Oddly, Herndon and his colleagues do not mention another data omission. This one was intentional on our part. Back in 2010, we were still sorting inconsistencies in Spanish G.D.P. data from the 1960s from three different sources. Our primary source for real G.D.P. growth was the work of the economic historian Angus Madison. But we also checked his data and, where inconsistencies appeared, refrained from using it. Other sources, including the I.M.F. and Spain’s monumental and scholarly historical statistics, had very different numbers. In our 2010 paper, we omitted Spain for the 1960s entirely. Had we included these observations, it would have strengthened our results, since Spain had very low public debt in the 1960s (under 30 percent of G.D.P.), and yet enjoyed very fast average G.D.P. growth (over 6 percent) over that period. We later reconciled this problem for our 2012 paper. This is just an example of what our archival research involves; it is not simply a matter of filling in cells on an Excel spreadsheet from sanitized, easy-to-use databases.

We conclude with a few thoughts to supplement our broader discussion of the issues in our Op-Ed piece. First, we reiterate that the frontier question for research is the issue of causality. Clearly, recessions can cause higher debt, and in some extreme cases drive debt to over 90 percent, though such extreme jumps are rare outside of a financial crisis. We ourselves, in our 2009 book, showed that for postwar systemic financial crises, the average rise in the debt-to-G.D.P. ratio after three years is 86 percent. But in our Journal of Economic Perspectives paper, we show that the duration of high-debt episodes (debt over 90 percent of G.D.P.) is very long indeed. The paper contains a case-by-case description of each debt overhang episode in advanced economies since 1800. As we note in our essay for The Times, the long duration of the overhangs, averaging 23 years, makes it hard to argue that they are simply the result of recessions driving up debt. We also note in that article that roughly half of all debt overhang episodes are associated with elevated real interest rates, suggesting the kind of vicious feedback loop between debt and growth that the periphery countries of the euro zone are currently suffering. In our view, the only way to break this feedback loop is to have dramatic write-downs of debt.

We also note that a little under half of all cases do not involve higher real interest rates, such as the recent Japanese experience. Our Op-Ed essay gives reasons debt might still matter, including the way in which it crowds out fiscal space and limits the economy’s capacity to respond to shocks. But the root of the problem is still probably the fact that as debt rises, so too does the risk that a turn in interest rates might suddenly take the country from a seemingly safe debt situation to an unsustainable one. The economic literature is replete with examples of this, and many forecasts suggest long-term interest rates will rise significantly over the next decade.

The basic problem for fiscal policy is that interest rates can turn very quickly but debt ratios cannot. So, most countries sensibly exercise some prudence as debt rises. Perhaps they are overly cautious. But the fact that debt levels over 90 percent of G.D.P. are rare (roughly 8 percent of postwar observation in advanced economies) and debt levels over 120 percent of G.D.P. are very rare. It is true that Japan has been an outlier since the 1990s, with gross public debt to G.D.P. exceeding 230 percent. But this ignores the fact that Japan, unlike the United States, is a creditor nation, holding massive dollar reserves that somewhat offset its debt. Until recently, it has always been running a current surplus with the rest of the world while the United States needs to borrow. Some have also used the example of Britain in the 18th century, when gross debt also exceeded 200 percent of G.D.P. Indeed, we include this and any other episode lasting longer than five years for which the data is available.

The graduate students now poring over debt data should consider using the five-year filter used in our 2012 paper. This does not turn out to exclude all that many debt overhang experiences, but it does filter out a few associated with short recessions and postwar remobilizations. The big question today is not how economies do with high debt after a war, but how to handle high debts in peacetime. After a war, when physical capital is destroyed, but human capital remains, it is often possible to rebuild faster. There are also many efficiency benefits from releasing wartime controls and bringing manpower to productive use. But the first few years of such experiences, in any event, might not necessarily capture the problem that one is interested in, of today’s peacetime deficits. Again, in our 2012 paper, we explore many reasons debt overhang might matter for growth, at least in theory. But much more needs to be understood.

We again turn readers to our Op-Ed essay to understand ideas for bringing debt down. To reiterate, there are four solutions: slow growth and austerity for a very long time, elevated inflation, financial repression and debt restructuring. We have long emphasized the need to use the whole tool kit creatively in the aftermath of a once-in-75-year financial crisis. One of us has widely discussed using financial repression as a means of dealing high debt. Even at the outset of the crisis, one of us advocated mildly high inflation. A Project Syndicate column in December 2008 advocated moderately elevated inflation as means of getting the economy moving again, in part by taking some edge off public and private debts. Bill Clinton’s 2011 book “Right to Work” cites our proposals to write down subprime mortgage debt on a large scale.

Early on in the financial crisis, in a February 2009 Op-Ed, we concluded that “authorities should be prepared to allow financial institutions to be restructured through accelerated bankruptcy, if necessary placing them under temporary receivership.”