If this theory is right, then as China's economy grows, the U.S. will be less able to handle the capital inflows that are necessary to remain the world's reserve currency. It would seem like a good idea for China to shoulder some of the burden of being the global reserve currency, just as the U.S. took over this duty from the United Kingdom a century ago. But China insists on maintaining its system of capital controls, making it hard to move money in and out of the country. That will prevent the yuan from joining or replacing the dollar in international markets. But as China further eclipses the U.S. in size, that could lead to greater instability in the international monetary system.