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The Grumpy Economist: The Phillips Curve


Behold the Phillips curve, one more statistical correlation treated as an eternal verity that our inflationary era has just undermined. 

From 2007 to 2019, the standard observation was “The Phillips curve has become flat.” Large changes in unemployment correspond to very little change in inflation, or small changes in inflation correspond to huge changes in unemployment, depending on which causal (mis) reading of the correlation you choose. To the optimist, allowing a tiny bit of inflation could dramatically reduce unemployment. To the pessimist, it would take immense unemployment to do anything about inflation, should we have to.

Then came the pandemic. Unemployment shot up with no change in inflation, right on the curve. 

Then came the inflation. The Phillips curve woke up. It’s almost vertical! (The scales of the two axes are different). 

Much Fed and commentator thinking relies on the Phillips curve. It’s the central way interest rates affect inflation, in conventional thinking. High interest rates raise real interest rates lower aggregate demand cause unemployment which causes via the Phillips curve, lower inflation. 

Clearly, something is very wrong here. Maybe expectations shift. Maybe supply shocks do matter after all. Surely one should start with a serious dynamic Phillips curve, as most macro literature does. Maybe the Phillips curve is flexible up but sticky down, and the natural rate shifts around.  Maybe prices are sticky until they aren’t. As Bob Lucas showed long ago, the slope of the Phillips curve depends on the volatility of inflation. Countries with volatile inflation get no output boost from additional inflation. Thousands of epicycles can be added, and this post is a bit of an invitation to do so. Or maybe the Phillips curve was just a correlation after all, hiding a deeper reality. (My view, but for another blog post). 

In the meantime, it’s another good warning not to take statistical correlations too seriously, and certainly not as causally as we tend to do. Such as inflation will always be 2%. Such as real interest rates are on a permanent downward trend? 

This time of inflation will lead us to rewrite an awful lot of macroeconomics. 

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