can also be made for an earlier epidemic in the Mediterranean a thousand years before this, around the second half of the fifth century BC. Records of it are scarce, consisting only of Thucydides detailed account for Athens and a brief mention by Roman chroniclers. However, it’s extent may have been much greater than is generally realised. Its possible consequences were the near annihilation of Rome by the Gauls (bad luck) and the subsequent rapid rise of Rome in the Samnite Wars (good luck), as well as Alexander’s great Eastern campaign, events which happened in the hundred or so years of population fall after the first outbreaks of epidemic.


History is not predestined. It’s not a predictable science like big-scale physics. Humans do change and that changes the way that they behave in comparable situations. However, I do believe that statistical patterns can be discerned and those have much to do with chance.

Personally, I have little doubt that depopulation periods of various magnitudes have happened throughout the last few thousand years but that little or no account of most of them survives. Such periods, if they exist, will, in my opinion, correlate with significant historical events. In the west we tend to play down the significance of such depopulation periods. The last one that happened, following The Great Pestilence or Black Death, which took place from the mid fourteenth to the sixteenth centuries AD, didn’t really correlate with any significant changes from a western point of view (well, maybe the rise of slavery). However, from the point of view of Eastern Europe and Asia the effects were much greater, with Turkish expansion, the collapse of the Tatars and the second serfdom.

My guesses at ancient depopulation periods in western Asia, including Europe, are 3000-2800 BC, ??2100-1900 BC, 1750-1550BC, 1350-1100 BC and 800-1000 AD. However, apart from some textual evidence for ancient epidemics (for the eighteenth and fourteenth centuries BC), they are largely based on hunches. Trying to find a good way to spot depopulation will be the job of radiocarbon specialists like Ian Shennan and geneticists like David Reich. Seeing if depopulation correlates with major events will be the job of archaeologists and geneticists. But for the first time such a thing seems possible.





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