President Trump has made radical changes to U.S. trade policy since taking office. The tariffs imposed by his administration have pushed the average tax rate on U.S. imports to its highest level in decades, causing some investors to lose confidence in American stocks.Consequently, the S&P 500 (^GSPC -0.67%), the best gauge for the U.S. stock market, has underperformed benchmark indexes in Asia, Canada, and Europe this year. The index has also underperformed stock markets in emerging economies like Brazil, M ...