What Is Confirmation Bias? Why We See What We Expect to See
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Give people a rule to discover and they will test it by looking for examples that fit rather than ones that would break it, and they will do so even when told that breaking it is the only way to be sure. The tendency to gather, interpret and remember evidence in a way that supports what one already thinks was named confirmation bias by the psychologist Peter Wason in the 1960s, and it has since turned out to be the most pervasive distortion in human reasoning, present in scientists, judges, doctors and everyone who has ever read the news.
Wason's experiments
Wason gave subjects the sequence 2, 4, 6 and told them it obeyed a rule they were to discover by proposing further sequences, which he would mark as fitting the rule or not. Most guessed the rule was numbers rising by two and tested it by offering 8, 10, 12 and 20, 22, 24, each of which was confirmed. Very few tried 1, 2, 3 or 10, 9, 8, which would have been confirmed and refuted respectively, and so very few found the actual rule, which was any three rising numbers. They had tested their hypothesis only in ways that could not show it to be wrong.
A second experiment showed four cards, each with a letter on one side and a number on the other, showing A, K, 4 and 7, and asked which must be turned over to test the rule that a card with a vowel on one side has an even number on the other. Most people turn A and 4. The correct answer is A and 7, since only a vowel with an odd number behind it could break the rule, and the 4 can prove nothing whatever is behind it. Fewer than one person in ten gets it right.
Three forms
The bias operates at each stage of dealing with evidence:
- •Biased search: seeking out sources and asking questions likely to confirm, as when someone who suspects a friend is angry asks questions that assume it
- •Biased interpretation: reading ambiguous evidence as support, so that supporters of opposite views can watch the same debate and each conclude their side won
- •Biased memory: recalling the instances that fit and forgetting the ones that did not, which is how a belief in lucky charms or in a horoscope survives a lifetime of counter-examples
Why it exists
The bias is not stupidity, and it is not cured by intelligence; more able reasoners are better at constructing arguments for what they already believe. One explanation is that testing a belief by looking for confirming cases is a reasonable strategy when most hypotheses are wrong and the world is complicated, since a wrong belief will usually fail to be confirmed quickly enough. Another is that reasoning did not evolve to find truth in solitude but to persuade others and to defend oneself in argument, tasks at which a one-sided search for evidence is a strength. On that view confirmation bias is a feature of an individual mind that becomes a bug only when the mind reasons alone.
The bias grows with emotion. On questions where nothing is at stake people update readily; on questions tied to identity, politics, religion or a decision already made, the same people examine evidence against their view with a rigour they never apply to evidence for it, a pattern called motivated reasoning. Experiments in which people are shown a study supporting their position and a study opposing it, of equal quality, typically end with both sides more convinced than before, each having found the flaws in the other's study only.
Where it does damage
In medicine, a doctor who forms an early diagnosis tends to order tests that would confirm it and to discount results that do not, which is why diagnostic checklists force alternatives to be considered. In criminal investigation, a suspect identified early attracts the evidence and other lines go cold, and several notorious wrongful convictions followed exactly that path. In science, researchers find what they expected in noisy data, one reason replication fails so often and why the best trials are designed so that nobody analysing the results knows which group is which. In everyday life it is the reason a news feed that shows people what they already agree with is so comfortable and so narrowing.
What limits it
Knowing about the bias helps less than one would hope, since people readily see it in others and rarely in themselves. What does work is structure. Actively asking what evidence would change one's mind, and looking for it, is the direct antidote, and Wason's subjects who were prompted to seek disconfirmation found the rule. Writing down a prediction before seeing the result removes the room to reinterpret. Working in groups that contain disagreement uses the bias against itself, since each side searches hard for the flaws in the other's case, which is the principle behind adversarial trials, peer review and a good editorial meeting. The scientific method itself, with its insistence on the test that could fail, is the most successful institutional answer to a flaw nobody escapes individually.
The takeaway
Confirmation bias is the tendency to search for, interpret and remember evidence in ways that support existing beliefs, demonstrated by Wason in the 1960s and found since in every field of judgement. It is strongest on questions bound up with identity, it is not reduced by intelligence, and it is best countered not by willpower but by structures that force the search for disconfirming evidence: predictions made in advance, blinded analysis, and disagreement built into the process.