Still, why didn’t the tax cuts have the desired effect? Perhaps they did but were overwhelmed by losses from other policies. Tariffs on foreign goods harmed growth by raising prices for consumers and producers and disrupting supply chains. Restrictive immigration policies helped cut U.S. population growth to less than 0.5% last year, the smallest increase since the 1918 Spanish flu epidemic. A growing population, especially of working-age immigrants, is crucial to economic growth.

There’s another likely deterrent as well. Mr. Trump’s erratic behavior and penchant for economic and diplomatic isolation have almost certainly dampened the enthusiasm of both foreigners and Americans to devote capital here. Foreign direct investment has dropped for five straight quarters.