ISLM, ISMP, DSGE and other models

From the Google Ngram Viewer.

Note that even though the ISLM is from 1937 (and yes it is in the GT, and Keynes did endorse Hicks formalization, as discussed here, and here), it is only in the 1970s that the term takes off. The ISMP, which substitutes a monetary policy rule for the LM, and includes as a result endogenous money into mainstream models (note again endogenous money is not central for heterodox models, since orthodox ones can incorporate it, as discussed here) takes over in the 2000s, as does the Dynamic Stochastic General Equilibrium models, in which the IS with a multiplier is substituted by a Ramsey model. So, given the trends, you should miss the good old ISLM indeed.

Lars P. Syll On What’s wrong with IS-LM?

By Lars. P. Syll

Yesterday, David Fields of Naked Keynesianism wondered what was my position on the fact that many heterodox economists would consider the IS-LM framework “to still be relevant if given enough flexibility without neoclassical synthesized elements.”

I will sure come back on this when time admits a more thorough analysis, but let me start by giving at least a tentative answer — focusing on where I think IS-LM doesn’t adequately reflect the width and depth of Keynes’s insights on the workings of modern market economies.

Read the rest here.

ISLM: a further explanation and a defense

I noted before  the traditional representation of the ISLM is problematic. Yet as I also noted the ISLM model can accommodate changes that incorporate the criticisms of classical-Keynesian, post-Keynesian and other heterodox groups. There is no need for an investment function based on the marginal productivity of capital and the principle of substitution. The accelerator can be incorporated, and the inverse relation with the rate of interest would result from the effects of interest rates on other components of demand. Also, endogenous money can be incorporated easily, and for the most part this has been done in New Keynesian models (the ISMP).

In the post (linked by David here) that prompted this sort of defense of a changed ISLM, Lars Syll correctly notes that New Keynesians are often right on policy, but incorrect on theory. And I for the most part agree with Lars intentions. Yet, he suggests that the problem lies in that:

"If macroeconomic models – no matter of what ilk – assume representative actors, rational expectations, market clearing and equilibrium, and we know that real people and markets cannot be expected to obey these assumptions, the warrants for supposing that conclusions or hypothesis of causally relevant mechanisms or regularities can be bridged, are obviously non-justifiable."

As I noted in my debate with Noah Smith, the problem with marginalism (neoclassical economics) is NOT rationality, utility maximization or supply and demand (not quite the same list raised by Lars). Here I would add that although one can certainly add heterogenous agents, assumptions that simplify and assume representative agents maximizing profits, for example, are not really problematic at all. Classical political economists and Marx did assume something like that and still did not reach the conclusion that the system was efficient in the sense of providing full utilization of resources.

Also, the idea that agents use all information per se is not necessarily bad (Tom Palley favors some sort of rational expectations, which he refers to as model consistent; see his old manual here). The problem is that the model used, by New Classical and other mainstream authors, has logical problems. Last but not least equilibrium per se is not a bad concept (on this there is the whole thing that Post Keynesians have inherited from Joan Robinson that makes things confusing for many heterodox economists). Equilibrium is actually quite essential for long-term analysis. And I would actually argue that it is relevant since it DOES have real world applications. In other words, real economies do fluctuate around long-term equilibrium positions that are sub-optimal.

The problem with mainstream theory is the notion of a natural rate, which is based on the principle of substitution which allows for 'factors of production' to be fully utilized. These are the problems that Keynes, by negating the idea of a natural rate, and Sraffa, by showing the logical problems of the principle of substitution, undermined. An ISLM without the natural rate is not only possible, but actually reasonably good as a tool for analyzing real economies.

PS: Note that Keynes wrote to Hicks on the ISLM that: "I found it very interesting and really have next to nothing to say by way of criticism." Keynes did not criticize the investment function in Hicks model, but note that this problem also was integral to the General Theory (GT). And yes Keynes was being nice, but he was nice too about Harrod's review of the GT, but did tell him that he did not mention effective demand.

Teaching macro after the crisis

Wendy Carlin and David Soskice have an interesting post at Voxeu.org on "How should macroeconomics be taught to undergraduates in the post-crisis era." They explicitly follow, as in their manual, a New Keynesian framework. Which is an ISMP model with price rigidities, which are incorporated in a slow adjusting Phillips curve with imperfections (they refer to it as the ISPCMR model). Also, they add the financial system. I should make clear that in many respects it is an improvement on some of the newer manuals around, which since the 1990s have given less attention to the possibilities of crises, and have emphasized long term growth (presenting the Solow model in the initial chapters).

From a methodological point of view I would also agree with the notion that "undergraduates need a unified integrated model through which they can understand the major business cycle events of the past century and see how economic theories and policy regimes have evolved in response to these events." Mind you I would go further, it's not just the undergrads, but the profession that needs a unified integrated model to understand the functioning of the macroeconomy. The ad hoc nature of much of the macroeconomic field has been caused by the wrongheaded search for neoclassical microfoundations for macroeconomics. And the authors should be more forceful in denying the relevance of microfoundations.

Having said that, there are significant problems with their way of teaching macro, from my point of view. Carlin and Soskice suggest that "the most glaring absence in macroeconomic models and courses, [is] that of the financial sector." Mind you I would disagree, even if the financial sector is often an afterthought in most macro discussions for undergrads. The most glaring problem is the presence of a natural rate, which implies that in the absence of imperfections markets would produce full utilization of resources. Note that, in their model, Carlin and Soskice have a level of the rate of interest that equilibrates the goods market (IS) with stable inflation, and note that this would be for Woodford "the Wicksellian or natural rate of interest."

They think that they avoid the problem because in their model the equilibrium rate "changes whenever the IS curve shifts," which is of course a confusion, since that would be true of the Wicksellian rate of interest too. With changes in investment and savings (IS) behavior, that is, productivity and thrift the neoclassical natural rate must shift. The role of the central bank is to adjust the rate of interest to its natural level. Note that Carlin and Soskice confuse the fact that monetary and real shocks affect the economy with the notion that the economy described by their model is "an economy which is not self-stabilising."

In their view, the monetary authority can move the economy to the equilibrium rate, but the equilibrium rate is not controlled by the authorities. I would suggest that the central bank actually has control of the key variable, which is not natural at all, and as Keynes suggested is conventional, the normal rate of interest. And that could be set at a level which will not lead to full employment, particularly for distributive reasons. In my view the essential feature of a more relevant macro model would be the elimination of the natural rate, the idea that the system has a tendency to bounce back to the trend, and the notion still in the Carlin and Soskice model that the output gap, the deviation from optimal output, has significant impact on inflation.

Further, in order to introduce the effects of the financial system, while it is welcome that the behavior of banking is introduced, the most important development of the last three decades, the increased financialization, and the shadow banking system, would be better incorporated by introducing the effects of debt and income distribution on the consumption function, which remains based on the permanent income in their model. And by the way, the model says nothing about how income distribution affects spending.Yet, the main reason behind the crisis has been the fact that workers had to get indebted, since income has stagnated. So income distribution has to be brought back to macroeconomics. Not enough Kalecki, with their New (but still neoclassical) version of Keynesianism, that is the problem with Carlin and Soskice.

ISLM: what is it good for?

The ISLM model persists in most undergraduate textbooks, and even though it has vanished from graduate courses*, it remains central to the way economists think about policy issues.  The main reason it survives is not so much that it allows a relatively simple intro to policy issues to undergrads, as some argue, but the fact that it captures the interaction between real and monetary phenomena which was central for Keynes’ General Theory (still worth reading), and is flexible enough to accommodate divergent views, often reflected in different inclinations of the curves.

It is also true that the conventional interpretation of the ISLM is fraught with problems.  But the ISLM model is incredibly flexible and can accommodate most critiques.  For example, a few years back a reader asked Mankiw why he still taught that the Fed controls money supply, rather than control the rate of interest, which would be more relevant.  His reply was not particularly good (just an ad for his book), but the fact remains that it is easy to change the LM to reflect the fact that central banks control the rate of interest.  The ISLM with a horizontal LM is sometimes referred to as the ISMP, where MP stands for monetary policy rule, and for the most part this is done for the benefit of students (i.e. you can buy the new text with the updated ISMP model for $150!).

The ISLM can also accommodate an investment function in which the level of activity, rather than the rate of interest, is central, which is more empirically accurate, and different consumption functions in which other elements besides disposable income appear.  More importantly the ISLM does not imply a natural rate of unemployment, which often appears in the supply side part of the macro course.  In other words, the ISLM allows for relevant discussion of policy issues, and clarification of policy differences.  Besides, compare this model with the consumption theory you get in micro, and this is still one of the most relevant things you can learn in economics.  The other would be not to trust economists!

* David Colander suggests that it’s not taught in graduate courses because more sophisticated dynamic stochastic general equilibrium models (DSGE) are more important.  DSGE models are based on the aggregation of individual maximizing behavior, and have been less relevant for policy analysis than old fashion macroeconometric models based on the old Keynesian theory.  The whole conference on the ISLM, in which you can find Colander’s paper is available here

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