Here’s Matt Yglesias discussing Robert Lucas:
Changing one’s mind is a great sign of discipline and rigorous thinking. But as Noah Smith says, it’s telling and remarkable that a leading practitioner in the field would twice flip-flop on the nature of the business cycle over the course of his career. First [Lucas] thinks all business cycles have the same cause and the causes are monetary. Then he’s persuaded by Kydland & Prescott that actually all business cycles are driven by real factors. But then a whole new event occurs that he thinks can’t be explained in the Kydland/Prescott framework and that causes him to revise his longstanding view that all business cycles are the same. And that has the secondary consequence of causing him to return to his initial view that the Great Depression wasn’t caused by real factors either.
Unless I’m mistaken, this statement is inaccurate. Lucas never abandoned his belief that the Great Depression was triggered by a negative monetary shock. He has shifted his views on the postwar recessions, and now believes that real shocks are the dominant factor in these smaller slumps.
The Noah Smith post that Yglesias links to quotes Lucas as saying the following:
If the Depression continues, in some respects, to defy explanation by existing economic analysis (as I believe it does), perhaps it is gradually succumbing under the Law of Large Numbers.
The “in some respects” phrase refers to the slow recovery. Lucas believes that the sharp fall in NGDP contributed to both the Great Contraction of 1929-32 and the contraction of 2008-09. He also believes that monetary factors alone cannot explain why the two recoveries took so long. I’ve argued that supply-side factors played a role in slowing the recovery of 1933-41, and I am pretty sure that Lucas agrees with that view. So I don’t see any significant changes in his views on the Great Depression.