Foreign stock mutual funds have had a performance edge over the broad U.S. stock market during the past 15 years. But the S&P 500 has outperformed them since March 2009. A $10,000 investment in the average foreign stock mutual fund on June 30, 1999, would have grown to $26,574 by Sept. 16 this year vs. $18,387 for the same $10,000 in the S&P 500, according to Morningstar.