There is a superficial analogy between pyramid or Ponzi schemes and pay-as-you-go insurance 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. 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.

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. 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.

—U.S. Social Security Administration