One mistake made by many investors is that of overconfidence in one’s ability to forecast and outperform the markets. This mistake also transcends into other areas of life, such as business management, driving and dating.

Need proof that people tend to overestimate their proficiency and prowess? Here are 10 examples from a recent presentation by Whitney Tilson:

1) 19% of people think they belong to the richest 1% of U.S. households

2) 82% of people say they are in the top 30% of safe drivers

3) 80% of students think they will finish in the top half of their class

4) When asked to make a prediction at the 98% confidence level, people are right only 60-70% of the time

5) 68% of lawyers in civil cases believe that their side will prevail

6) Doctors consistently overestimate their ability to detect certain diseases

7) 81% of new business owners think their business has at least a 70% chance of success, but only 39% think any business like theirs would be likely to succeed

Graduate students were asked to estimate the time it would take them to finish their thesis under three scenarios: best case, expected, and worst case. The average guesses were 27.4 days, 33.9 days, and 48.6 days, respectively. The actual average turned out to be 55.5 days.

9) Mutual fund managers, analysts, and business executives at a conference were asked to write down how much money they would have at retirement and how much the average person in the room would have. The average figures were $5 million and$2.6 million, respectively. The professor who asked the question said that, regardless of the audience, the ratio is always approximately 2:1

10) 86% of my Harvard Business School classmates say they are better looking than their classmates

Source: Whitney Tilson (T2 Partners)

…here’s his full presentation:

Tilson Prez May 2011