The idea that the government of a major advanced country would default on its debt—that is, tell lenders that it won't repay them all they're owed—was, until recently, a preposterous proposition. Argentina or Russia might stiff their creditors, but surely not the likes of the United States, Japan, or Great Britain. Well, it's still a very, very long shot, but it's no longer entirely unimaginable. Governments of rich countries are borrowing so much that it's conceivable that one day the twin assumptions underlying their burgeoning debt (that lenders will continue to lend and that governments will continue to pay) might collapse. What happens then? (Click here to follow Robert Samuelson).

The question is so unfamiliar that the past provides few clues to the future. Psychology is decisive. To take a parallel example: the dollar. The fear is that foreigners (and Americans, too) lose confidence in its value and dump it for yen, euros, gold, or oil. If too many investors do that, a self-fulfilling stampede could trigger sell-offs in U.S. stocks and bonds. People have predicted such a crisis for decades. It hasn't happened yet. The currency's decline has been orderly, because the dollar retains a bedrock confidence based on America's political stability, openness, huge wealth, and low inflation. But something could shatter that confidence, tomorrow or 10 years from tomorrow.

The same logic applies to exploding government debt. We have moved into uncharted territory and are prisoners of psychology. Consider Japan. In 2009, its budget deficit—the gap between spending and taxes—amounts to about 10 percent or more of gross domestic product (GDP). Its total government debt—the borrowing to cover all past deficits—is approaching 200 percent of GDP. That's twice the size of the economy. The mountainous debt reflects years of slow economic growth, many "stimulus" plans, an aging society, and the impact of the global recession. By 2019, the debt-to-GDP ratio could hit 300 percent, says a report from JPMorgan Chase.

No one knows how to interpret these numbers. If someone had predicted 20 years ago that Japan's government debt would rise so spectacularly, the forecast would doubtlessly have inspired alarms: "The debt will be unmanageable; Japan will pay crushing interest rates as fearful lenders demand high returns to compensate for the risk that government might default or inflate away its debt." Instead, just the opposite has happened. Japanese investors—households, banks, insurers—have absorbed 94 percent of the debt. Interest rates on 10-year Japanese government bonds (JGBs) have dropped from 7.1 percent in 1990 to 1.4 percent now.

Superficially, it's possible to explain this. Japan has ample private savings to buy bonds; slight deflation—falling prices—makes low interest rates acceptable; and investors remain confident that new and maturing debt will be financed. But the correct conclusion to draw is not that major governments (such as Japan and the United States) can easily borrow as much as they want. It is that they can easily borrow as much as they want until confidence that they can do so evaporates—and we don't know when, how, or whether that may happen.

Wealthy societies everywhere face a similar dilemma. Debt is ballooning from already-high levels. In the United States, the Congressional Budget Office reckons the Obama administration's planned budgets would increase the debt-to-GDP ratio from 41 percent in 2008 to 82 percent in 2019. Annual interest payments on the debt would rise from $170 billion in 2009 to $799 billion in 2019. But to contain deficits and debt by cutting spending or increasing taxes would involve wrenching and unpopular measures that might, perversely, weaken the economy and worsen deficits. In Japan, the existing Value Added Tax (national sales tax) of 5 percent would have to go to 12 percent, says JP Morgan, along with deep spending cuts. Against choices like that, some advanced country might decide that a partial or complete default, though dire, would be less dire economically and politically than the alternatives.

Deprived of domestic or international credit, defaulting countries in the past have suffered deep economic downturns, hyperinflation, or both. The odds may be against a wealthy society tempting that fate, but even the remote possibility underlines the precariousness and novelty of our present situation. The arguments over whether we need more stimulus (and debt) obscure the larger reality that past debt increasingly constricts governments' room for economic maneuvering.

Robert Samuelson is also the author of The Great Inflation and Its Aftermath: The Past and Future of American Affluence and Untruth: Why the Conventional Wisdom Is (Almost Always) Wrong .

© 2009