In his preface to Benjamin Graham's book The Intelligent Investor, Warren Buffett states: "To invest successfully over a lifetime does not require a stratospheric IQ, unusual business insights, or inside information. What's needed is a sound intellectual framework for making decisions and the ability to keep emotions from corroding that framework. You must supply the emotional discipline."
In his best selling book The Little Book of Common Sense Investing, John Bogle cites numerous examples illustrating the disparity between the returns of funds and the returns achieved by investors in the funds. He then goes on to say: "Fund investors have been chasing past performance since time eternal, allowing their emotions - perhaps even their greed - to overwhelm their reason."
Dalbar, a leading financial services market research firm, published an 2013 article Quantitative Analysis of Investor Behavior where they showed that investors severely underperformed equity and bond indices for the 20 year period through December 2012. Investors in equity funds averaged a return of just 4.25% while the S&P 500 index returned 8.21%. Investors in bonds funds averaged a return of just 0.98% while the Barclays Aggregate Bond Index returned 6.34%.