It's also tough to think rationally yourself. "It's hard to keep your emotions in check when your money is on the line," Shefrin says.
And, even if you're confident the panicked market is giving you a buying opportunity, you're likely to want to wait until it hits bottom. If a market is in free fall, buying stocks on the way down is likely to give you instant losses.
Not only will buyers hold back. A falling market will bring many more sellers out of the woodwork. Leverage is one reason: Many investors buy stocks on borrowed money, so they can't afford to lose as much without facing bankruptcy.
This is one explanation for the temporary, sharp drops in many financial markets in the summer of 2007. Losses on leveraged mortgage debt prompted many hedge funds to dump all sorts of assets to raise cash.
Comment: Only a few transactions occur at the lowest price. It is not realistic to buy at the lowest price but one should start buying when the price is already low by your valuation.