New Delhi, March 4 -- With tariffs poised to take effect on Tuesday on products from Canada and Mexico, plus threats of levies on imports from the European Union, one thing is clear: President Donald Trump likes the number 25.

All of those tariffs are set at 25%, assuming no last-minute deal to lower them. Trump said on Monday that there's "no room left for Mexico or for Canada" to make a deal before midnight.

Don't read too much into that number, however.

"There's nothing magical, useful, or relevant about the number 25%," University of Michigan professor of public policy and economics Justin Wolfers tells Barron's. "It doesn't come from a model, from careful evaluation of the consequences of a tariff, or from history. It doesn't solv...