Arise ‘cliodynamics’

OPINION

NATURE|Vol 454|3 July 2008

ESSAY Arise ‘cliodynamics’ hat caused the collapse of the Roman Empire? More than 200 explanations have been proposed1, but there is no consensus about which explanations are plausible and which should be rejected. This situation is as risible as if, in physics, phlogiston theory and thermodynamics coexisted on equal terms. This state of affairs is holding us back. We invest in medical science to preserve the health of our bodies, and in environmental science to maintain the health of ecosystems. Yet our understanding of what makes societies healthy is in the pre-scientific stage. Sociology that focuses on the past few years or decades is important. In addition, we need a historical social science, because processes that operate over long timescales can affect the health of societies. It is time for history to become an analytical, and even a predictive, science.

Let histor y continue to focus on the particular. Cliodynamics, meanwhile, will develop unifying theories and test them with data generated by history, archaeology and specialized disciplines such as numismatics (the study of ancient coins). Is this proposal feasible? The most compelling argument against the possibility of scientific history goes like this. Human societies are extremely complex. They consist of many different kinds of individuals and groups that interact in complex ways. People have free will and are therefore unpredictable. Moreover, the mechanisms that underlie social dynamics vary with historical period and geographical region. Medieval France clearly differed in significant ways from Roman Gaul, and both were very different to ancient China. It is all too messy, argue the naysayers, for there to be a unifying theory. If this argument were correct, there would be no empirical regularities. Any relationships between important variables would be contingent on time, space and culture.

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Splitters and lumpers Every scientific discipline has its share of splitters, who emphasize the differences between things, and lumpers, who stress similarities in search of organizing principles. Lumpers dominate physics. In biology, splitters, who care most for the private life of warblers or the intricate details of a chosen signalling molecule, are roughly matched in numbers by lumpers, who try to find fundamental laws. Social sciences such as economics and sociology are rich in lumpers. Sadly, few are interested in applying analytical approaches to the past. History has an alarmingly small proportion of lumpers. Rather than trying to reform the historical profession, perhaps we need an entirely new discipline: theoretical historical social science. We could call this ‘cliodynamics’, from Clio, the muse of history, and dynamics, the study of temporally varying processes and the search for causal mechanisms2,3 .

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Empirical empires 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 © 2008 Macmillan Publishers Limited. All rights reserved

D. PARKINS

If we are to learn how to develop a healthy society, we must transform history into an analytical, predictive science, argues Peter Turchin. He has identified intriguing patterns across vastly different times and places.

OPINION

NATURE|Vol 454|3 July 2008

France, the Thirty Years War in Germany, and This explanation — the ‘demographic– the English Civil War and Glorious Revolu- structural’ theory — is a work-in-progress. tion. Similarly, population growth during the Our tests with the eight case studies6 supeighteenth century was followed by the ‘age of port some of its predictions: for example, revolutions’, ranging from the French Revolu- élite overproduction preceded the crisis in tion of 1789 to the pan-European revolutions every case. The tests also identify areas where of 1848–49 (ref. 5). the theory needs to be modified. Perhaps we Such oscillations between population need an entirely new theory to explain the growth and instability have been termed observed patterns and predict new ones, but ‘secular cycles’6. Given the limitations of his- that is the business of science. The important torical data, we need an appropriately coarse- thing is that societies as different as medigrained method to determine the statistical eval France, the Roman Empire and China significance, and the generality, of the pattern. under the Han dynasty share dynamics, when The basic idea is to demarcate population viewed in an appropriately coarse-grained growth and decline phases, way. Not everything in and to count the instability history is contingent and “Societies as different incidents (such as peasant particular. as medieval France, the uprisings and civil wars) that Even so, theories develoccur during each phase. Roman Empire and China oped and tested on preindustrial data must be Wi t h my c o l l e a g u e s under the Han dynasty modified before they can Sergey Nefedov and Andrey share dynamics.” Korotayev, I have collected be applied to contemporary quantitative data on demosocial dynamics. Happily, graphic, social and political variables for there are indications that our theories will not several historical societies. Applying the need to be replaced wholesale. Rapid demoabove approach to eight secular cycles in graphic change and élite overproduction were medieval and early modern England, France, still important factors in twentieth-century the Roman Empire and Russia, we find that revolutions10. the number of instability events per decFurthermore, over the past 200 years, ade is always several times higher when the political instability in the United States has population was declining than when it was waxed and waned in a pattern reminiscent of increasing6. The probability of this happen- that in preindustrial societies. Political violence ing by chance is vanishingly small. The same — urban riots, lynchings, violent labour dispattern holds for the eight dynasties that putes and so on — was almost absent in the unified China, from the Western Han to the early nineteenth century, increased from the Qing7, and for Egypt from the Hellenistic to 1830s and reached a peak in around 1900. the Ottoman periods8. The American Civil War occurred during this period of growing unrest. The instability then Making waves subsided during the 1930s, and the following Such strong regularity points to the presence two decades were remarkably calm. Finally, in of some fundamental principles. Population the 1960s, political violence increased again11. growth beyond the means of subsistence leads It remains to be seen whether a modito declining levels of consumption and popular fied version of the demographic–structural discontent, but this is not enough to destabilize theory can explain this pattern. The point is agrarian societies. Peasant uprisings have little that the study of such slow-moving processes chance of success when the governing élites are requires a long-term view and an explicitly unified and the state is strong9. historical approach. The connection between population dynamics and instability is indirect, medi- Learning lessons ated by the long-term effects of population Any claim that history can become a predicgrowth on social structures. One effect is the tive science raises eyebrows. But scientific increasing number of aspirants for élite posi- prediction is a broader concept than merely tions, resulting in rivalry and factionalism. forecasting the future. It can be used to test Another consequence is persistent inflation, theories. For example, two rival theories may which causes a decline in real revenues and a make different predictions about the behavdeveloping fiscal crisis of the state. As these iour of some variable, such as birth rate, trends intensify, they result in state bank- under certain social conditions. We then ask ruptcy and a loss of military control; conflict historians to explore the archives, or archaeamong élite factions; and a combination of ologists to dig up data, and determine which élite-mobilized and popular uprisings, lead- theory’s predictions best fit the data. Such ing to the breakdown of central authority3,9. retrospective prediction, or ‘retrodiction’, is © 2008 Macmillan Publishers Limited. All rights reserved

the life-blood of historical disciplines such as astrophysics and evolutionary biology. Cliodynamic theories will not be able to predict the future, even after they have passed empirical tests. Accurate forecasts are often impossible because of phenomena such as mathematical chaos, free will and the selfdefeating prophecy. But we should be able to use theories in other, perhaps more helpful, ways: to calculate the consequences of our social choices, to encourage the development of social systems in desired directions, and to avoid unintended consequences. Like other systems with nonlinear feedback, societies often respond to interventions in surprising ways. When the Assembly of Notables refused to approve a new land tax in 1787, they did not intend to start the French Revolution, in which many of them lost their heads. When Tony Blair was Britain’s prime minister, he set out to increase the proportion of youth getting higher education to 50%. He was presumably unaware that the overabundance of young people with advanced education preceded the political crises of the age of revolutions in Western Europe12, in late Tokugawa Japan and in modern Iran and the Soviet Union9,10. It is time we heeded the old adage that those who do not learn from history are doomed to repeat it. We must collect quantitative data, construct general explanations and test them empirically on all the data, rather than on instances carefully selected to prove our pet narratives. To truly learn from history, we must transform it into a science. ■ Peter Turchin is professor of ecology and mathematics at the University of Connecticut, Storrs, Connecticut 06269, USA. He is the author of War and Peace and War: The Life-Cycles of Imperial Nations (Pi Press, 2006). 1. Demandt, A. Der Fall Roms: die Auflösung des Römischen Reiches im Urteil der Nachwelt (Beck, Munich, 1984). 2. Turchin, P. Historical Dynamics: Why States Rise and Fall (Princeton Univ. Press, 2003). 3. Turchin, P. War and Peace and War: The Life Cycles of Imperial Nations (Pi Press, 2006). 4. Mack, A. (ed.) Human Security Report 2005: War and Peace in the 21st Century (Oxford Univ. Press, 2005). 5. Fischer, D. H. The Great Wave: Price Revolutions and the Rhythm of History (Oxford Univ. Press, 1996). 6. Turchin, P. & Nefedov, S. Secular Cycles (Princeton Univ. Press, 2008). 7. Nefedov, S. PhD dissertation [in Russian] (Ekaterinburg Univ., 1999). 8. Korotayev, A. & Khaltourina, D. Introduction to Social Macrodynamics: Secular Cycles and Millennial Trends in Africa (URSS, 2006). 9. Goldstone, J. A. Revolution and Rebellion in the Early Modern World (Univ. California Press, 1991). 10. Goldstone, J. A. J. Int. Affairs 56, 3–21 (2002). 11. Levy, S. G. Political Violence in the United States, 1819-1968. (Computer file, Inter-University Consortium for Political and Social Research, Ann Arbor, 1991). 12. O’Boyle, L. J. Modern Hist. 42, 471–495 (1970).

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