Confirmation bias

Confirmation bias is the tendency to seek, interpret, and remember information that confirms one's existing beliefs while ignoring or undervaluing information that contradicts them. It was demonstrated experimentally by Peter Wason in 1960 and is one of the most-replicated findings in cognitive psychology.

Confirmation bias matters because it is the bias most directly opposed to the scientific method. Science depends on actively seeking evidence that could falsify a hypothesis; confirmation bias inclines us to seek evidence that would confirm it. The bias scales from individual reasoning to institutional decision-making to political discourse to information consumption. It is amplified by personalized feeds, filter bubbles, and any environment where evidence is selected by the seeker rather than presented by an adversarial process. In medicine, confirmation bias contributes to diagnostic errors. In law, it contributes to wrongful convictions. In science, it contributes to publication bias and the replication crisis.

Peter Wason's 1960 four-card selection task is the classic experimental demonstration of confirmation bias. Participants were given four cards and a hypothesis, and asked which cards needed to be turned over to test the hypothesis. Most people selected cards that could only confirm the hypothesis, not cards that could falsify it. Wason's later 2-4-6 task extended the demonstration: participants given a numerical sequence and a rule consistently tested only confirming sequences, missing the broader rule. Klayman and Ha (1987) reframed the phenomenon as positive test strategy — a more general tendency to seek information that would arise if the hypothesis were true. Earlier philosophical roots trace to Francis Bacon's Novum Organum (1620), which warned of the human mind's tendency to support its first conclusions.

Confirmation bias is among the most-replicated findings in cognitive psychology. Nickerson's (1998) comprehensive review documented it across hypothesis testing, memory recall, evidence evaluation, and motivated reasoning. The bias is robust to expertise, intelligence, and stakes. It is amplified when the hypothesis is identity-relevant: people are particularly prone to seek confirming evidence for beliefs that are central to their political identity, professional self-concept, or personal worldview. Mercier and Sperber's (2011) argumentative theory proposes that confirmation bias evolved not as a flaw in individual reasoning but as a feature of group reasoning — useful when conclusions are tested adversarially in dialogue, problematic when reasoning is solitary.

Confirmation bias is not the same as outcome-driven motivated reasoning, though the two overlap. Confirmation bias operates even when the seeker has no emotional stake in the answer; it reflects a general cognitive default toward positive testing. The bias also is not eliminated by being well-intentioned. Researchers, judges, and clinicians who care deeply about getting the right answer still exhibit it. Reducing confirmation bias requires structural practices: pre-registration, blind review, devil's advocacy, deliberate consideration of disconfirming evidence, exposure to good-faith disagreement.

The Cognitive Bias Susceptibility tool measures confirmation bias using a four-card selection task adapted from the CART (Stanovich, West, & Toplak, 2016). The user is asked to identify which cards are needed to test a rule; bias scoring reflects whether the user selects only confirming cards, only falsifying cards, or both.

• Cognitive bias
• Heuristic
• Bias blind spot
• Decision hygiene