Seeing what you expect: confirmation bias
We look for, notice and remember evidence that fits what we already believe. Learn to test your ideas by looking for what would prove them wrong.
PsychologyThink clearlyAbout 7 min: 4 reading, 3 practiceUpdated
Educational, not medical or mental-health advice.
- Explain confirmation bias and the three places it acts (searching, interpreting, remembering)
- Test an idea by looking for evidence against it
- Ask "what would change my mind?" before deciding
Builds toward: Think clearly. Catch common thinking errors in yourself and in other people's arguments, and make better calls when you can't be sure.
Think clearly: thinking errors and better decisions: lesson 3 of 7
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Pairs with Nickerson (1998), "Confirmation bias: A ubiquitous phenomenon in many guises"; Wason (1960); OpenStax, Psychology 2e, section 7.3 "Problem Solving"
What it is
Confirmation bias is the tendency to look for, interpret and remember information in ways that support what you already believe. It works at three points:
- Searching: you look for evidence that would fit your idea, not evidence that could break it.
- Interpreting: you read mixed evidence as support, and pick holes only in evidence against you.
- Remembering: the hits stick; the misses fade.
Quick check
Tap an answer to check it.
Priya believes her horoscope is accurate. She remembers the weeks it was right and forgets the rest. Which part of confirmation bias is this?
- She isn't choosing what to look at; she is choosing what to remember
Show answer
Remembering: hits are recalled, misses fade. Selective memory makes a belief look better supported than it is.
Why it matters
The 2-4-6 puzzle. In a classic study (Wason, 1960), people were told that the numbers 2, 4, 6 follow a rule, and that they could test any three numbers to find it. A typical guess was "even numbers going up by two", tested with 8, 10, 12, then 20, 22, 24. Every test said yes, so people announced their rule, and most first announcements were wrong. The real rule was simply "any three numbers in increasing order". Their tests could only ever confirm. A test like 1, 2, 3 or 6, 4, 2, which might have said no, would have taught them more.
Same evidence, opposite conclusions. Lord, Ross and Lepper (1979) gave people who supported and people who opposed the death penalty the same two studies: one seemed to show it deterred crime, the other that it didn't. Each side rated the study that agreed with them as better done, and both sides reported that their views had become stronger. Mixed evidence seemed to push them further apart.
Confirmation bias helps explain why doctors can stick with a first diagnosis, why sports fans see the referee as biased against their team, and why arguments online rarely change minds. It feels like thinking, but it is mostly collecting.
Worked example
You're sure your new colleague, Tom, doesn't like you: he didn't say hello on Monday.
- State the belief as something you can test: "Tom doesn't like me."
- Ask what you would expect to see if it were false. He'd help when asked, include you in emails, chat at lunch sometimes.
- Look for that evidence on purpose. This week: he answered your question in detail on Tuesday and copied you in on Wednesday's email. He didn't say hello on Monday, but he also missed two other people; he arrived late.
- Weigh both sides. One ambiguous moment against several friendly ones. The evidence doesn't support your belief.
The key step is 2: deciding in advance what evidence would count against you.
Common mistake
Searching in a way that can only agree. Typing "proof that coffee is bad for you" finds proof that coffee is bad for you. Search with neutral words ("coffee health effects research review"), or search for both sides, and notice which results you are tempted to skip.
Key terms
- confirmation bias
- seeking, interpreting and remembering evidence in ways that support what you already believe
- disconfirming evidence
- evidence that could show a belief is wrong
- belief polarisation
- when people on both sides see the same mixed evidence and become more certain
- steelman
- the strongest version of the view you disagree with
Practise the key terms
Tap a card to turn it over. 4 cards.
| Front | Back |
|---|---|
| confirmation bias | seeking, interpreting and remembering evidence in ways that support what you already believe |
| disconfirming evidence | evidence that could show a belief is wrong |
| belief polarisation | when people on both sides see the same mixed evidence and become more certain |
| steelman | the strongest version of the view you disagree with |
Practice
Quick quiz
Tap an answer to check it.
In Wason's 2-4-6 puzzle, why were most first answers wrong?
- The rule was simple; the tests just couldn't reveal it
Show answer
They only tested numbers that fitted their guess, so every answer said yes. Tests that can only confirm never show you where your idea is wrong.
Which test would teach you most if you guessed "even numbers going up by two"?
- This fits your guess, so a yes tells you nothing new
- Also fits your guess
Show answer
1, 2, 3. A test that breaks your rule but might still get a yes is the one that can prove you wrong.
In Lord, Ross and Lepper's study, what happened after both sides read the same two studies?
- That is what you'd expect from mixed evidence, but it isn't what they reported
Show answer
Both sides reported that their views had become stronger. Each side judged the agreeing study better done, and reported becoming more certain.
What is the best question to ask before deciding something you feel sure about?
- That is confirmation bias in action
Show answer
What evidence would change my mind?. Deciding in advance what would count against you makes it possible to notice it.
Sort it
Which group does this belong to?
Only reading reviews of the phone you've already chosen
You choose what evidence to look atShow answer
Searching. You choose what evidence to look at
Calling a study "badly done" only when it disagrees with you
You judge evidence by whether it agreesShow answer
Interpreting. You judge evidence by whether it agrees
Recalling the times your lucky socks worked, not the times they didn't
Hits stick, misses fadeShow answer
Remembering. Hits stick, misses fade
Following only accounts that share your politics
Your sources can only agree with youShow answer
Searching. Your sources can only agree with you
The other view
Looking for cases that fit your idea is not always a mistake. Klayman and Ha (1987) showed that this "positive test" strategy works well in many real situations; it failed in the 2-4-6 puzzle partly because the true rule was so much broader than people's guesses. The problem is not testing cases that fit; it is never testing cases that could prove you wrong.
The "both sides got more certain" result also needs care: in Lord, Ross and Lepper's study it was people's own report of how their views had changed. Later studies that measured views before and after (for example Guess and Coppock, 2020) found that people usually shift a little towards evidence they read, even when they disagree with it. Mixed evidence rarely changes minds much, but it seldom pushes people further away.
In short
- Confirmation bias: we search for, interpret and remember evidence that fits what we believe.
- In Wason's 2-4-6 puzzle, people tested only what fitted their guess, so they never found their mistake.
- Mixed evidence can leave both sides feeling more certain (Lord, Ross and Lepper, 1979).
- Defences: decide what would change your mind, look for that evidence on purpose, and search with neutral words.
- Testing cases that fit is fine; never testing cases that could prove you wrong is the problem.
Mastery check
Answer without looking back. Score 80% or more and the lesson is marked done. Your result is kept only in this browser.
Check yourself
Answer in your head first, then open the answer. Nothing is saved.
Name the three points at which confirmation bias works.
Show answer
Searching for evidence, interpreting it, and remembering it.
What was the real rule in Wason's 2-4-6 puzzle?
Show answer
Any three numbers in increasing order.
You're convinced a diet works. What would a fair test look like?
Show answer
Decide in advance what result would show it doesn't work, then look for good studies or records on both sides, including results that went against it, rather than only success stories.
Further reading
- Wason, P. C. (1960). On the failure to eliminate hypotheses in a conceptual task. Quarterly Journal of Experimental Psychology, 12(3), 129–140.
- Lord, C. G., Ross, L., & Lepper, M. R. (1979). Biased assimilation and attitude polarization: The effects of prior theories on subsequently considered evidence. Journal of Personality and Social Psychology, 37(11), 2098–2109.
- Klayman, J., & Ha, Y.-W. (1987). Confirmation, disconfirmation, and information in hypothesis testing. Psychological Review, 94(2), 211–228.
- Guess, A., & Coppock, A. (2020). Does counter-attitudinal information cause backlash? Results from three large survey experiments. British Journal of Political Science, 50(4), 1497–1515.
- Nickerson, R. S. (1998). Confirmation bias: A ubiquitous phenomenon in many guises. Review of General Psychology, 2(2), 175–220.
- OpenStax, Psychology 2e, section 7.3, free online.
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