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    Thinking Dictionary

    Fast and slow thinking

    What it is

    We have two broad ways of thinking.

    • Fast thinking is quick and automatic. Recognising a friend's face, or finishing the phrase "bread and…," happens without effort.
    • Slow thinking is deliberate and takes effort. Working out 17 × 24 in your head, or comparing two phone contracts, needs it.

    The psychologist Daniel Kahneman made this idea famous in his 2011 book Thinking, Fast and Slow.¹ He described the two modes as "System 1" and "System 2." These are a helpful way of talking, not two separate parts of the brain.

    Fast thinking runs most of our lives very well. But it can be confidently wrong. The skill is noticing when a situation calls for slowing down.

    A classic puzzle

    Try to answer this quickly:

    A bat and a ball cost 1.10 in total. The bat costs 1.00 more than the ball. How much does the ball cost?

    Many people's first answer is 0.10. It feels right. But check it: if the ball cost 0.10, the bat would cost 1.10, and together they would cost 1.20. The right answer is 0.05. The ball costs 0.05, the bat costs 1.05, and the total is 1.10.

    This puzzle comes from a short test designed to measure whether people stop and check an answer that pops into their head.² It shows how fast thinking can serve up a wrong answer that feels completely right.

    When can you trust your gut?

    Sometimes fast, intuitive judgement is excellent. Think of an experienced firefighter sensing danger, or a nurse who spots that a patient is getting worse before the tests show it.

    Research suggests that expert intuition is reliable only under two conditions. First, the situation must be regular enough to learn from, with real patterns. Second, the person must have had lots of practice with clear feedback on whether they were right.³ A doctor who sees thousands of rashes and learns which ones were serious can develop good intuition. Someone making one-off predictions with little feedback has far less chance to build that kind of skill.

    And feeling confident is not a reliable sign that you are right.³

    A warning about some famous findings

    Some well-known studies often told alongside fast and slow thinking have not held up when other scientists tried to repeat them. Repeating a study to check its result is called replication.

    • "Willpower runs out like fuel." This idea, called ego depletion, claimed that self-control is a limited resource that gets used up. Two very large replication attempts, run across dozens of labs with thousands of people, found little or no effect.⁴˒⁵
    • Priming behaviour. One famous study claimed that reading words linked to old age made people walk more slowly. A later attempt didn't reproduce it, except when the experimenters expected to see the effect.⁶ Another set of studies found that being reminded of "professors" did not make people score higher on a quiz.⁷ Kahneman, who had featured priming in his book, later publicly urged psychologists to tackle these doubts.⁸
    • Hard-to-read fonts. A claim that hard-to-read text makes people think more carefully also failed when many studies were pooled together.⁹

    None of this undermines the basic distinction between fast and slow thinking, or the bat-and-ball puzzle. But be wary of anyone who presents these specific effects as settled science.

    In everyday life

    • Shopping. "Was 99, now 49!" Fast thinking sees a bargain. Slow thinking asks whether you need it, and what it cost elsewhere.
    • Headlines. A shocking headline triggers a quick reaction. Slowing down means reading past the headline before sharing it.
    • Big decisions. Choosing a mortgage or a medical treatment deserves slow thinking. Choosing a sandwich usually doesn't.

    The common trap

    The trap is trusting an answer because it feels obvious. Fast thinking is good at producing answers that feel right, whether or not they are.²

    Some ways out:

    • Check quick answers when the stakes are high. Ask: "How would I know if this is wrong?"
    • Ask whether your gut has earned trust here. Have you had lots of practice with clear feedback in this kind of situation?³
    • Notice the feeling of certainty. Feeling sure is not evidence on its own.

    Try it

    Ask a friend or family member the bat-and-ball puzzle, and tell them to answer quickly. Then ask them to check their answer. Did their first answer change?

    Afterwards, write down three situations from your coming week where your first answer might deserve a second look. At the end of the week, look back. Did slowing down change anything?

    Related concepts

    Confirmation bias · Cognitive shortcuts · Actively open-minded thinking · Probabilistic reasoning · Credible sources

    Sources

    1. Kahneman, D. (2011). Thinking, fast and slow. Farrar, Straus and Giroux.

    2. Frederick, S. (2005). Cognitive reflection and decision making. Journal of Economic Perspectives, 19(4), 25–42. https://doi.org/10.1257/089533005775196732

    3. Kahneman, D., & Klein, G. (2009). Conditions for intuitive expertise: A failure to disagree. American Psychologist, 64(6), 515–526. https://doi.org/10.1037/a0016755

    4. Hagger, M. S., Chatzisarantis, N. L. D., Alberts, H., Anggono, C. O., Batailler, C., Birt, A. R., Brand, R., Brandt, M. J., et al. (2016). A multilab preregistered replication of the ego-depletion effect. Perspectives on Psychological Science, 11(4), 546–573. https://doi.org/10.1177/1745691616652873

    5. Vohs, K. D., Schmeichel, B. J., Lohmann, S., Gronau, Q. F., Finley, A. J., Ainsworth, S. E., Alquist, J. L., Baker, M. D., et al. (2021). A multisite preregistered paradigmatic test of the ego-depletion effect. Psychological Science, 32(10), 1566–1581. https://doi.org/10.1177/0956797621989733

    6. Doyen, S., Klein, O., Pichon, C.-L., & Cleeremans, A. (2012). Behavioral priming: It's all in the mind, but whose mind? PLoS ONE, 7(1), e29081. https://doi.org/10.1371/journal.pone.0029081

    7. Shanks, D. R., Newell, B. R., Lee, E. H., Balakrishnan, D., Ekelund, L., Cenac, Z., Kavvadia, F., & Moore, C. (2013). Priming intelligent behavior: An elusive phenomenon. PLoS ONE, 8(4), e56515. https://doi.org/10.1371/journal.pone.0056515

    8. Yong, E. (2012). Nobel laureate challenges psychologists to clean up their act. Nature (news). https://doi.org/10.1038/nature.2012.11535

    9. Meyer, A., Frederick, S., Burnham, T. C., Guevara Pinto, J. D., Boyer, T. W., Ball, L. J., Pennycook, G., Ackerman, R., et al. (2015). Disfluent fonts don't help people solve math problems. Journal of Experimental Psychology: General, 144(2), e16–e30. https://doi.org/10.1037/xge0000049