Last semester, we discussed biases and irrationalities that violate the rationality assumption in economics. Another common bias is confirmatory bias. This describes the tendency of people to look for information that confirms their preconceived notions or a decision they made.
An example used by Tyler Cowen on his blog is how people eagerly read advertisements for a car after they have already bought the car: they do it as a way of confirming that they made the right decision. Another example that is a little different is when people notice or comment on a piece of jewelry or an expensive piece of clothing on a Walker student because they believe that all students from private schools are all rich. In that case, they are looking for information to confirm their preconceived notion.
Any other examples you can think of?
(Source: Marginal Revolution)