By Carmen M. Reinhart and Kenneth S. Rogoff

(Bloomberg News) As public debt in advanced countries reaches levels not seen since the end of World War II, there is considerable debate about the urgency of taming deficits with the aim of stabilizing and ultimately reducing debt as a percentage of gross domestic product.

Our empirical research on the history of financial crises and the relationship between growth and public liabilities supports the view that current debt trajectories are a risk to long-term growth and stability, with many advanced economies already reaching or exceeding the important marker of 90 percent of GDP. Nevertheless, many prominent public intellectuals continue to argue that debt phobia is wildly overblown. Countries such as the U.S., Japan and the U.K. aren't like Greece, nor does the market treat them as such.

Indeed, there is a growing perception that today's low interest rates for the debt of advanced economies offer a compelling reason to begin another round of massive fiscal stimulus. If Asian nations are spinning off huge excess savings partly as a byproduct of measures that effectively force low- income savers to put their money in bank accounts with low government-imposed interest-rate ceilings -- why not take advantage of the cheap money?

Although we agree that governments must exercise caution in gradually reducing crisis-response spending, we think it would be folly to take comfort in today's low borrowing costs, much less to interpret them as an "all clear" signal for a further explosion of debt.

Changing Interest Rates

Several studies of financial crises show that interest rates seldom indicate problems long in advance. In fact, we should probably be particularly concerned today because a growing share of advanced country debt is held by official creditors whose current willingness to forego short-term returns doesn't guarantee there will be a captive audience for debt in perpetuity.

Those who would point to low servicing costs should remember that market interest rates can change like the weather. Debt levels, by contrast, can't be brought down quickly. Even though politicians everywhere like to argue that their country will expand its way out of debt, our historical research suggests that growth alone is rarely enough to achieve that with the debt levels we are experiencing today.

While we expect to see more than one member of the Organization for Economic Cooperation and Development default or restructure their debt before the European crisis is resolved, that isn't the greatest threat to most advanced economies. The biggest risk is that debt will accumulate until the overhang weighs on growth.

Historical Precedents

At what point does indebtedness become a problem? In our study "Growth in a Time of Debt," we found relatively little association between public liabilities and growth for debt levels of less than 90 percent of GDP. But burdens above 90 percent are associated with 1 percent lower median growth. Our results are based on a data set of public debt covering 44 countries for up to 200 years. The annual data set incorporates more than 3,700 observations spanning a wide range of political and historical circumstances, legal structures and monetary regimes.