What are the social forces that hold together complex societies encompassing hundreds of millions of people? How did human ultrasociality – extensive cooperation among large numbers of unrelated individuals – evolve? The theory of cultural multilevel selection is a powerful theoretical framework for addressing these questions. I use this framework to investigate a major transition in human social evolution, from small-scale egalitarian groups to large-scale hierarchical societies such as states and empires. A key mathematical result in the theory is that large states should arise in regions where interpolity competition – warfare – is particularly intense, resulting in high probability of cultural trait extinction. In my talk I will describe how this theory fares when tested empirically against alternatives, using Seshat: Global History Databank.

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I propose a model for the evolution of large states during the Ancient and Medieval eras, motivated by the ideas of Ibn Khaldun. Ibn Khaldun primarily focused on the interaction between pastoralists and farmers in the Maghreb (Northern Africa), but I extend his theory to Afroeurasia as a whole. The ‘mirror-empires’ model proposes that antagonistic interactions between steppe pastoralists and settled agriculturalists within, or next to the Old World’s arid belt (running from the Sahara desert to the Gobi desert) result in an autocatalytic process, which pressures both pastoralist and farming polities to scale up in polity size and military power. Thus, location on a steppe frontier should correlate with the frequency of imperial genesis. I survey extensive historical data that support this prediction.

A useful approach to thinking about why outbreaks of political violence (scaling up to revolutions and civil wars) occur is to separate the causes into structural conditions and triggering events. Specific triggers of political upheaval, such as self-immolation of a Tunisian fruit vendor, are very hard, perhaps impossible to predict. On the other hand, structural pressures build up slowly and predictably, and are amenable to analysis and forecasting. Quantitative historical analysis reveals that complex human societies are affected by recurrent — and predictable — waves of political violence (P. Turchin and S. A. Nefedov. Secular Cycles. Princeton Univ. Press; 2009). The structural-demographic theory suggests that such seemingly disparate social indicators as stagnating or declining real wages, a growing gap between rich and poor, overproduction of young graduates with advanced degrees, and exploding public debt, are actually related to each other dynamically. Historically, such developments have served as leading indicators of looming political instability. In my presentation I will describe a dynamical model based on structural-demographic theory and illustrate it with data on economic, social, and political dynamics in nineteenth century America, including the most violent episode of political instability in the U.S. history, the American Civil War. I also discuss what this theory tells us about the U.S. today.