Why Believe In Keynesian Models?

A correspondent asks a good question: what evidence makes me believe that Keynesian economics is broadly right, given the relative absence of experience with large fiscal stimulus programs?
I’d answer that question with several points.
First, we’re talking about a model, not just a prediction about the impact of spending increases. So you can ask about the ancillary predictions of that model as opposed to rival models. Anti-Keynesians assured us that budget deficits would send interest rates soaring; Keynesian analysis said they’d stay low as long as the economy remained far from full employment. Guess who was right?
Also, there are some features of the approach that can be tested separately. Keynesianism isn’t just about sticky prices, but it does generally assume sticky prices — and there is overwhelming evidence, from a variety of sources, that prices are indeed sticky.
Also also: there’s plenty of evidence that monetary policy can move output and employment — and it’s very hard to devise a model in which that is true that doesn’t also say that fiscal policy can be effective, especially when you’re up against the zero lower bound.
Second, while we don’t have a lot of postwar experience with fiscal stimulus, we do have a lot of experience with anti-stimulus, that is, austerity — and that turns out to be reliably contractionary. Again, it’s hard to think of a model in which austerity is contractionary but stimulus isn’t expansionary.
Finally, there is evidence from fiscal expansions in the 1930s, which actually did lead to economic expansion too.

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