At a gathering of investment bank trainees in suburban New York last night, I was asked my opinion of the most common problem among traders. My answer was neither fear nor greed. It was overconfidence.

Studies in behavioral finance find that about 3/4 of all traders rate their prowess as "above average", despite the obvious reality that only half of us are better than the other half. This overconfidence, moreover, affects actual trading performance. Research by Terence Odean and colleagues finds that overconfidence affects frequency of trading, which in turn contributes to poor trading results. In one study of online traders, the group of traders favored high beta (volatile) small cap companies and tended to not diversify their portfolios. Their actual trading results slightly beat the small cap index, but after trading costs were factored in, they significantly underperformed the index. The most frequent traders were the ones who underperformed the index by the greatest margin.

One of my favorite studies of overconfidence came from the London Business School. Traders were shown price patterns and asked to figure out the market's next direction and indicate their confidence in their prediction. The price patterns were generated entirely randomly. The traders with the highest confidence in their predictions traded the most frequently and incurred the greatest losses. A completely random trader--50% right, 50% wrong--who trades once a day will have runs of five consecutive winners about six times during a year. It is difficult to not think you have the hot hand after such a string, become overconfident, and raise your trading size. Of course, the random trader will have an equal number of strings of losers. That is likely to burst confidence and lead the trader to cut trading size--assuming, of course, that he's still in the game at that point.Is it any wonder that traders seek help from coaches and psychologists? Few of those coaches and psychologists, however, will tell the trader the truth: You're trading random patterns and your problem is overconfidence in them.