As Karl Popper demonstrated, evaluating a scientific proposition requires falsifiability—theories or hypotheses can’t be proved or disproved if they can’t be subjected to empirical tests. When the 2017 Tax Cuts and Jobs Act was passed, we were criticized for being overly optimistic about the effects we predicted it would have. Now the evidence is in. Our critics were wrong, and the economic data have met or even exceeded our predictions. In 2017, we predicted that reducing the federal corporate tax rate to 21% from 35% and introducing full expensing of new-equipment investment would boost productivity-enhancing business investment by 9%. Though...