Ponzi is, as Ponzi does

Or maybe Paul Krugman's mama once said, but he's forgotten;

Take the common claim on the right that Social Security is a Ponzi scheme because the system has few real assets. It’s true that Social Security is mainly a system in which each generation pays for the previous generation’s retirement, in the expectation that it will receive the same treatment from the next generation. But like monetary circulation, this process can go on forever; there’s nothing unsustainable about it (yes, demography, but that’s about the levels of taxes and benefits, not the fundamental nature of the scheme). So there’s nothing Ponziesque at all.

Except that he once thought it was;

"Social Security ... does not look like a redistributionist scheme. In practice it has turned out to be strongly redistributionist, but only because of its Ponzi game aspect, in which each generation takes more out than it put in.  

 "Well, the Ponzi game will soon be over, thanks to changing demographics, so that the typical recipient henceforth will get only about as much as he or she put in (and today's young may well get less than they put in)."    -- Paul Krugman, in the Boston Review, December/January, 1996-7.