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What is so bad about the RBC model?

Author(s): Simon Wren-Lewis

From Mainly Macro:

DSGE models, the models that mainstream macroeconomists use to model the business cycle, are built on the foundations of the Real Business Cycle (RBC) model. We (almost) all know that the RBC project failed. So how can anything built on these foundations be acceptable? As Donald Trump might say, what is going on here?

The basic RBC model contains a production function relating output to capital (owned by individuals) and labour plus a stochastic element representing technical progress, an identity relating investment and capital, a national income identity giving output as the sum of consumption and investment, marginal productivity conditions (from profit maximisation by perfectly competitive representative firms) giving the real wage and real interest rate, and the representative consumer’s optimisation problem for consumption, labour supply and capital. (See here, for example.)

What is the really big problem with this model? Not problems along the lines of ‘I would want to add this’, but more problems like I would not even start from here. Let’s ignore capital, because in the bare bones New Keynesian model capital does not appear. If you were to say giving primacy to shocks to technical progress I would agree that is a big problem: all the behavioural equations should contain stochastic elements which can also shock this economy, but New Keynesian models do this to varying degrees. If you were to say the assumption of labour market clearing I would also agree that is a big problem.

However none of the above is the biggest problem in my view. The biggest problem is the assumption of continuous goods market clearing aka fully flexible prices. That is the assumption that tells you monetary policy has no impact on real variables. Now an RBC modeller might say in response how do you know that? Surely it makes sense to see whether a model that does assume price flexibility could generate something like business cycles?

The answer to that question is no, it does not. It does not because we know it cannot for a simple reason: unemployment in recessions is involuntary, and this model cannot generate involuntary unemployment, but only voluntary variations in labour supply as a result of short term movements in the real wage. Once you accept that higher unemployment in recessions is involuntary (and the evidence for that is very strong), the RBC project was never going to work.

So how did RBC models ever get off the ground? Because the New Classical revolution said everything we knew before that revolution should be discounted because it did not use the right methodology. And also because the right methodology - the microfoundations methodology - allowed the researcher to select what evidence (micro or macro) was admissible. That, in turn, is why the microfoundations methodology has to be central to any critique of modern macro. Why RBC modellers chose to dismiss the evidence on involuntary unemployment I will leave as an exercise for the reader.

The New Keynesian (NK) model, although it may have just added one equation to the RBC model, did something which corrected its central failure: the failure to acknowledge the pre-revolution wisdom about what causes business cycles and what you had to do to combat them. In that sense its break from its RBC heritage was profound. Is New Keynesian analysis still hampered by its RBC parentage? The answer is complex (see here), but can be summarised as no and yes. But once again, I would argue that what holds back modern macro much more is its reliance on its particular methodology.


One final point. Many people outside mainstream macro feel happy to describe DSGE modelling as a degenerative research strategy. I think that is a very difficult claim to substantiate, and is hardly going to convince mainstream macroeconomists. The claim I want to makeis much weaker, and that is that there is no good reason why microfoundations modelling should be the only research strategy employed by academic economists. I challenge anyone to argue against my claim. 

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