In the 1990s, two young French economists then affiliated with the Massachusetts Institute of Technology, Thomas Piketty and Emmanuel Saez, began the first rigorous effort to gather facts on income inequality in developed countries going back decades. In the wake of the 2007 financial crash, fundamental questions about the economy that had long been ignored again garnered attention. Piketty and Saez’s research stood ready with data showing that elites in developed countries had, in recent years, grown far wealthier relative to the general population than most economists had suspected. By the past decade, according to Piketty and Saez, inequality had returned to levels nearing those of the early 20th century. Last fall, Piketty published his magnum opus, Capital in the Twenty-First Century , in France. The book seeks to model the history, recent trends, and back-to-the-19th-century future of capitalism. The American Prospect asked experts and scholars in the field of inequality to...