We shouldn’t judge a book by its cover. Nor judge a research paper by its title. “Discussion paper No.5: Public investment and potential output”, just published by the Office for Budget Responsibility, doesn’t sound like a barrel of laughs. But unenticing is not the same as unimportant.

This is the official forecaster telling us what economic impact it expects public investment – public spending that creates an asset, like a road or hospital – to have.

Policy debates normally focus on public investment’s growth........

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