The French economist Thomas Piketty's book, "Capital in the 21st Century," has caused quite a firestorm in the U.S. Paul Krugman, writing in the New York Review of Books, described it as "a call to arms" for those who wish "to restrain the growing power of inherited wealth." Conservative commentators have fretted about Mr. Piketty's "soft Marxism" (in the words of James Pethokoukis of the American Enterprise Institute) and the obvious allusion, in the book's title, to Marx's own "Das Kapital."

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