As a follow-up to this:
Ponzi Schemes vs. Social Security, SSA, January 2009: ...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.
In this context, it would be most accurate to describe Social Security as a transfer payment--transferring income from the generation of workers to the generation of retirees--with the promise that when current workers retiree, there will be another generation of workers behind them who will be the source of their Social Security retirement payments. So you could say that Social Security is a transfer payment, but it is not a pyramid scheme. There is a huge difference between the two, and only a superficial similarity.
If the demographics of the population were stable, then a pay-as-you-go system would not have demographically-driven financing ups and downs and no thoughtful person would be tempted to compare it to a Ponzi arrangement. However, since population demographics tend to rise and fall, the balance in pay-as-you-go systems tends to rise and fall as well. During periods when more new participants are entering the system than are receiving benefits there tends to be a surplus in funding (as in the early years of Social Security). During periods when beneficiaries are growing faster than new entrants (as will happen when the baby boomers retire), there tends to be a deficit. This vulnerability to demographic ups and downs is one of the problems with pay-as-you-go financing. But this problem has nothing to do with Ponzi schemes, or any other fraudulent form of financing, it is simply the nature of pay-as-you-go systems. ...