Real output per worker, 1921-1939:
The Depression didn’t just see a fall in employment, it saw a fall in the output of those still employed, reversing much of the productivity gains of the 1920s. (This surprised Keynes, among others, who still believed in the declining marginal product of labor, which predicted the opposite.) Recovery in the late 1930s, conversely, didn’t just mean higher employment, it involved a sharp acceleration in labor productivity. There’s a widespread idea that output per worker necessarily reflects supply-side factors — technology, skills, etc. But if demand had such direct effects on labor productivity in the Great Depression, why not in the Lesser Depression too? But for some reason, people who scoff at the idea of the “Great Forgetting” of the 1930s have no trouble believing that the drastic slowdown in productivity growth of recent years has nothing to do with the economic crisis it immediately followed.
EDIT: I should add: While the decline in production during the Depression was, of course, primarily a matter of reduced employment, the decline in productivity was not trivial. If output per employee had continued to rise in the first half of the 1930s at the same rate as in 1920s, the total fall in output would have been on the order of 25 percent rather than 33 percent.
Note also that the only other comparable (in fact larger) fall in GDP per worker came in the immediate postwar demobilization period 1945-1947. I’ve never understood the current convention that says we should ignore the depression and wartime experience when thinking about macroeconomic relationships. Previous generations thought just the opposite — that we can learn the most about how the system operates from these kinds of extreme events, that “the prime test of Keynesian theory must be the Great Depression.” Isn’t it logical, if you want to understand how shifts in aggregate demand affect economic outcomes, that you would look first at the biggest such shifts, where the effects should be clearest? The impact of these two big demand shifts on output per worker, seem like good reason to expect such effects in general.
And it’s not hard to explain why. In real economies, there are great disparities in the value of the labor performed by similar people, and immense excess capacity in the form of low-productivity jobs accepted for lack of anything better. Increased demand mobilizes that capacity. When the munitions factories are running full tilt, no one works shining shoes.