Peter Turchin is a highly respected evolutionary biologist who has specialized in the synthesis of theory and empirical data (see his book Complex Population Dynamics for his work in that area). He has now turned the skills he honed explaining animal societies to human societies, and particularly to explaining the rise and fall of empires. In broad terms I would describe his approach as Malthus meets Marx meets social constructionism meets evolutionary game theory. While his model is strictly applicable only to agrarian empires, his explanations of phenomena such rising income equality, intra-elite conflict, and even increased demand for university admissions, resonate so strongly with modern society that it is clear that a modified version of his model will go a long way towards explaining our current political and economic circumstances. There are few aspects of his work that are individually wholly new; Turchin's contribution is a rigorous synthesis of historical case-studies with evolutionary theory and quantitative empirical evidence. His work has the potential to transform our understanding of "macro" social issues in the same way that behavioral economics has transformed our understanding of decision making at the "micro" level. I'll go out on a limb and predict that Turchin will eventually win a Nobel prize in economics.
I'll provide a quick overview of Turchin's work, but this synopsis doesn't do it justice; if you find my overview implausible, please read his books for yourself.
How groups manage to escape the prisoners' dilemma and cooperate is a central question of evolutionary biology. Turchin argues that the social construction of "other" along meta-ethnic frontiers (which are often defined in terms of factors other than ethnicity, in particular religion), is necessary to enable group cooperation which allows empire building. This is why empires almost invariably arise along frontiers. A ruling class with a high potential for collective action ("asabiya" - a term Turchin borrows from the 14th century political philosopher Ibn Khaldun), will expand while financing its wars by taxing the peasants. In the early days of the empire, the elite are relatively austere warriors, and low population densities allow peasants to produce a significant surplus, so elite requirements do not overburden peasant production. As population densities increase, the surplus produced per peasant decreases because each has less land, but at the same time rents charged by the elites increase as land becomes scare. Peasants become poorer, though the elite continue to do well. Wealth inequality increases, and eventually the peasant base cannot sustain the high expectations of the growing elite population. Consequently, some of the elite class find themselves without land to sustain their lifestyle, while others become extremely wealthy due to control of scarce resources. This gives rise to intra-elite conflict. Social cohesion declines due to increasing inequality, both between elite and peasant classes and within the elite. The result is that peasants who are desperate and weakened by poverty are drawn into elite infighting. A combination of civil war, famine and plague reduces the population of the weakened state. The population decline ultimately leads to lower food prices and increased wages for the poor, but the loss of social cohesion is not so easily reversed. The recovery is thus impeded by continued infighting, and sometimes an outside group with higher asabiya takes over before another expansion phrase is triggered.
Turchin has three books developing his approach. "War and Peace and War: The Life Cycles of Imperial Nations" is the popular introduction. It describes the approach without any math or equations, and applies it to a range of historical empires. This is the place to start for a general introduction, particularly if you are not mathematically inclined. However, it is not formally rigorous and will not convince you if you are sceptical. "Secular Cycles" (with Sergey Nefedov) supports the theory with quantitative empirical data. It applies the model to two cycles in each of England, France, Rome and Russia. This is the book to read if you are comfortable with numbers and need to be convinced by empirical evidence. "Historical Dynamics: Why States Rise and Fall" provides the theoretical framework, discussing, for example, why an explanation of cyclical dynamics requires a feedback loop. It is quite mathematical, and while you don't have to work your way through all the equations, you should be comfortable with mathematical models generally. Turchin's model was inspired by Jack A Goldstone, "Revolution and Rebellion in the Early Modern World." This is also an excellent book. It is written in a more traditional historical style; the model is informal, rather than formal, and the argument is supported by historical analysis of particular revolutions, rather than by quantitative data. In these respects it is similar to "War and Peace and War," though it is substantially longer. If you are looking for an extended analysis in a more traditional style of social history, this a great book.
This review pertains to all three of Turchin's books, and I am posting the same review for all of them.