To believe in a privatization-friendly rate of return, you have to believe that half a century from now, the average stock will be priced like technology stocks at the height of the Internet bubble - and that stock prices will nonetheless keep on rising.
Social Security privatizers usually defend their bullishness by saying that stock investors earned high returns in the past. But stocks are much more expensive than they used to be, relative to corporate profits; that means lower dividends per dollar of share value. And economic growth is expected to be slower.
Which brings us to the privatizers' Catch-22.
They can rescue their happy vision for stock returns by claiming that the Social Security actuaries are vastly underestimating future economic growth. But in that case, we don't need to worry about Social Security's future: if the economy grows fast enough to generate a rate of return that makes privatization work, it will also yield a bonanza of payroll tax revenue that will keep the current system sound for generations to come.
Alternatively, privatizers can unhappily admit that future stock returns will be much lower than they have been claiming. But without those high returns, the arithmetic of their schemes collapses.
It really is that stark: any growth projection that would permit the stock returns the privatizers need to make their schemes work would put Social Security solidly in the black.
And I suspect that at least some privatizers know that. Mr. Baker has devised a test he calls 'no economist left behind': he challenges economists to make a projection of economic growth, dividends and capital gains that will yield a 6.5 percent rate of return over 75 years. Not one economist who supports privatization has been willing to take the test.

But the offer still stands. Ladies and gentlemen, would you care to explain your position?