It's no secret that momentum is a powerful force in the stock market. It can drive real estate prices too, and may even account for all those color-coded rubber wristbands everyone is wearing today. Problem is, momentum is often irrational, and advertisers know it. In a new study, people were asked to buy and sell virtual stocks that had long histories of either increasing or decreasing earnings. For the most part, folks bought winners and sold losers. If that result doesn't surprise you, then you're just the sort of investor that Joseph Johnson University of Miami wants to warn.