Of Voles and Men

David Meskill

Can the study of voles—meadow mice or field mice (genus Microtus)—prepare one to produce valuable insights into human history? Peter Turchin thinks so. With Historical Dynamics: Why States Rise and Fall, the theoretical ecologist joins Jared Diamond, Peter Richerson, and a growing number of other biological scientists who have recently turned their sights on human history. Turchin believes his work on the dynamics of vole and other animal populations has given him the basic tools not only to illuminate a particular topic in history—the rise and fall of agrarian empires—but even more ambitiously, to advance history as a discipline.

In Historical Dynamics, Turchin develops and compares several theories in order to explain the dynamics of empires during the long agrarian phase of recorded human history. Early on he introduces a basic and very helpful distinction from population ecology, that between three “orders” of dynamic change or growth: 1) linear or exponential; 2) asymptotic or logistic; and 3) oscillatory. The first kind, limitless growth, almost never occurs, not only because growth requires resources and almost all resources are limited, but also because growth triggers debilitating feed-back mechanisms. For example, in the classic predator-prey situation, the growth of prey populations inevitably allows predators to run wild. This leaves the asymptotic/logistic dynamic, in which growth (whether of a linear or exponential kind) slows until it stabilized around an equilibrium, and the oscillatory dynamic, in which there is either a boom and bust or repeated ups and downs. In both the asymptotic/logistic and the oscillatory patterns, the nature of the feed-back mechanisms plays the decisive role. The key difference between the two—something not at first self-evident, but then readily grasped—is that the former can occur only if the feed-back is immediate and singular, whereas oscillations occur either if there is a lagged feed-back or if more than one feed-back is in operation. To explain the rise and fall of states, Turchin thus argues, we need to look for lagged or multiple feed-back mechanisms.

Turchin considers four processes that may contribute to the rise or fall of states. These are 1) the logistics of expansion, 2) ethnic assimilation, 3) ethnic frontiers as sources of internal cohesion, and 4) the interactions between population growth and political stability. For the first process, Turchin draws on and elaborates Randall Collins’ work on the geopolitics of expansion (which is similar to Paul Kennedy’s theory of imperial overreach). This theory focuses on the gains of expansion, such as increased resources and people to exploit, as well as the drawbacks, such as longer borders to defend and longer routes between core and periphery. Because these negative feedbacks should act fairly quickly, Turchin argues, Collins’ theory accounts at most for asymptotic dynamics: it may explain why states reach a point where they stop expanding, but it can’t explain why they eventually collapse. The second process—ethnic assimilation or lack thereof—also isn’t the main focus of Turchin’s book. It’s not rejected as a possible explanation for the fall of states, as geopolitics was, but rather it is not developed fully enough to address the central question adequately. Even so, Turchin’s brief treatment of processes of assimilation is fascinating and demonstrates the power of mathematical models to reveal surprising patterns. Relying, for example, on Rodney Stark’s argument and data about the social networks that were crucial in the spread of Christianity in the Roman Empire, Turchin shows that the “threshold” or “take-off” arguments that have often been proposed to explain Christianity’s “sudden” popularity in the third century are simply superfluous. The explanations they offer lead to a dead-end. The same exponential rate of growth (which Stark explains in terms of social networks) can account for both the increase of Christians from, say, .0017% of the Roman Empire’s population in 40 CE to still less than 1% in 200 CE, as well as the apparent “take-off” from 1.9% in 250 CE to 10.5% in 300 CE and 56.5% in 350 CE. An understanding of exponential growth takes the mystery out of something like Christianity’s only apparently explosive development in the third century—and thereby redirects our search for causes.

Turchin’s book is most valuable for two other explanations of the rise and fall of empires. His most original contribution relates to the role of what he calls “meta-ethnic frontiers.” Symptomatic of the breadth and productive eclecticism of his thought, Turchin begins here with an idea borrowed from the medieval Muslim thinker Ibn Khaldun, who is sometimes called the world’s first sociologist. Ibn Khaldun, in trying to explain the cyclical rise and fall of Arab states in North Africa, pointed to the key role of asabiya, the internal cohesion and moral strength of a group. The originally high asabiya of mountain tribesmen allowed them to conquer effete low-lying cities, but after some time the allures of urban life would reduce their own collective fortitude—opening the way for the cycle to begin all over again. To explain where asabiya comes from in the first place, Turchin points to Frederick Barth’s seminal study of tribal identity in Afghanistan: group identity is always highest in conflict with another group. When things like religion, language, and way of life (agricultural vs. pastoral) are radically different, the people on each side of the border—which Turchin refers to as a meta-ethnic boundary—will develop a fierce sense of loyalty to their own people.

After developing his model of the waxing of asabiya along meta-ethnic frontiers and its waning in the imperial interior, Turchin tests the theory in considerable detail against the history of state formation and decline in Europe from the Roman Empire to 1900 C.E. (sweeps of this kind are frequent in the book). He does so by dividing Europe into more than 50 regions and coding each of them in century-long intervals for the elements forming a meta-ethnic frontier (religious conflict, etc.), together yielding more than 2000 data points. He then plots these results against the later emergence of states above a certain threshold size. Meta-ethnic frontiers, he finds, explain most—though not all—subsequent instances of later formation of large states. The unexplained exceptions remain a problem, Turchin readily admits, and he explicitly solicits improvements to his theory. For now, however, he argues, what is crucial is that the meta-ethnic frontiers theory has much greater relative explanatory power than its main rival, Collins’ geopolitical approach. (A possible criticism at this point is that Turchin’s test of his theory may have involved some ad hoc adjustments. Already knowing the outcome to be explained [in this case, where states did, in fact, form] Turchin may have tweaked his parameters [for example, the choice of the boundaries of the sub-regions] in such a way as to confirm his theory. Such adjustments, often unintentional, are hard to avoid. Thus, other tests, in which the parameters are chosen without knowledge of the outcomes, would be desirable. Turchin, ever aware of methodological matters and modest in his claims, would almost certainly agree and, indeed, himself repeatedly encourages improvements on and challenges to his theories.)

If asabiya-generation on the frontier accounts mainly for the expansion of states up to a certain equilibrium, excessive population growth largely accounts for states’ subsequent collapse. Here Turchin borrows from and modifies Jack Goldstone’s demographic-structural account of the collapse of early modern states. Goldstone’s theory revolves around the indirect effects of Malthusian growth. I.e. a growing population doesn’t lead directly to starvation, but to a strain on state capacity, inflation, and both more intense elite exploitation of non-elites and greater intra-elite competition.

In the end, Turchin admits that he hasn’t been able to synthesize the various strands into a single unified account. This may be disappointing, but it also fits well with his refreshing modesty and, more than that, with his broader intellectual agenda. Turchin wants to bring to history the mind-set of the scientist. Theories are not (or are only rarely) complete or simply “right”; rather, we usually only have the choice between better and worse theories, those that explain more or less. Hence, in order to make progress, Turchin argues, it is crucial that we frame our arguments in as explicit a way as possible so that we can compare their explanatory power. Constructing mathematical models, he believes, is invaluable for these purposes of testing and comparison. Furthermore, history needs math because its dynamics often involve non-linear, lagged feedbacks—as demonstrated in the rise and fall of agrarian states. The underlying mechanisms of these oscillations are usually too complex to be captured by merely verbal models, and can only be grasped with the aid of explicit mathematical ones.

While becoming a leading ecologist, Turchin has somehow also found the time and energy to acquire vast erudition not only in historical, but also sociological, anthropological, and economic, literatures. This natural scientist doesn't simply or crudely apply lessons drawn from his studies of voles and other animals to human populations; rather, he attempts to combine some of those insights into dynamics with an awareness of factors that make human history even more complex. While he advocates a bold methodological position, the modesty with which he makes his case should absolve him of the charge of “intellectual imperialism” often thrown at earlier generations of social and natural scientists, who encroached on the turf of historians. Turchin goes to considerable lengths to explain his models in ways that are generally comprehensible (though one may have to dust off little-used knowledge from high school or college math). Historians who focus on the many fields from which Turchin draws his case studies (France, Russia, China, Islam, Christianity, Egypt, England) may, or may not, find his explanations convincing. Regardless, he will surely welcome the scrutiny and debate.