When Standard & Poor's reduced the nation's credit rating from AAA to AA-plus, the United States suffered the first downgrade to its credit rating ever. S&P took this action despite the plan Congress passed this past week to raise the debt limit.

The downgrade, S&P said, "reflects our opinion that the fiscal consolidation plan that Congress and the administration recently agreed to falls short of what, in our view, would be necessary to stabilize the government's medium-term debt dynamics."

It's those medium- and long-term debt problems that also worry economics professor Laurence J. Kotlikoff, who served as a senior economist on President Reagan's Council of Economic Advisers. He says the national debt, which the U.S. Treasury has accounted at about $14 trillion, is just the tip of the iceberg.

"We have all these unofficial debts that are massive compared to the official debt," Kotlikoff tells David Greene, guest host of weekends on All Things Considered. "We're focused just on the official debt, so we're trying to balance the wrong books."

Kotlikoff explains that America's "unofficial" payment obligations — like Social Security, Medicare and Medicaid benefits — jack up the debt figure substantially.

"If you add up all the promises that have been made for spending obligations, including defense expenditures, and you subtract all the taxes that we expect to collect, the difference is $211 trillion. That's the fiscal gap," he says. "That's our true indebtedness."

We don't hear more about this enormous number, Kotlikoff says, because politicians have chosen their language carefully to keep most of the problem off the books.