Looking back 10 years, international stock mutual funds had a solid lead over the U.S. stock market until mid-2011. Then the S&P 500 began a period of outperformance. A $10,000 investment in the average international stock mutual fund made on June 30, 2005, would have grown to $19,838 by July 14 this year, according to Morningstar Inc. data. That would have trailed the $21,867 that the same investment in the S&P 500 would have generated.