Coronavirus shows the problem with Trump's stock-market boasting

He is, in effect, experiencing the downside of having spent the last three years personalizing much of what happens in the markets and the economy, saying that the soaring stock values under his watch are a reflection of his special ability, and a central part of his case for re-election in November.

Most presidents avoid boasting about a rising stock market because they know how fragile it is, and how little control over stock prices they really have, and how stock prices can move sharply for reasons outside their control, or sometimes for no clear reason at all.

The cost of claiming personal credit for stock market gains comes when you get stock market losses. And that is particularly relevant after a 7 percent drop in the S&P 500 since its peak last Wednesday, seemingly caused by a recognition on Wall Street that the spread of coronavirus could disrupt the world economy.