The first of these forces was very much a creature of the industrial age. Earlier wars had produced mixed results, as victors profited and losers paid. The Civil War is another example: It launched the careers of John D. Rockefeller, Andrew Carnegie, and other Northern plutocrats, but ruined Southern slave-owners. Not since the times of the ancient Greeks had intense popular military mobilization (paired with egalitarian norms and institutions) helped curb economic inequality.

Second are revolutions that truly transformed societies—the sort that were born of the two world wars. From 1917 on, communists in Russia, China, and elsewhere confiscated, redistributed and collectivized private wealth, and set wages, leveling inequality on an unprecedented scale. Revolutions before these, by contrast, were rarely extreme enough to have the same effect: The French Revolution, by comparison a far less bloody affair, made more modest headway.

Third, violent turmoil sometimes destroyed states altogether, taking the rich and powerful down with them. While everyone stood to suffer in times of collapse, the richest simply had more to lose. Records of equalizing misery reach back thousands of years: The last Roman aristocrats lined up for handouts from the Pope, and Mayan nobility had to make do with the same diet as commoners. More recently, Somalia’s anarchy reduced the inequalities of the brutal kleptocracy that had preceded it.

Humans have long faced competition in inflicting damage serious enough to rebalance the scales, which brings up the fourth leveling force. The first pandemic of bubonic plague at the end of antiquity, the Black Death in the late Middle Ages, and the merciless onslaught of smallpox and measles that ravaged the New World after 1492 claimed so many lives that the price of labor soared and the value of land and other capital plummeted. Workers ate and dressed better, while landlords were reduced to complaints that, as one English chronicler put it, “such a shortage of laborers ensued that the humble turned up their noses at employment, and could scarcely be persuaded to serve the eminent for triple wages.” Surviving tax registers from late medieval Italy also bear witness to the sweeping erosion of elite fortunes.

But what of less murderous mechanisms of combating inequality? History offers little comfort. Land reform often foundered or was subverted by the propertied. Successful programs that managed to parcel out land to the poor and made sure they kept it owed much to the threat or exercise of violence, from Mexico during its revolution to postwar Japan, South Korea, and Taiwan. Just as with the financial crisis of 2008, macroeconomic downturns rarely hurt the rich for more than a few years. Democracy on its own does not consistently lower inequality. And while improving access to education can indeed narrow income gaps, it is striking to see that American wage premiums for the credentialed collapsed precisely during both world wars.