The obligatory Krugman link

Here you go:

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.