Scott Sumner writes,

As you may know I view inflation as an almost worthless concept… In contrast Krugman discusses the original version of the Phillips curve…which used wage inflation instead of price inflation. Whereas price inflation is a useless concept, wage inflation is a highly useful concept.

Fine. But Krugman also draws attention to how the level of the unemployment rate affects the level of the wage inflation rate. This takes us back to the original, pre-1970 Phillips Curve, from Act I in my terminology (Act I was the Forgotten Moderation, from 1960-1969, Act II was the Great Stagflation, from 1970-1985. Act III was the Great Moderation, from 1986-2007, and Act IV is whatever you want to call what we are in now.) The Act I Phillips Curve says flat-out that (wage) inflation will be high when unemployment is low, and vice-versa.

The Phillips Curve was revised in Act II, when the specification became that the rate of wage inflation increases when the unemployment rate is above below the NAIRU and decreases when it is below above the NAIRU. In other words, it relates the change in the rate of wage inflation to the unemployment rate. At the time, cognoscenti were saying that Friedman had moved the Phillips Curve one derivative.

Some comments.

1. The Act I Phillips Curve works better over the 27-year period (Acts III and IV) that Krugman covers. Within the sample period, in 9 out of the 10 years when unemployment is near the bottom of its range (less than 5 percent), wage inflation is near the top of its range (3.5 percent or higher). In all three high-unemployment years, wage inflation is less than 2.5 percent.

2. Although the rate-of-change in wage inflation is also correlated with the unemployment rate, the relationship is not as impressive. In the late 1990s, we had the lowest unemployment rate, but wage inflation actually declined (admittedly by only a small amount). More troubling is the fact that the very high rate of unemployment in recent years produced a decline in wage inflation hardly larger than that of the much milder previous recessions.

3. The overall variation in wage inflation over the 27 years is remarkably low. It ranges from 1.5 percent to 4 percent. When there is this little variation to explain, the actual magnitude of the effect of variations in unemployment on inflation is going to be pretty small. See the post by Menzie Chinn. If you do not have any data points that include high inflation, then you cannot use the Phillips Curve to explain high inflation. Chinn argues that the relationship is nonlinear. I would say that we do not know that there exists a nonlinear relationship. What we know is that we observe a relationship that, if linear, has a shallow slope. The most we can say is that if there is a steep slope somewhere, then there is a nonlinear relationship.

4. If you had given a macroeconomist only the information that wage inflation varied between 1.5 percent and 4 percent, that macroeconomist would never have believed that such a time period included the worst unemployment performance since the Great Depression. In terms of wage inflation, the last five years look like a continuation of the Great Moderation.

Some larger points concerning market monetarism, paleo-Keynesianism, and the microfoundations debate:

5. Concerning Scott’s view of things, I have said this before: Arithmetically, nominal GDP growth equals real GDP growth plus growth in unit labor costs plus the change in the price markup. If you keep the price markup constant and hold productivity growth constant, then nominal GDP growth equals real GDP growth plus wage growth. So it is nearly an arithmetic certainty that when nominal GDP grows more slowly than wages, then real GDP declines. But to me, this says nothing about a causal relationship. You could just as easily say that a decline in real GDP causes nominal GDP to grow more slowly than wages. What you have are three endogenous variables.

Scott insists on treating nominal GDP growth as the exogenous variable controlled by the central bank. To me, that is too much of a stretch. I am not even sure that the central bank can control any of the important interest rates in the economy, much less the growth rate of nominal GDP. Yes, if they print gobs and gobs of money, then inflation will be high and variable, and so will nominal GDP growth. But otherwise, I am skeptical.

6. I view paleo-Keynesianism as being hostile to Act III macro. I share this hostility. However, right now, you have saltwater economists saying, “Freshwater economists reduce macroeconomics to a single representative agent with flexible prices solving stochastic calculus problems. Hah-hah. That is really STOOpid.”

The way I look at it, the Act III New Keynesians reduced macroeconomics to a single representative agent with sticky prices solving stochastic calculus problems. They should not be so proud of themselves.

Paul Krugman calls Act III macro a wrong turn. (Pointer from Mark Thoma.) I would not be so kind. I also would not be as kind as he is to the MIT macroeconomists who emerged in that era.

You cannot just blame Lucas and Prescott for turning macro into a useless exercise in mathematical…er…self-abuse. You have to blame Fischer and Blanchard, too. Personally, I blame them even more.

Having said all that, I do not share Krugman’s paleo-Keynesianism. Just because the Lucas critique was overblown does not mean that other critiques are not valid. I have developed other doubts about the Act I model, and these lead me to believe that PSST is at least as plausible a starting point for thinking about macro.