Critics of President Trump worry that he doesn’t listen to anybody any more. Moderate advisers such as Gary Cohn, H.R. McMaster and Rex Tillerson are gone, replaced by nationalist hard-liners more likely to tell Trump what he wants to hear than what might be good policy.

But Trump still has one master: the stock market, which he regards as a barometer measuring how he’s doing as president. “He views the stock market as the ultimate arbiter of his policies,” Greg Valliere of Horizon Investments tells Yahoo Finance. “He believes that his tax cuts and regulatory reform are a big reason for last year’s explosive rally.”

Trump has tweeted about the stock market at least 47 times since being elected, often taking credit for handsome gains. But he hasn’t tweeted on the stock market since Feb. 7, when he complained that stocks should be going up, not down. His silence of late makes sense: stocks have been volatile and are slightly negative for the year—in part because of Trump himself.

Stocks sold off on the two occasions so far this year when Trump announced new tariffs—the steel and aluminum tariffs he announced March 8, and a broader set of tariffs against Chinese imports announced March 22. And guess what: In the aftermath of those selloffs, Trump modified his policies, perhaps in response to the poor marks the stock market gave to the protectionism he promised as a candidate.

Trump has significantly dialed back the steel and aluminum tariffs, issuing exemptions for many countries that leave just one-third of imported steel and 45% of imported aluminum subject to the new tariffs. They now only apply to imports from Russia, China and a handful of other Asian countries.

Spooked investors

The S&P 500 stock index fell 5% in the two days following the announcement of new tariffs on $50 to $60 billion worth of imports from China. Then came news that American and Chinese negotiators were engaged in fresh talks to tamp down trade tensions and avoid a trade war. Voila! Stocks rebounded, recovering some of the losses caused when Trump first announced the China tariffs.

Markets still seem cautious about Trump’s protectionist leanings. Of all Trump’s economic policies, tariffs, protectionism and the risk of trade wars spook investors and economists the most. Trump famously said earlier this month that “trade wars are good, and easy to win.” The markets are schooling him otherwise.

China has already announced one set of retaliatory tariffs on American imports, as a proportionate response to the impact Trump’s steel and aluminum tariffs are likely to have on Chinese imports to the United States. China has not yet responded to the China-only tariffs Trump announced on March 22. But most trade experts expect that to happen within a few weeks, probably after the Trump administration specifies which Chinese imports will bear the new tariffs. Assuming the China retaliation is proportionate, market reaction could be severe, since the new China tariffs cover at least 17 times as much trade with China as the steel and aluminum tariffs do.