This intriguing book examines states/empires from -3500 to the present. The authors identify relationships in features such as population, size, technology, and polity types and over time trends, and examine if these can be summarized in equations as a focus for possible explanations of causes. Technology is a key factor, allowing for population growth as well as increasing size of empires, as larger distances can be covered moving from messages sent by runners, riders, and advances through engineering. Different technologies and endowments favor different polities and changes in empire growth and decline curves. The authors note a declining trend in the effective number of states and increasing share of area and population in the largest state, which if projected suggest that the world will converge in a single state in 4616. They caution that prediction is difficult given the number of things that can intervene, just as the expected lifespan of a rabbit can be cut short if encountering a fox. Many factors that could affect future population and state sizes are discussed, including climate change and the earth’s carrying capacity. The authors emphasize the value of evaluating possible futures against observed trends, and I entirely agree. But I missed a discussion of possible changes from territorial conquest through war as a convention (uto possidetis) to territorial integrity norms after 1945. Apart from a few mergers, no large state has grown substantially since then, and further splits seem more likely than large scale expansions.