(Early) Monday DeLong Smackdown: Larry Summers on how we know more than we write down in our lowbrow (or highbrow) economic models

Larry Summers: Thoughts on Delong and Krugman Blogs: “I think the issue is more on the supply side than the demand side…

…If I believed strongly in the vertical long run Phillips curve with a NAIRU around five percent and in inflation expectations responsiveness to a heated up labor market, I would see a reasonable case for the monetary tightening that has taken place. I am sure Paul and Brad are right that a desire to be ‘sound’ also influences policy.  I am not nearly as hostile to this as Paul. I think maintaining confidence is an important part of the art of policy.

A good example of where market thought is, I think, right and simple model based thought is I think dangerously wrong is Paul’s own Mundell-Fleming lecture on confidence crises in countries that have their own currencies.  Paul asserts that a damaging confidence crisis in a liquidity trap country without large foreign debts is impossible, because if one developed the currency would depreciate, generating an export surge.

Paul is certainly correct in his model, but I doubt that he is in fact. Once account is taken of the impact of a currency collapse on consumers’ real incomes, on their expectations, and especially on the risk premium associated with domestic asset values, it is easy to understand how monetary and fiscal policymakers who lose confidence and trust see their real economies deteriorate, as Olivier Blanchard and his colleagues have recently demonstrated.  Paul may be right that we have few examples of crises of this kind, but if so this is, perhaps, because central banks do not in general follow his precepts.

I do not think this is a pressing issue for the US right now. But the idea that policymakers should in general follow the model and not worry about considerations of market confidence seems to me as misguided as the view that they should be governed by market confidence to the exclusion of models.

Larry is taking the side that our economists’ models are primarily filing systems rather than truth-generating mechanisms. As I have already written twice in the past week, the question is: Are models properly idea-generating machines, in which you start from what you think is the case and use the model-building process to generate new insights? Or are models merely filing systems–ways of organizing your beliefs, and whenever you find that your model is leading you to a surprising conclusion that you find distasteful the proper response is to ignore the model, or to tweak it to make the distasteful conclusion go away?

Both can be effectively critiqued. Models-as-discovery-mechanisms suffer from the Polya-Robertson problem: It involves replacing what he calls “plausible reasoning”, where models are there to assist thinking, with what he calls “demonstrative reasoning”. in which the model itself becomes the object of analysis. The box that is the model is well described but, as Dennis Robertson warned,there is no reason to think that the box contains anything real. Models-as-filing-systems are often used like a drunk uses a lamp post: more for support than illumination.

In the real world, it is, of course, the case that models are both: both filing systems and discovery mechanisms. Coherent and productive thought is, as the late John Rawls used to say, always a process of reflective equilibrium–in which the trinity of assumptions, modes of reasoning, and conclusions are all three revised and adjusted under the requirement of coherence until a maximum level of comfort with all three is reached. The question is always one of balance.

The particular fight Larry wants to pick this weekend is over Paul Krugman’s Mundell-Fleming lecture, and Paul’s claim that a floating-rate sovereign that borrows in its own currency and is in a liquidity trap should not worry about maintaining “confidence”. Paul’s argument is that, in the model, pursuing aggressive expansionary policies will eat first to currency depreciation, then to an export boom, then to full employment, and only then will any downsides emerge. Thus the process can be stopped before it begins to generate risks. And it is only after full employment is attained that policy concern should shift to avoiding such risks.

Larry, however, says: We know things that are not in the model. Those things make Paul’s claim wrong.

My problem with Larry is that I am not sure what those things are. Paul notes that in normal times–away from the interest-rate zero lower bound–a loss of “confidence” in a country that floats its and borrows in its own currency an be contractionary:

Now,,, we can examine the effect of a loss of investor confidence… which we can view as a sudden stop… reduction in capital inflows at any given interest and exchange rate…. What would happen to a country with its own currency and a floating rate confronted with a foreign loss of confidence?… The depreciation of the currency at any given interest rate would increase net exports, and hence shift the IS curve out. This might be the end of the story. As I noted… however, the central bank might be concerned about the possible inflationary consequences of depreciation, and would therefore lean against it; in that case the MP schedule would shift up. So interest rates would rise due to rising demand for domestic goods and, possibly, tight money driven by inflation concerns. It is possible, if the latter motive is strong enough, that output could actually fall…

But, Paul says, at the zero lower bound things are different because the central bank has a cushion between what it wants the real interest rate to be and what the zero lower bound forces the real interest rate to be:

Right now the United States, the United Kingdom, and Japan are all stuck in the liquidity trap…. [with] policy… constrained by the zero lower bound…. This in turn means that any shift in the MP schedule, unless very large, won’t lead to a rise in interest rates…. All that matters is the rightward shift in the IS curve…. In short, under current conditions the much-feared loss of confidence by foreign investors would be unambiguously expansionary, raising output and employment in nations like the United States, the United Kingdom, and Japan…

It’s not clear what we know that is not in the model that would reverse this conclusion of Paul’s in the case of a floating exchange rate in a country that borrows in its own currency and happens to be in a liquidity trap. Paul notes that his conclusion goes against market wisdom:

Many people will, I know, object to this conclusion…. [It] seems very counterintuitive… and is very much at odds with what almost every policymaker and influential figure has been saying…. when I have tried laying out this argument to other economists, I have found that in general they recognize the point but argue that real-world complications mean that a sudden stop will nonetheless be contractionary even in countries with independent currencies and floating rates. Why? They offer a variety of reasons….

And Paul goes on:

The short-term/long-term interest rate distinction does not appear to offer any channel through which a nation with an independent currency can suffer a decline in output due to reduced foreign willingness to hold its debt….

Several commentators—for example, Rogoff (2013)—have suggested that a sudden stop of capital inflows provoked by concerns over sovereign debt would inevitably lead to a banking crisis…. [But] why, exactly… should [we] believe that a sudden stop leads to a banking crisis…. [It is] is only a problem for the banks if they have large liabilities denominated in foreign currency…. Inflation—and fear of inflation—is a potential channel through which sudden stops can end up being contractionary even for countries with independent currencies and domestic-currency debt…. Large foreign-currency debt can effectively undermine monetary independence, as can fears of depreciation-led inflation. However, the major nations with large debts but independent currencies don’t have large foreign-currency debt, and are currently quite remote from inflation pressure. So the crisis story remains very hard to tell.

I would say that the major nations with large debts don’t have large foreign-currency debt that we know of…

January 1, 2016


Brad DeLong
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