‘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...