In academic economics as in electoral politics, inequality has become a topic of pre-eminent importance. Yet economics has no accepted theory of inequality—no scientific understanding of the mechanisms and processes that create it. Until recently, wealth distribution was regarded by many economists as a minor issue compared with growth. Too much focus on distribution was even considered a threat to growth. The economists who have offered theories of inequality tend to be those on the heterodox fringes: David Ricardo, Karl Marx, Henry George and, more recently, Thomas Piketty. Broadly convincing explanations remain elusive.

Into this vacuum steps an outsider, Walter Scheidel, a historian of ancient Rome and an ingénue in the brash world of economic analysis. “The Great Leveler” is an astonishing tour de force, a dense 500-page survey of the known history of inequality in all societies, from hunter-gatherers to hedge-fund profiteers. The author believes that settled, stable societies have an inevitable tendency toward greater inequality. The only substantive countervailing forces are all bad news: mass-mobilization wars, social revolution, plague and state collapse. The cure is worse than the disease. Thus in Mr. Scheidel’s telling Lenin, Hitler and Gavrilo Princip did more than the embrace of social democracy to make the 20th century an era of equality.

Across the globe and across centuries, the author documents inequality increasing during conditions of peace and security in society after society, starting with Ancient Rome. As the empire expanded, Mr. Scheidel explains, the citizen-soldier state was replaced by the imperium, and the senatorial class magnified its wealth across the far-flung territories of the Pax Romana. Even at the level of provincial society, we can see evidence of increasing wealth concentration: Archaeological evidence from Pompeii shows how, in the period of about 160 years before the town’s destruction after the eruption of Mount Vesuvius, house sizes became steadily more unequal, as upper-class houses expanded their footprint and middle-class housing disappeared. The scholar Robert Stephan has looked at the variation of square footage in dwelling footprints elsewhere in the empire as a systematic measure of inequality. The dispersion in dwelling sizes in England was larger under Roman rule in England than in the preceding Iron Age or the later early Middle Ages. Centralized states become increasingly unequal states.

The re-establishment of stronger states in Europe in the later Middle Ages was associated, Mr. Scheidel argues, with rising inequality—though here the evidence is thin. Before the onset of the Black Death in 1348, political stability created conditions that helped markets and towns flourish, generating a highly unequal society. As the population expanded, many independent peasant cultivators in England and France lost their land and were reduced to day labor. In France, for example, the typical plot size of peasants fell from 25 acres to less than 7 acres between the ninth century and the early 14th century. At the top, great lords magnified their possessions.

Then came the bad news (or good news from the point of view of equality): the catastrophic population losses of the Bubonic Plague era. All across Europe, these losses acted as a great equalizer. England’s population, for example, went from around six million people in 1315 to 2.2 million by 1450. The resulting shortage of workers in the deserted countryside meant that the real wages of unskilled laborers in England soared to levels not seen again until the 20th century. As land rents declined and cities contracted, the incomes and wealth of property owners shrank relative to those of workers. This equalizing trend, Mr. Scheidel notes, was particularly evident in the wealth-distribution records of northern Italian cities.