On the surface, it looks as though international tax competition is heating up. Trump's move can potentially change the business logic of U.S. multinationals, which now prefer to stash international profits -- more than $2.5 trillion of them -- overseas. May's pledge appears to be aimed at making it more attractive to invest those profits in the U.K. It's difficult to calculate the exact effect of lowering the top-line rates, though, because hardly anyone pays them. Now that business-friendly governments appear to have some leeway, they should go back to the old idea of eliminating corporate levies and just taxing personal income and consumption.

In the U.S., according to the Treasury Department, the average rate paid by profitable companies with more than $10 million in assets is 22 percent, not 35. The U.K. doesn't calculate the average effective corporate tax rate for its companies. Early this year, however, an investigation by the Sunday Times turned up a stunning fact: six out of the country's 10 biggest multinationals, including Lloyds, British American Tobacco and Shell, didn't pay any corporate tax at all in 2014. 

In the U.S., the U.K. and elsewhere corporate taxation is a crazy quilt of deliberate exemptions, accidental loopholes and, in some cases, special sweetheart deals (just ask Europe's competition commissioner, Margrethe Vestager, about Apple and Ireland). Because of everything companies do to game it, the tax distorts financial reporting and creates a lot of needless work for lawyers, accountants and tax agents, imposing a high transaction cost on business and society. It also leads to ugly, unfathomable corporate structures that abuse globalization. International bickering because of what is perceived as unfair competition is by now white noise at G20 meetings, where a lot of time is spent on international corporate tax harmonization. 

At the same time, in part because of the widespread gaming and the plethora of exemptions, corporate taxes don't contribute much to national budgets. According to the Organization for Economic Cooperation and Development, the average contribution in developed nations is just 2.8 percent of gross domestic product. In both the U.K. and the U.S. it's even lower, 2.45 percent and 2.6 percent, respectively. In 1968, corporate taxes accounted about a quarter of all federal tax revenues in the U.S. today, that share has shrunk to less than 10 percent.

Perhaps because of corporate taxation's diminished importance, big changes to the headline rate are so popular: They look impressive in newspaper stories and election platforms, but they don't have a huge effect on the budget deficit. For example, the change Trump proposes -- from the effective rate of 22 percent to 15 percent -- would shave off just 0.8 percent of GDP in revenue.