Here's a great talk on investing that was given at Harvard Business School by Mark Sellers. I highly recommend it.
"Personally, I'm amazed at how little conviction most investors have in the stocks they buy. Instead of putting 20% of their portfolio into a stock, as the Kelly Formula might say to do, they'll put 2% into it. Mathematically...it can be shown that a 2% position is the equivalent of betting on a stock that has only a 51% chance of going up, and a 49% chance of going down. Why would you waste your time even making that bet?"
Sellers MBA Talk