I disagree 0.1% with Wren-Lewis (I object to his use of the word "need") and about 0.01% with Kling who suggests a claim about what was necessary without quite making it.
Now I cut and paste an over long comment with which I polluted Kling's comment thread. I do not recommend reading the same old same old. I seriously considered posting this to my hard drive (but not my trash can -- I'm a word horder).
Thanks for the link. I think that you and Wren-Lewis agree 100%.
The 0.1% is that I think Friedman's story (in which backward looking expectations were pretty explicit) is sufficient to explain stagflation in the USA in the 1970 but not necessary. I think the pre-Friedman and contra Friedman Keynesians could fit the facts too.
[this "comment" has become much too long for a comments thread. The rest is over here]
My now old tired argument is that a model in which expected inflation is a constant plus 0.5 times lagged inflation fits the 1970s US data fine, but implies a long term inflation unemployment tradeoff and does not imply acceleration.
The model is obviously not the truth and not just because all models are false by definition. We can be sure it isn't a true claim about expectations by introspection, that is a thought experiment. No one can believe that after a thousand years of exactly 10% inflation year after year, expected future inflation will be 5%. I know of no evidence that anyone ever believed any such thing (although I'm sure someone somewhere did, because tinfoil hats).
It was obvious to prominent Keynesians (I am thinking of Solow and Tobin) that the one lag autoregressive expectations model wasn't the truth. It is also obvious that in around 1970 and 1971, they thought something like it was an approximation useful to US policy makers. This view seems to me to be supported by US aggregate data through 1980 (and through 2014). I think that Friedman's position is that Keynesians should not be allowed to use friedman's methodology of positive economics and their claim that an equation was useful there and then must be interpreted as a claim that it is a universal truth. If that was his view, he was much more generoust to Solow Samuelson and Tobin than well to get personal I ever was in the 20th century (and at least the first 10 years of the 21st). I believed that they believed in a Phillips curve with no expectations term at all. I was clearly totally wrong.
As I note from time to time to time to time, it was standard to include price inflation in estimates of the Phillips relation from say Phillips's second paper on the topic on. There was a debate which can be translated into contemporary econospeak as "in around 1970, Solow believed that US inflation expectations (unlike Latin American inflation expectations) were anchored."
This doesn't mean that he predicted that they would remain anchored. Like Keynes, he clearly and definitely said that the relationship between inflation and real variables was not stable and that they decoupled given high inflation.
Solow's position was very explicitly that inflation expectations are sometimes anchored and sometimes not. So high or steady inflation is eventually reflected one for one in expectations, but low and variable inflation isn't (so the average forecast error can increase in the average inflation rate). I think that this is very devinitely the view currently expressed by, among others, Ben Bernanke, Janet Yellen and Narayana Kocherlakota. I also think it is consistent with the evidence. In any case, it is the current view of many top status economists, who remember the 1970s as we do, and is the standard view among monetary policy makers.
For example, I just noticed this
In this Economic Letter, we focus on two simple extensions that are potentially important to the current inflation outlook.note especially "Cogley, Timothy, Giorgio E. Primiceri, and Thomas J. Sargent. 2010. “Inflation-Gap Persistence in the U.S.” American Economic Journal: Macroeconomics 2(1), pp. 43–69 (which I haven't read).
The first extension incorporates anchored inflation expectations with the constraint that long-run inflation eventually returns to the Fed’s inflation target of 2% (see Williams 2006, Stock and Watson 2010, and Cogley, Primiceri, and Sargent 2010).
The semi new point I would like to make here is that Solow's position is entirely consistent with the argument Friedmans AEA presidential address. Friedman discussed only the case of a FOMC which set a very low unemployment target (3% IIRC). If one accepts his assertion that this would lead to accelerating inflation (as I do) nothing much follows. It doesn't follow that the Fed can't stabilize. It doesn't follow that the Fed can't cause long term average unemployment to be lower or higher (within limits).
It is an example which makes it clear that belief in an expectations unaugmented Phillips curve is unreasonable. Almost nobody ever believed in an expectations unaugmented Phillips curve. Samuelson and Solow definitely did not. Hicks made an argument almost identical to Friedman's in 1967. http://www.economics.ox.ac.uk/materials/working_papers/paper399.pdf
I think the key event was the spread of the strange belief that Samuelson, Solow et al believed in an expectations unaugmented Phillips curve. It was believed by, well for example, Robert Waldmann that looking at the data they saw a pattern and just decided to assume it was a structural relationship. I think that this strange delusion (about what Samuelson and Solow thought) was extremely influential exactly because they were famously brilliant economists. Also they and especially Samuelson were top figures in the formalization of economic theory, so a perceived error by Samuelson due to insufficient respect for theory was shocking. The take home lesson was that you better not trust data without formal theory -- that looking at data and thinking in English could lead very smart people to think very stupid things.
I don't know exactly when or how the strange delusion about old Keynsians began. It is certainly expressed in Friedman's Nobel lecture (in which he doesn't name his straw Keynesians).
Tnis all means I agree with Krugman about the role of Friedman Phelps and stagflation in causing the rational expectations revolution. However I also think that incorrect beliefs about what people who disagreed with Friedman said were crucial too.