Professional investors tend to move the fastest when a market suddenly turns. That’s largely out of self-preservation, because the biggest risk they face is being so out-of-step with the market that their clients fire them. That can lead the pros to chase the market trend too far and too long. “Institutions sell more than individuals when there is a large stock-market drop,” finance professors Patrick Dennis and Deon Strickland found in a 2002 study. They also showed that the more widely a stock is held by big investors, the greater its trading volume during sharp market drops.