Selling Winners and Holding Losers

Extracted from the book: Quantitative Trading Strategies,  Lars Kestner
Studies of human bias in economic situations shed light on how the mind affects trading
decisions. In one such study conducted in 1998, Terrance Odean, professor of finance at the University of California, examined 10,000 accounts at a large discount brokerage firm to determine if individuals’ trading styles differed between winning trades and losing trades made from 1987 and 1993. He found a significant tendency for investors to sell winning stocks too early and hold losing stocks too long. Over his test period, investors sold approximately 50 percent more of paper profits on winning trades than they sold of paper losses in losing trades.
Based on the data, Odean concluded that winning stocks were sold quicker and more frequently than losing stocks. Although the results were a bit surprising, this behavior by investors could make sense. When we buy stocks, we’re placing a bet that a company is undervalued. Stocks that increase in value are logically
becoming less undervalued as they rise, while stocks that decrease in value are logically becoming more undervalued. Winning stocks that have increased in value could be considered not as cheap as when they were purchased. Losing stocks that have declined in value could be considered cheaper than when purchased. In this case, it makes sense to sell the winning stocks that have become less cheap and
hold losing stocks that have become cheaper.
Although the logic is sound, Odean’s results show that the opposite actually occurs. Winning stocks that were sold continued to rise, while losing stocks that were held continued to fall in value. In the year following sales, stocks sold with gains by individual investors outperformed the market by an average 2.35 percent. At the same time, losing stocks that were held underperformed the market by an average of 1.06 percent. Odean discovered, on average, that investors underperform the market by selling their winners too early and holding on to their losers too long. Based on purely economic terms, it’s unclear why they behaved in this manner; psychology may provide the missing link. Clearly, the more profitable course of action suggested by the study is to buy winning stocks and sell losing ones. Two theoretical underpinnings dominate the tendency to sell winners and ride losers: prospect theory and mean reversion theory.