The best performing stock mutual fund for the first decade of this century was Ken Heebner's CGM Focus Fund. He managed to average 18% per annum over the 10 years. So no doubt investors in his fund would be pretty happy, right? Wrong. The average return achieved by investors in this fund over the same time period was a shocking -11% per annum. Yes, MINUS ELEVEN percent per year, a total of 94% less than what they should have made. The herd behaviour of investors and how they mess things up for themselves is the #1 story in finance. Understand this and everything else is just details.