We filter incoming sensory information in line with our beliefs – evidence that supports an existing belief will be amplified, while information that contradicts it will be downplayed or ignored – leaving us in our own “reality tunnel” which may be quite different to someone else’s. Consequently we may miss useful information if it doesn’t support our belief system.
This tendency gets worse with “groupthink”. We are influenced by other people, and if we only have contact with people who believe the same as us, our filters get even stronger (“social proof” in Cialdini’s Influence: The Psychology of Persuasion).
Remedies: talk to people with different beliefs. Read a newspaper with a differing political point of view to your own occasionally. Actively look for evidence which counters your own beliefs. Ask yourself, “How would I know if this wasn’t true?”