The 6 biggest investing mistakes

At our two favorite universities, Yale and Princeton, psychologists are fond of giving students questionnaires asking how they compare with their classmates. For example, students are asked: "Are you a more skillful driver than your average classmate?" Invariably, the overwhelming majority answer that they are above-average drivers. Even when asked about their athletic ability, where one would think it more difficult to delude oneself, students generally say they're above average. They see themselves as above-average dancers, conservationists, friends, and so on.

And so it is with investing. In recent years, a group of behavioral psychologists and financial economists have created the important new field of behavioral finance. Their research shows that we are not always rational. We tend to be overconfident. If we do make a successful investment, we confuse luck with skill. It was easy in early 2000 to delude yourself that you were an investment genius when your Internet stock doubled and then doubled again.

To deal with the pernicious effects of overconfidence, think about amateur tennis. The player who steadily returns the ball, with no fancy shots, is usually the player who wins. And the prudent buy-and-hold investor who holds a diversified portfolio through thick and thin is the investor most likely to achieve his long-term goals.

NEXT: Mistake #2: Following the herd