Psychologists have identified a tendency in people to think they have control over events even when they have none. That can lead investors to overvalue a losing stock in their portfolio. It also can lead them to imagine trends when none exist or believe they can spot a pattern in a stock chart and thus predict the future. In fact, the changes in stock prices are very close to a “random walk”: There is no dependable way to predict the future movements of a stock’s price from its past wanderings.
The same holds true for supposed seasonal patterns, even if they appear to have worked for decades. Once everyone knows there is a Santa Claus rally in the stock market between Christmas and New Year’s Day, the “pattern” will evaporate. Investors will buy one day before Christmas and sell one day before the end of the year to profit from the supposed regularity. But then investors will have to jump the gun even earlier, buying two days before Christmas and selling two days before the end of the year. Soon all the buying will be done well before Christmas and the selling will take place right around Christmas. Any apparent stock market pattern that can be discovered will not last as long as there are people around who will try to exploit it.