Those who use French economist Thomas Piketty’s work to justify redistributionist public policies face challenges from more than just leading classical liberals. As Robert Rosenkranz writes in the Wall Street Journal, Piketty himself believes people have learned the wrong lessons from his scholarship.

Now in an extraordinary about-face, Mr. Piketty has backtracked, undermining the policy prescriptions many have based on his conclusions. In “About Capital in the 21st Century,” slated for May publication in the American Economic Review but already available online, Mr. Piketty writes that far too much has been read into his thesis.

Though his formula helps explain extreme and persistent wealth inequality before World War I, Mr. Piketty maintains, it doesn’t say much about the past 100 years. “I do not view r>g as the only or even the primary tool for considering changes in income and wealth in the 20th century,” he writes, “or for forecasting the path of inequality in the 21st century.”

Instead, Mr. Piketty argues in his new paper that political shocks, institutional changes and economic development played a major role in inequality in the past and will likely do so in the future.

When he narrows his focus to what he calls “labor income inequality”—the difference in compensation between front-line workers and CEOs—Mr. Piketty consigns his famous formula to irrelevance. “In addition, I certainly do not believe that r>g is a useful tool for the discussion of rising inequality of labor income: other mechanisms and policies are much more relevant here, e.g. supply and demand of skills and education.” He correctly distinguishes between income and wealth, and he takes a long historic perspective: “Wealth inequality is currently much less extreme than a century ago.”

All of this takes the wind out of enraptured progressives’ interpretation of Mr. Piketty’s book, which embraced the r>g formulation as relevant to debates playing out in Congress.