Outperforming the stock market in the United States is difficult. Using the S&P 500, the benchmark index of U.S. equities, as the standard of performance, only 27% of actively managed large-capitalization equity funds beat the passive benchmark over the 12-month period ending June 30. Over the decade ending in June, just 13% of actively managed stock funds beat the broad S&P 500 index.

Why do active portfolio managers struggle to beat the index?