The stock market has been turning cartwheels since Donald Trump’s election. They say it’s the prospect of faster economic growth that got stock traders so excited. Really?

Mr. Trump has promised a huge jump in the growth rate of real gross domestic product, to perhaps 3.5%, 4% or more. As he said in his Las Vegas debate with Hillary Clinton, “I actually think we can go higher than 4%. I think you can go to 5% or 6%.” Good luck with that.

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