Confirmation Bias
From SA Mafia Wiki
Confirmation Bias is the tendency for human beings to favor unfairly information which supports their hypotheses and beliefs, and reject information which is contrary. Because Mafia Games often become emotionally charged affairs, the risk of confirmation bias becomes very high. In severe cases, it can result in tunneling, where a player interprets everything another player does as Scummy, regardless of their actual alignment.
Confirmation bias encourages players to find Scum through illogical or unreasonable means - e.g. "I dislike that post they made [before they got their Role PM ]." It also encourages players to only point out posts that are Scummy, while ignoring, dismissing, or otherwise negating posts which are Townie. If the accused player flips Scum, it only deepens the bias, since it retroactively makes the accuser feel their belief actually was justified, and can result in many false positives as they connect innocent townies to the scum player using similarly illogical reasoning.
The best way to overcome confirmation bias in mafia games is to take a step away from the computer and calmly attempt to falsify one's own beliefs.
Note that "gut" is not in and of itself confirmation bias - a player might suspect another is scum without being able to put into words why they feel that way. But if they use their gut to falsely characterize the player's posting, then it becomes victim to confirmation bias.
