A large and growing body of academic research suggests there are market anomalies that can be exploited to beat a strict index approach. Some of that research has been recognized with Nobels in economic science - William F. Sharpe in 1990 and Eugene F. Fama in 2013.

September 2001 turned out to be an unusually bad time to sell stocks: By New Year's Day 2002, little more than three months after the post-9/11 low reached on Sept. 21, the S&P 500 had gained close to 20 percent.

Low-volatility funds, which tend to smooth out performance, have been especially popular since the financial crisis. The PowerShares S&P 500 Low Volatility Fund is the oldest, begun in 2011.

James B. Stewart Quotes