People naturally tend to seek information that confirms their expectations and hypothesis, even when evidence is disconfirming or falsifying. When we encounter information that is consistent with our beliefs, we usually accept it with an open mind and glad heart. We accept information uncritically unless there is an unavoidable reason to doubt it.
Yet, when we discover facts that force us to question our beliefs, we ask a very different question: must I believe this? We wonder if we can dismiss this troublesome information or if the evidence is too overwhelming to reject.
Why people fall to confirmation trap
Human mind is designed to retrieve information that is consistent with hypothesis selectively accessible. We tend to entertain provisional hypothesis as true. It is even possible to plant people with false memories.
Loftus (1975) had participants to watch a film of automobile accident. They were asked about a barn that was not even in the film. People, however, were able to recall having seen a barn even though it did not exist.
Furthermore, due to a process how we search information, we have limits to our attention and cognitive processing. We must search for information selectively. People tend to look first where they are most likely to find the most useful information. This results in retrievability bias that leads us to come to the conclusions we desire to reach.
Lord et. al. (1979) asked participants to review evidence for and against effectiveness of death penalty (balanced research). Supporters were not persuaded by information that death penalty is ineffective. They criticised studies were poorly designed and findings were unreliable. However, opponents found the same evidence to be valid and persuasive. Similarly, they criticised research showing the effectiveness of death penalty at deterring crime and came up with reasons to disagree with evidence. Both groups left the experiment even more assured of opening opinions.
People naturally tend to believe the things that confirm their expectations. Once you become aware of confirmation trap, you are likely to find that it pervades your decision making processes.
For instance: when you seek to confirm your decision to hire a particular employee, you have no trouble in finding positive information. However, it may be more important to determine if negative information is also available (e.g. criminal records, positive information of another potential applicant).
Bazerman, M.H. and Moore, D.A., 1994. Judgment in managerial decision making (p. 226). New York: Wiley.