Researcher Peter Turchin sees two cycles driving political instability. The secular cycle, lasting two to three centuries, starts with a relatively egalitarian society (supply and demand for labour roughly balance). But over time, population grows, labour supply outstrips demand, elites form and the living standards of the poorest fall. Then the society becomes top-heavy with elites, who start fighting for power. Political instability ensues, leading to collapse, and the cycle begins again. The shorter fathers-and-sons cycle, spanning 50 years or two generations, interacts with the longer cycle. Turchin sees this cycle peaking around 1870 (ethnic strife, class resentment), 1920 (race...