Mark Thoma sends us to:
The Logic of Pay-As-You-Go Systems In contrast to a Ponzi scheme, dependent upon an unsustainable progression, a common financial arrangement is the so-called "pay-as-you-go" system. Some private pension systems, as well as Social Security, have used this design. A pay-as-you-go system can be visualized as a pipeline, with money from current contributors coming in the front end and money to current beneficiaries paid out the back end.
There is a superficial analogy between pyramid or Ponzi schemes and pay-as-you-go programs in that in both money from later participants goes to pay the benefits of earlier participants. But that is where the similarity ends. A pay-as-you-go system can be visualized as a simple pipeline, with money from current contributors coming in the front end and money to current beneficiaries paid out the back end.
So we could image that at any given time there might be, say, 40 million people receiving benefits at the back end of the pipeline; and as long as we had 40 million people paying taxes in the front end of the pipe, the program could be sustained forever. It does not require a doubling of participants every time a payment is made to a current beneficiary, or a geometric increase in the number of participants. (There does not have to be precisely the same number of workers and beneficiaries at a given time--there just needs to be a fairly stable relationship between the two.) As long as the amount of money coming in the front end of the pipe maintains a rough balance with the money paid out, the system can continue forever. There is no unsustainable progression driving the mechanism of a pay-as-you-go pension system and so it is not a pyramid or Ponzi scheme...