Our current social security system operates on a pay-as-you-go basis; benefits are paid almost entirely out of current revenues. As the ratio of retirees to taxpayers increases, concern about the high costs of providing benefits in a pay-as-you-go system has led economists to explore other options. One involves "prefunding," in which a person's withholdings are invested in financial instruments, such as stocks and bonds, the eventual returns from which would fund his or her retirement. The risks such a system would introduce--such as the volatility in the market prices of investment a.