When banks become members of the Federal Reserve System, which is mandatory for national banks and voluntary for state banks, they have to purchase stock in their local Federal Reserve Bank equal to six percent of their own capital and surplus.
Surplus countries have little choice but to place most of their spare funds in the reserve currency since it is used to settle trade and has the most liquid bond market.
Even Alan Greenspan, the cautious chairman of the Federal Reserve, had started to fret about what the government would do with all the surplus revenues filling its coffers.