Kai Pfaffenbach / Reuters In 1961, the economist Robert Mundell published a paper laying out, per the title, “A Theory of Optimum Currency Areas.” In it, he inquired about the appropriate geographic extent of a shared unit of money. Was it the world? A country? Part of a country? A border-spanning region of, say, the western parts of the United States and Canada, with a separate currency circulating in the eastern parts of the two countries?