Once emotion is involved in cognition (and emotionless cognition is quite rare), our mental system no longer operates as an ideal scientist : carefully weighing the pros and cons of the belief implied by the emotion. Instead the emotional person acts more like a prosecutor or defense lawyer seeking by any means to find evidence for the belief.
We also tend to interpret ambiguous information - and I think that most incoming information is of that sort! - in such a way that it lines up with our feelings and desires and prejudices.
Psychologists call that motivated cognition. And I think that we are all guilty of it - some more, some less.
The next book I'm going to read is called The Master And His Emissary. It's about how the two sides of the brain work together to understand the world outside us. The main idea is that one side deals with the big picture while the other focuses on necessary features. I think the master is the right-brain and the emissary the left. If I understand it correctly, the master uses the emissary to focus on the finer details which are then transferred to the big picture, amending it.
This is an edit. I forgot to conclude that somehow or other, the emissary's information is not transferred back objectively, or else the master refuses to deal with it honestly and that might allow for the continuance of bias in some situations. I'll know better after I've read the book.