Overconfidence Bias
The tendency for investors to overestimate their knowledge, skills, and ability to predict market outcomes.
What is Overconfidence Bias?
Overconfidence bias causes investors to believe they have superior information, analytical skill, or market-timing ability compared to peers — leading them to trade excessively, under-diversify, and take on more risk than is justified by their actual edge. Studies consistently show that high trading frequency reduces returns, largely due to transaction costs and the false belief that each trade reflects genuine insight. Overconfidence also manifests as underestimating downside scenarios and over-concentrating in familiar assets ('home bias'). It is among the most documented behavioral biases in individual investor behavior.
Example
Barber and Odean (2001) analyzed 35,000 brokerage accounts and found that the most active traders earned 6.5 percentage points less per year than the least active traders, after controlling for risk. The overconfident traders believed their trades were information-driven, but excessive transaction costs and poor timing eroded their returns significantly.
Source: Barber & Odean — Trading Is Hazardous to Your Wealth (2000)