What might be the true costs of a default on the U.S.'s debt or the loss of its AAA credit rating?

Consider this: In 1979, when Washington was engaged in a similar fight over raising the borrowing ceiling, the U.S. went into technical default because the Treasury had trouble making timely interest payments. Academics who have studied that episode say the result was a long-term increase in interest rates of 0.6 percent.

No one knows what might happen to interest rates this time, but for the sake of argument, let's think about how a similar increase might affect Americans today.

Start with mortgages, now at some of the lowest rates in history -- about 4.5 percent for a 30-year fixed-rate loan. Suppose we end up with a rise in rates comparable to 1979. For someone borrowing $180,000, that would add about $23,000 over the life of the loan.

Overall consumer debt would become more expensive. Americans have total debt, including mortgages, credit cards, auto and student loans and other obligations of about $13 trillion. Higher rates would add about $80 billion a year in additional interest costs, or $800 billion over the next decade.

Analysts predict that higher borrowing costs for the federal government would add to the nation's deficit and lower economic growth -- bad news for an economy whose growth the Commerce Department today said had almost come to a halt.

It's not unreasonable to think, too, that any slowdown would lead to an increase in the ranks of the jobless. Economists now estimate that for each 1 percentage point decline in growth there is roughly a 0.5 percent increase in the unemployment rate. That works out to a loss of roughly 650,000 jobs.

And then there are the markets. A default or downgrade would surely depress financial markets. The Standard & Poor's 500 has declined about 2 percent this month, partly because of concerns over the stalemate in Washington. Wall Street estimates of how much the stock market might fall range from 6.3 percent to 9 percent. This is in line with the 9 percent drop the S&P 500 experienced the day the Republican-led House first rejected the planned bailout of the banking industry in the 2008 financial crisis. Bond markets might also take a hit in the event of a default or downgrade.

A market decline comparable to that of 2008 would be a blow to retirement savings. For example, American now hold about $4.5 trillion in 401(k)s and similar accounts. Say markets declined 7 percent, not an unrealistic amount in light of how destabilizing a default or downgrade might be. Such a drop implies a $310 billion loss for retirement savings accounts.

To be sure, these are back-of-the-envelope estimates and it's hard to believe that what happened a generation ago will play out in the same way today. "History does not repeat itself, but it does rhyme," Mark Twain once said. Still, there's enough data and enough history to conclude that a default or downgrade would be unpleasant, harmful to American consumers, and possibly even catastrophic for the U.S. economy.

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