Robert Rosenkranz in WSJ

‘Capital in the 21st Century,” a dense economic tome written by French economist Thomas Piketty, became a publishing sensation last spring when Harvard University Press released its English translation. The book quickly climbed to the top of best-seller lists, and more than 1.5 million copies are now in circulation in several languages.

The book’s central proposition, that inequality in capitalist societies will inevitably grow, can be summed up with a simple equation: r>g. That is, the return on capital (r) outpaces the growth rate of the economy (g) over time, leading inexorably to the dominance of inherited wealth. Progressives such as Princeton economist Paul Krugman seized on Mr. Piketty’s thesis to justify policies they have long wanted—namely, very high taxes on the wealthy.

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…

Well, duh!