The Washington Post fills in more details on how private accounts are not a free lunch. Note: The Post has corrected the story linked above, so I deleted the quote from the original version that was here before and added the new one below, which explains how the “benefit offset” works. Somehow, this didn’t make it into the State of the Union…

[U]nder the proposal, workers who opt to invest in the new private accounts would lose a proportionate share of their guaranteed payment from Social Security plus interest. They should be able to recoup those lost benefits through their private accounts, as long as their investments realize a return greater than the 3 percent that the money would have made if it had stayed in the traditional plan.

That 3 percent level is the interest rate earned by Treasury bonds currently held by the Social Security system.

The Post mistakenly reported that the balance of a worker’s personal account would be reduced by the worker’s total annual contributions, plus 3 percent interest. In fact, the balance in the account would belong to the worker upon retirement, according to White House officials.

…What Bush did not detail is how contributions in the account would reduce workers’ monthly Social Security checks. Under the system, described by an administration official, every dollar contributed to an account would be taken from the guaranteed Social Security benefit, with interest.

“The person comes out ahead if their personal account exceeds a 3 percent real rate of return, which is the rate of return that the trust fund bonds receive,” the senior administration official said. “So, basically, the net effect on an individual’s benefits would be zero if his personal account earned a 3 percent real rate of return. To the extent that his personal account gets a higher rate of return, his net benefit would increase.”

If a worker sets aside $1,000 a year for 40 years, and earns 4 percent annually on investments, the account would grow to $99,800 in today’s dollars. All of that money would be the worker’s upon retirement. But guaranteed benefits over the worker’s lifetime would be reduced by approximately $78,700 — the amount the worker would have contributed to Social Security but instead contributed to his private account, plus 3 percent interest above inflation. The remainder, $21,100, would be the increase in benefit the worker would receive over his lifetime above the level he would have received if he stayed in the traditional system.

Under the system, total benefit gains may be minimal. The Social Security Administration, in projecting benefits under a partially privatized system, assumes a 4.6 percent rate of return over inflation. Thus gains in an account would be offset by a reduction in guaranteed benefits equal to 70 percent of the account’s balance.

The Congressional Budget Office, Capitol Hill’s official scorekeeper, assumes a 3.3 percent rate of return. Under that scenario, the full amount in a worker’s account would be reduced dollar for dollar from his Social Security checks, for a net gain of virtually zero.

If investments earned less than 3 percent a year above inflation, a worker would do worse in total benefits than he would have done in the traditional system.