Over the next few days, lots of people are going to be poring over their investment portfolio, trying to figure out which stocks to keep and which stocks to sell. Unfortunately, many of these investors will make the exact same mistake, causing them to lose vast sums of money over the long term.

The problem is loss aversion. Kahnemanandtversky stumbled upon loss aversion after giving their students a simple survey, which asked whether or not they would accept a variety of...