The point of Arjun's and my paper on debt dynamics was to show that for household debt, borrowing and changes in debt don't line up well at all. While some periods of rising household leverage -- like the housing bubble of the 2000s -- were also periods of high household borrowing, only a small part of longer-term changes in household debt can be explained this way. This is because interest, income growth and inflation rates also affect debt-income ratios, and movements in these other variables often swamp any change in household borrowing.
As far as I know, we were the first people to make this argument in a systematic way for household debt. For government debt, it's a bit better known -- but only a bit. People like Willem Buiter or Jamie Galbraith do point out that the fall in US debt after World War II had much more to do with growth and inflation than with large primary surpluses. You can find the argument more fully developed for the US in papers by Hall and Sargent or Aizenman and Marion, and for a large sample of countries by Abbas et al., which I've discussed here before. But while many of the people making it are hardly marginal, the point that government borrowing and government debt are not equivalent, or even always closely linked, hasn't really made it into the larger conversation. It's still common to find even very smart people saying things like this:
We didn’t have anything you could call a deficit problem until 1980. We then saw rising debt under Reagan-Bush; falling debt under Clinton; rising under Bush II; and a sharp rise in the aftermath of the financial crisis. This is not a bipartisan problem of runaway deficits!
Note how the terms "deficits" and "rising debt" are used interchangeably; and though the text mostly says deficits, the chart next to this passage shows the ratio of debt to GDP.
What we have here is a kind of morality tale where responsible policy -- keeping government spending in line with revenues -- is rewarded with falling debt; while irresponsible policy -- deficits! -- gets its just desserts in the form of rising debt ratios. It's a seductive story, in part because it does have an element of truth. But it's mostly false, and misleading. More precisely, it's about one quarter true and three quarters false.
Here's the same graph of federal debt since World War II, showing the annual change in debt ratio (red bars) and the primary deficit (black bars), both measured as a fraction of GDP. (The primary deficit is the difference between spending other than interest payments and revenue; it's the standard measure of the difference between current expenditure and current revenue.) So what do we see?
It is true that the federal government mostly ran primary surpluses from the end of the war until 1980, and more generally, that periods of surpluses were mostly periods of rising debt, and conversely. So it might seem that using "deficits" and "rising debt" interchangeably, while not strictly correct, doesn't distort the picture in any major way. But it does! Look more carefully at the 1970s and 1980s -- the black bars look very similar, don't they? In fact, deficits under Reagan were hardy larger than under Ford and Carter -- a cumulative 6.2 percent of GDP over 1982-1986, compared with 5.6 percent of GDP over 1975-1978. Yet the debt-GDP ratio rose by just a single point (from 24 to 25) in the first episode, but by 8 points (from 32 to 40) in the second. Why did debt increase in the 1980s but not in the 1970s? Because in the 1980s the interest rate on federal debt was well above the economy's growth rate, while in the 1970s, it was well below it. In that precise sense, if debt is a problem it very much is a bipartisan one; Volcker was the appointee of both Carter and Reagan.
Here's the same data by decades, and for the pre- and post-1980 periods and some politically salient subperiods. The third column shows the part of debt changes not explained by the primary balance. This corresponds to what Arjun and I call "Fisher dynamics" -- the contribution of growth, inflation and interest rates to changes in leverage. [*] The units are percent of GDP.
|Totals by Decade|
|Primary Deficit||Change in Debt||Residual Debt Change|
|Primary Deficit||Change in Debt||Residual Debt Change|
Over the longer run, it is true that there is a shift from primary surpluses before 1980 to primary deficits afterward. (This is different from our finding for households, where borrowing actually fell after 1980.) But the change in fiscal balances is less than 25 percent the change in debt growth. In other words, the shift toward deficit spending, while real, only accounts for a quarter of the change in the trajectory of the federal debt. This is why I said above that the morality-tale version of the rising debt story is a quarter right and three quarters wrong.
By the way, this is strikingly consistent with the results of the big IMF study on the evolution of government debt ratios around the world. Looking at 60 episodes of large increases in debt-GDP ratios over the 20th century, they find that only about a third of the average increase is accounted for by primary deficits.  For episodes of falling debt, the role of primary surpluses is somewhat larger, especially in Europe, but if we focus on the postwar decades specifically then, again, primary surpluses accounted for only a about a third of the average fall. So while the link between government debt and deficits has been a bit weaker in the US than elsewhere, it's quite weak in general.
So. Why should we care?
Most obviously, you should care if you're worried about government debt. Now maybe you shouldn't worry. But if you do think debt is a problem, then you are looking in the wrong place if you think holding down government borrowing is the solution. What matters is holding down i - (g + π) -- that is, keeping interest rates low relative to growth and inflation. And while higher growth may not be within reach of policy, higher inflation and lower interest rates certainly are.
Even if you insist on worrying not just about government debt but about government borrowing, it's important to note that the cumulative deficits of 2009-2011, at 22 percent of GDP, were exactly equal to the cumulative surpluses over the Clinton years, and only slightly smaller than the cumulative primary surpluses over the whole period 1947-1979. So if for whatever reason you want to keep borrowing down, policies to avoid deep recessions are more important than policies to control spending and raise revenue.
More broadly, I keep harping on this because I think the assumption that the path of government debt is the result of government borrowing choices, is symptomatic of a larger failure to think clearly about this stuff. Most practically, the idea that the long-run "sustainability" of the debt requires efforts to control government borrowing -- an idea which goes unquestioned even at the far liberal-Keynesian end of the policy spectrum -- is a serious fetter on proposals for more stimulus in the short run, and is a convenient justification for all sorts of appalling ideas. And in general, I just reject the whole idea of responsibility. It's ideology in the strict sense -- treating the conditions of existence of the dominant class as if they were natural law. Keynes was right to see this tendency to view of all of life through a financial lens -- to see saving and accumulating as the highest goals in life, to think we should forego real goods to improve our financial position -- as "one of those semicriminal, semi-pathological propensities which one hands over with a shudder to the specialists in mental disease."
On a methodological level, I see reframing the question of the evolution of debt in terms of the independent contributions of primary deficits, growth, inflation and interest rates as part of a larger effort to think about the economy in historical, dynamic terms, rather than in terms of equilibrium. But we'll save that thought for another time.
The important point is that, historically, changes in government borrowing have not been the main factor in the evolution of debt-GDP ratios. Acknowledging that fact should be the price of admission to any serious discussion of fiscal policy.
 Strictly speaking, debt ratios can change for reasons other than either the primary balance or Fisher dynamics, such as defaults or the effects of exchange rate movements on foreign-currency-denominated debt. But none of these apply to the postwar US.
 The picture is a bit different from the US, since adverse exchange-rate movements are quite important in many of these episodes. But it remains true that high deficits are the main factor in only a minority of large increases in debt-GDP ratios.