What is Confirmation Bias?
Confirmation bias is the tendency to search for, interpret, favor, and recall information in ways that confirm pre-existing beliefs or hypotheses. It operates at every stage of reasoning — from which questions people ask, to how they weigh evidence, to what they remember after the fact. A hiring manager who believes a candidate is strong may unconsciously focus on interview answers that support that impression while discounting signs of weakness. Unlike motivated reasoning, which serves emotional goals, confirmation bias can arise even when people are trying to be objective, making it one of the most pervasive and difficult-to-correct cognitive biases documented in psychology.
How it works
Peter Wason identified confirmation bias experimentally in 1960 using what became known as the 2-4-6 task. Participants were told that the number sequence 2, 4, 6 conformed to a rule, and they had to discover it by proposing new sequences. Most people hypothesized “ascending even numbers” and then tested only sequences consistent with that hypothesis — such as 8, 10, 12 — rather than testing sequences that might disconfirm it. The actual rule was simply “any ascending numbers,” but participants rarely discovered it because they failed to seek disconfirming evidence. Raymond Nickerson’s comprehensive 1998 review later documented how this same pattern manifests across scientific reasoning, legal judgment, medical diagnosis, and everyday belief formation.
Applied example
In criminal investigations, confirmation bias has contributed to wrongful convictions. The case of Brandon Mayfield, a Portland attorney falsely linked to the 2004 Madrid train bombings, illustrates the pattern. After an initial (incorrect) fingerprint match, FBI analysts who reviewed the same evidence consistently confirmed the identification rather than challenging it. An independent review by the Office of the Inspector General found that the initial misidentification created an anchoring effect that biased subsequent examinations — a textbook demonstration of confirmation bias in expert judgment.
Why it matters
Confirmation bias threatens the quality of decisions wherever evidence must be evaluated — in hiring, investment analysis, product research, medical diagnosis, and policy design. Effective countermeasures include structured decision-making processes, pre-mortems, red teams, and deliberate consideration of alternatives, making awareness of this bias essential for anyone designing systems that depend on accurate information processing.
