People are often sloppy when they talk about the adverse effects of economic uncertainty, frequently using “uncertainty” to mean “an increased probability of something bad happening.” That’s not really about uncertainty: it means that average expectations of what’s going to happen are worse, so it’s a fall in the mean, not a rise in the variance.

But uncertainty properly understood can have serious adverse effects, especially on investment.

Let me offer a hypothetical example. Suppose there are two companies, Cronycorp and Globalshmobal, that would be affected in opposite ways if Trump imposes a new set of tariffs. Cronycorp would like to sell stuff we’re currently importing, and would build a new factory to make that stuff if assured that it would be protected by high tariffs. Globalshmobal has already been considering whether to build a new factory, but it relies heavily on imported inputs, and wouldn’t build that factory if those imports will face high tariffs.

Suppose Trump went ahead and did the deed, imposing high tariffs and making them permanent. In that case Cronycorp would go ahead, while Globalshmobal would call off its investment. The overall effect on spending would be more or less a wash.

On the other hand, suppose that Trump were to announce that we’ve reached a trade deal: all tariffs on China are called off, permanently, in return for Beijing’s purchase of 100 million memberships at Mar-a-Lago. In that case Cronycorp will cancel its investment plans, but Globalshmobal will go ahead. Again, the overall effect on spending is a wash.

But now introduce a third possibility, in which nobody knows what Trump will do — probably not even Trump himself, since it will depend on what he sees on Fox News on any given night. In that case both Cronycorp and Globalshmobal will put their investments on hold: Cronycorp because it’s not sure that Trump will make good on his tariff threats, Globalshmobal because it’s not sure that he won’t.

Technically speaking, both companies will see an option value to delaying their investments until the situation is clearer. That option value is basically a cost to investment, and the more unpredictable Trump’s policy, the higher that cost. And that’s why trade tantrums are exerting a depressing effect on demand.

Furthermore, it’s hard to see what can reduce this uncertainty. U.S. trade law gives the president huge discretionary authority to impose tariffs; the law was never designed to deal with a chief executive who has poor impulse control. A couple of years ago many analysts expected Trump to be restrained by his advisers, but he’s driven many of the cooler heads out, many of those who remain are idiots, and in any case he’s reportedly paying ever less attention to other people’s advice.

None of this guarantees a recession. The U.S. economy is huge, there are a lot of other things going on besides trade policy, and other policy areas don’t offer as much scope for presidential capriciousness. But now you understand why Trump’s tariff tantrums are having such a negative effect.

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