Not everything in history is contingent and particular, Peter Turchin notes. Societies as different as medieval France, the Roman Empire and China under the Han dynasty share dynamics, when viewed in an appropriately coarse-grained way:
In fact, several patterns cut across periods and regions3. For example, agrarian, preindustrial states have seen recurrent waves of political instability — not interstate warfare, but lethal collective violence occurring within states, ranging from small-scale urban riots, in which just a few people are killed, to a full-blown civil war. This is just the sort of violence we need to understand: many more people are killed today in terrorist campaigns, civil wars and genocides than in wars between nations4.Recent comparative research shows that agrarian societies experience periods of instability about a century long every two or three centuries. These waves of instability follow periods of sustained population growth. For example, in Western Europe, rapid population growth during the thirteenth century was followed by the ‘late-medieval crisis’, comprising the Hundred Years War in France, the Hussite Wars in the German Empire, and the Wars of the Roses in England. Population increase in the sixteenth century was followed by the ‘crisis of the seventeenth century’ — the wars of religion and the Fronde in France, the Thirty Years War in Germany, and the English Civil War and Glorious Revolution. Similarly, population growth during the eighteenth century was followed by the ‘age of revolutions’, ranging from the French Revolution of 1789 to the pan-European revolutions of 1848–49 (ref. 5).
Such oscillations between population growth and instability have been termed ‘secular cycles’6. Given the limitations of historical data, we need an appropriately coarse-grained method to determine the statistical significance, and the generality, of the pattern. The basic idea is to demarcate population growth and decline phases, and to count the instability incidents (such as peasant uprisings and civil wars) that occur during each phase.
With my col leagues Sergey Nefedov and Andrey Korotayev, I have collected quantitative data on demographic, social and political variables for several historical societies. Applying the above approach to eight secular cycles in medieval and early modern England, France, the Roman Empire and Russia, we find that the number of instability events per decade is always several times higher when the population was declining than when it was increasing6. The probability of this happening by chance is vanishingly small. The same pattern holds for the eight dynasties that unified China, from the Western Han to the Qing7, and for Egypt from the Hellenistic to the Ottoman periods8.