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French economist Thomas Piketty’s 700-page opus “Capital in the Twenty-First Century”—a blockbuster in Europe and the U.S. for its conclusions about rising inequality—says Japan is no exception to the rule.

Japan has long prided itself on relative social equality, and Mr. Piketty says the country, like his native France, has a much smaller gap between rich and poor than the U.S. But he believes the gap in Japan is poised to widen in coming decades.