Brad DeLong, the blogging economics professor who specializes in this subject, ran the numbers. "The safest long-term investment the U.S. Treasury offers is the 20-year, inflation-protected TIP. ... What Bush is not telling you is that, under the Bush plan, if you divert $1,000 from your Social Security to private accounts, that amount is clawed back -- charged to an account associated with your normal Social Security benefit, that amount is then compounded at 3 percent per year plus the rate of inflation, and then after you retire, deducted over time from you normal Social Security benefit.

"If you are 45 and if Bush's plan were available today ... follow George W. Bush's advice, divert $1,000 into your private account, invest it in TIPS, and at the 1.85 percent per year interest rate you will indeed be able to collect an extra amount worth $10.11 a month in today's dollars when you retire at 65. ...

"But the clawback would reduce your normal Social Security benefit by $14.16 a month. You're $4.05 a month behind."

That's why privatizers never mention the clawback.

Basically, you have to beat 3 percent plus inflation to come out ahead, and the only way to do that is to gamble in the stock market.