The glossary Bias
A bias is an illogical preference or prejudice that the portfolio manager takes. It’s a uniquely human foible, and since investors are human, they can be affected by it as well.
There is an entire field in social sciences that focuses on the psychology of investing. Thanks to the advances made, we can understand some interesting aspects of the investor psyche such as: why do we tend to be more averse to lose than gain, why we tend to make the same investment mistakes, why we are attracted by free things, etc.