The Casebook
The cognitive bias casebook
A working register of the reasoning patterns that quietly distort serious writing. Each case records how the pattern operates, the conditions it favours, what its presence looks like on the page, and one practical counter — the same 175 cards every audit is scored against.
175 cases on file
Need To Act Fast
Pressure to decide and move turns confidence into a substitute for evidence.
Actor-Observer BiasWe explain our own behavior by the situation but other people's similar behavior by their character.Ambiguity EffectWe avoid options when the odds, rules, or outcome distributions are unclear, even if the expected payoff may be better.Appeal to NoveltyWe treat something as better mainly because it is new.Backfire EffectWhen a belief is tied to identity or status, contradictory evidence can sometimes strengthen commitment instead of weakening it.Barnum EffectPeople accept vague, flattering descriptions as uniquely accurate for themselves.Belief BiasWe judge an argument by whether we like its conclusion, not by whether its logic is sound.Conjunction FallacyWe judge a detailed, specific scenario as more likely than a broader, simpler one that actually contains it.Decoy EffectAdding a third option that is clearly worse than one option can shift people toward that favored option.Defensive Attribution HypothesisWhen something bad happens to someone else, we explain it in ways that help us feel safer or less personally vulnerable.Disposition EffectPeople tend to sell winners too early and hold losers too long.Dunning-Kruger EffectPeople with low skill in a domain often overestimate their performance because they lack the knowledge needed to spot their own mistakes.Effort JustificationWe value an outcome more because we worked hard, paid dearly, or suffered to get it.Egocentric BiasWe overweight our own perspective, effort, and role when judging shared events.Endowment EffectOnce we own something, we value it more than we did before owning it.Escalation of CommitmentWe intensify commitment to a bad decision after negative feedback instead of cutting losses.False Consensus EffectWe overestimate how much other people share our beliefs, preferences, and habits.Fundamental Attribution ErrorWe overattribute other people's behavior to personality and underattribute it to situation.Generation EffectWe remember and often value ideas more when we generate them ourselves rather than simply receive them.Hard-Easy EffectOn hard tasks we are usually too confident, and on easy tasks we are often not confident enough.Hyperbolic DiscountingWe strongly prefer smaller-sooner rewards over larger-later rewards, especially when both options are close to the present.Identifiable Victim EffectWe are more moved to help one vivid, named person than many anonymous people represented statistically.IKEA EffectWe overvalue things we partly built ourselves.Illusion of ControlWe overestimate how much our actions can influence outcomes that are largely random or system-driven.Illusory SuperiorityWe judge our own abilities or qualities as better than they really are relative to others or to objective standards.Information BiasWe seek more information even when it is unlikely to improve the decision.Lake Wobegon EffectMost people rate themselves as above average on desirable qualities, even when that cannot be true for everyone.Law of TrivialityGroups spend disproportionate time on easy, low-stakes details and too little on hard, high-stakes issues.Less-is-Better EffectWhen options are judged separately, a smaller or objectively worse option can seem better if it scores higher on an easy-to-judge cue.Loss AversionLosses usually hurt more than equivalent gains feel good, so we work harder to avoid losses than to pursue gains.Occam's RazorWe can overprefer the simplest explanation or plan because simplicity feels elegant and manageable, even when reality is more complex.Optimism BiasWe expect our future to go better than base rates justify, especially for risks, timelines, and outcomes under our control.Overconfidence EffectPeople's confidence in their judgments often exceeds their actual accuracy, especially for predictions, estimates, and plans.Peltzman EffectSafety rules or protective measures can lead people to behave less cautiously, reducing the net gain from the intervention.Processing Difficulty EffectInformation that is slightly harder to process can receive deeper attention, making it feel more memorable, serious, or earned than equally…Pseudocertainty EffectIn multi-stage decisions, we overweight options that remove uncertainty at one stage even when total odds are no better.ReactanceWe push back when we feel our freedom to choose is being limited.Reverse PsychologyPeople may choose the opposite of what they are pushed toward, simply to reassert autonomy.Rhyme-as-Reason EffectStatements that rhyme are judged as more truthful or wise than equivalent non-rhyming statements.Risk CompensationWhen people feel safer, they often take more risk, offsetting some of the intended safety benefit.Self-Serving BiasWe credit our successes more to skill and effort, and our failures more to bad luck or circumstance.Social Comparison BiasWe evaluate ourselves and our choices relative to nearby others, often letting status comparisons outweigh absolute value.Social Desirability BiasPeople report attitudes or behaviors that make them look good to others instead of what is most accurate or true.Status Quo BiasWe prefer the current state or default, even when better alternatives exist.Sunk Cost FallacyWe continue a failing course of action because we have already invested time, money, or effort in it.System JustificationPeople tend to defend existing systems and arrangements as fair, natural, or necessary even when they are flawed.Third-Person EffectWe tend to believe persuasive messages, misinformation, or manipulation affect other people more than they affect us.Trait Ascription BiasWe see other people's behavior as reflecting fixed traits while treating our own behavior as more flexible and context-dependent.Unit BiasWe assume the provided unit, package size, or chunk is the right amount to consume, buy, or complete.Zero-Risk BiasWe prefer eliminating a small risk completely over achieving a larger total reduction in risk.
Not Enough Meaning
Faced with gaps, the mind fills them with a coherent story rather than the missing facts.
Anecdotal FallacyAnecdotal fallacy is letting one or two vivid stories outweigh broader and better-quality evidence.AnthropomorphismAnthropomorphism is attributing human motives, feelings, or intentions to nonhuman systems, objects, or abstractions.Appeal to ProbabilityAppeal to probability fallacy is assuming that because something could happen or is plausible, it probably will happen.Argument from FallacyArgument from fallacy is rejecting a conclusion simply because one argument for it is flawed.Authority BiasAuthority bias is giving extra weight to a view because it comes from a person with status, title, or prestige.Automation BiasAutomation bias is trusting automated output too much and neglecting contrary evidence or obvious context.Bandwagon EffectBandwagon effect is adopting a belief or behavior because many other people appear to hold it.Cheerleader EffectCheerleader effect is the tendency for people to seem more attractive or appealing when seen in a group than when evaluated alone.Clustering IllusionClustering illusion is seeing meaningful streaks or clumps in data that are actually compatible with randomness.ConfabulationConfabulation is unintentionally filling gaps in memory or explanation with details that feel true but were not actually observed or…Cross-Race EffectCross-race effect is the tendency to be worse at distinguishing faces of races one has had less experience individuating.Curse of KnowledgeCurse of knowledge is the difficulty of imagining what it is like not to know what you already know.DeclinismDeclinism is seeing the past as better than it was and the future as likely worse than it will be.Denomination EffectDenomination effect is being more reluctant to spend one large unit of money than an equivalent value split into smaller units.EssentialismEssentialism is believing categories of people or things have a deep fixed essence that explains how they behave.Extrinsic Incentive ErrorExtrinsic incentive error is assuming other people are driven mainly by money, perks, or pressure while undervaluing intrinsic motives like…Functional FixednessFunctional fixedness is seeing an object, team, or process only in its usual role and missing other workable uses.Gambler's FallacyGambler's fallacy is expecting a random process to reverse soon because one outcome has happened repeatedly.Group Attribution ErrorGroup attribution error is assuming what is true of one member is true of the whole group, or that group-level traits cleanly describe each…Halo EffectHalo effect is when one positive trait or first impression spills over into unrelated judgments about the same person or thing.Hindsight BiasHindsight bias is the tendency to feel, after an outcome is known, that it was predictable all along.Hot-Hand FallacyHot-hand fallacy is believing success will keep continuing because someone or something seems hot, beyond what the evidence supports.Illusion of Asymmetric InsightIllusion of asymmetric insight is believing you understand other people better than they understand you.Illusion of external agencyIllusion of external agency is misattributing your own actions, urges, or interpretations to outside agents or forces.Illusion of TransparencyIllusion of transparency is overestimating how much other people can tell what you are thinking or feeling.Illusion of ValidityIllusion of validity is feeling highly confident in a judgment because the evidence forms a neat story, even when it predicts poorly.Illusory CorrelationIllusory correlation is perceiving a relationship between two variables when the evidence is weak, selective, or absent.Impact BiasImpact bias is overestimating how intense and how long your future emotional reaction to an event will be.In-Group FavoritismIn-group favoritism is preferring, trusting, or rewarding people seen as part of your own group.Insensitivity to Sample SizeInsensitivity to sample size is treating small samples as if they are just as reliable as large ones.Just-World HypothesisJust-world hypothesis is the tendency to assume people generally get what they deserve and deserve what they get.Masked-Man FallacyMasked-man fallacy is treating differences in what is known about something under one description versus another as proof they are…Mental AccountingMental accounting is treating money or resources differently based on arbitrary labels instead of on total value and best use.Moral Credential EffectMoral credential effect is using a past moral act as psychological permission to behave less ethically later.Moral LuckMoral luck is judging people differently for similar choices because luck changed the eventual outcome.Murphy's Law (as cognitive bias)As a cognitive bias, Murphy's Law is the tendency to overexpect that things will go wrong, often in chains, because negative possibilities…Neglect of ProbabilityNeglect of probability is reacting to how vivid or scary an outcome is while giving too little weight to how likely it actually is.Normalcy BiasNormalcy bias is underestimating the possibility and impact of disaster because the present still feels normal.Not Invented HereNot invented here is rejecting outside ideas or solutions mainly because they came from outside the group.Out-Group Homogeneity BiasOut-group homogeneity bias is seeing people outside your group as more similar to each other than they really are.Outcome BiasOutcome bias is judging the quality of a decision mainly by its result instead of by the information and process available when it was made.PareidoliaPareidolia is seeing a meaningful image, signal, or message in vague or noisy input.Pessimism biasPessimism bias is expecting worse outcomes than the evidence justifies.Placebo EffectPlacebo effect is improvement caused by expectations and context rather than by the active ingredient alone.Planning FallacyPlanning fallacy is underestimating how long, costly, and messy future tasks will be even when similar tasks ran over before.Positivity EffectPositivity effect is giving relatively more weight to positive information or memories than negative ones, especially for people or brands…Pro-Innovation BiasPro-innovation bias is assuming a new technology or process is better simply because it is new.Projection BiasProjection bias is assuming your future self or other people will want what you want right now.Reactive DevaluationReactive devaluation is devaluing a proposal because it comes from a rival, opponent, or distrusted source.Recency IllusionRecency illusion is the feeling that something you only recently noticed must itself be new or newly common.Restraint BiasRestraint bias is overestimating your ability to resist temptation, stay calm, or stick to rules in the heat of the moment.Rosy RetrospectionRosy retrospection is remembering past experiences as better than they felt at the time.Self-Consistency BiasSelf-consistency bias is reconstructing past beliefs and behavior so they fit your current identity and story.Spotlight EffectSpotlight effect is overestimating how much other people notice and remember your appearance, mistakes, or behavior.StereotypingStereotyping is assigning traits or likely behavior to an individual based mainly on group membership.Subadditivity EffectSubadditivity effect is assigning higher total probability when a broad category is unpacked into specific parts.Survivorship BiasSurvivorship bias is drawing lessons from the cases that remain visible while missing the failures that disappeared from view.Telescoping EffectTelescoping effect is misremembering when past events happened, often pulling distant events closer and shifting recent ones around.The Magical Number Seven, Plus or Minus TwoThis bias is treating around seven items as a natural safe limit for thinking or design, even though working memory depends on chunking and…Time-Saving BiasTime-saving bias is misjudging how much time faster speed or throughput actually saves, usually by overvaluing gains at already high speeds…Ultimate Attribution ErrorUltimate attribution error is interpreting your own group's bad behavior as situational but another group's bad behavior as revealing…Well-Traveled Road EffectWell-traveled road effect is underestimating the time or difficulty of familiar routes and familiar tasks.Zero-Sum ThinkingZero-sum bias is assuming one person's or group's gain must come at another's expense, even when mutual gain is possible.
Too Much Information
A finite attention budget makes us over-weight what is vivid, recent, or already in mind.
AnchoringAn initial number, label, or piece of information pulls later estimates toward it, even when it is arbitrary or only weakly relevant.Attentional BiasWe selectively notice certain kinds of information while overlooking the rest, especially information tied to our worries, goals, or habits.Availability HeuristicWe judge how likely or common something is by how easily examples come to mind, not by actual frequency.Base Rate FallacyWe ignore general prevalence data and focus too much on case-specific details.Bias Blind SpotWe can see cognitive biases in other people more easily than in ourselves.Bizarreness EffectUnusual or bizarre information is remembered better than ordinary information.Choice-Supportive BiasWe remember the option we chose as better than it really was and the options we rejected as worse than they were.Confirmation BiasWe seek, interpret, and remember information in ways that support what we already believe.Congruence BiasWe test whether our favored idea fits instead of trying to find out whether it fails.Conservatism BiasWe revise beliefs too slowly when new evidence arrives.Context EffectOur judgment of an option shifts depending on what other options or surrounding cues are present.Continued Influence EffectOnce misinformation is encoded into a mental model, it can keep influencing reasoning even after it has been corrected.Contrast EffectSomething looks better or worse depending on what it is compared with immediately before or beside it.Cue-Dependent ForgettingInformation can be stored but hard to retrieve when the cues present at recall do not match the cues present during learning.Distinction BiasWhen options are compared side by side, we exaggerate small measurable differences that matter little in actual experience.Empathy GapWe underestimate how much behavior changes across emotional or visceral states and how hard it is to understand people in a different state…Focusing effectWhen one salient detail is in focus, it pulls judgment toward itself and crowds out other relevant factors.Framing EffectEquivalent information leads to different choices depending on how it is worded or packaged.Frequency IllusionAfter you notice or learn something once, you suddenly start seeing it everywhere and feel as if its frequency has increased.Humor EffectFunny material is remembered better than neutral material.Illusory Truth EffectRepeated statements start to feel true simply because they feel familiar.Mere-Exposure EffectWe tend to like things more after repeated exposure, even when the repetition provides no new value.Money IllusionWe think in nominal money terms and ignore inflation, purchasing power, or real value.Mood-Congruent Memory BiasWhen we are in a given mood, memories that match that mood come to mind more easily.Naive CynicismWe assume people who disagree with us are driven by selfish motives, hidden agendas, or bias more than they really are.Naive RealismWe experience our own view as a direct reading of reality, not as an interpretation.Negativity BiasNegative information has a stronger impact on attention, learning, and judgment than equally strong positive information.Observer-Expectancy EffectAn observer's expectations can subtly change what they notice, record, or even elicit from others.Omission BiasWe judge harmful actions as worse than equally harmful inactions, so doing nothing can feel morally safer even when outcomes are just as…Ostrich EffectWe avoid information that might be painful, threatening, or shame-inducing, especially when it could force action.Picture Superiority EffectPictures are usually remembered better than words.Selective PerceptionPeople perceive the same evidence differently because expectations, motives, and prior beliefs shape what they notice and how they…Self-Relevance EffectInformation tied to ourselves is encoded and recalled especially well.Semmelweis ReflexWe reflexively reject new evidence because it conflicts with established beliefs, norms, or identity.Subjective ValidationA statement feels accurate because it seems personally meaningful, even if it is vague or broadly applicable.Von Restorff EffectAn item that stands out from its surroundings is more likely to be noticed and remembered.Weber-Fechner LawWe notice differences in proportion to the starting level more than by absolute size.
What Should We Remember
What we keep and what we discard quietly rewrites the record we later reason from.
Absent-MindednessAbsent-mindedness is forgetting caused by weak attention during encoding or retrieval rather than by lack of capacity.CryptomnesiaCryptomnesia is when a forgotten memory returns but feels like a new original idea.Duration NeglectDuration neglect is the tendency to pay too little attention to how long an experience lasted when later evaluating it.Fading Affect BiasFading affect bias is the tendency for the emotional intensity of unpleasant memories to fade faster than that of pleasant memories.False MemoryA false memory is a recollection of an event or detail that did not happen, or did not happen the way it is remembered.Google EffectThe Google effect is the tendency to remember where to find information more readily than the information itself when we expect it to be…Implicit StereotypeStereotypical bias is the tendency to remember, interpret, and judge people through broad category-based expectations instead of…Leveling and SharpeningLeveling and sharpening describes how memories and retellings simplify some details while exaggerating others.Levels-of-Processing EffectThe levels-of-processing effect is the finding that information processed for meaning is remembered better than information processed only…Memory InhibitionMemory inhibition refers to the suppression or reduced accessibility of some memories when others are activated or retrieved.Misattribution of MemoryMisattribution happens when you remember information or an event but attach it to the wrong person, place, time, or context.Misinformation EffectThe misinformation effect is the distortion of memory after exposure to misleading post-event information.Modality EffectThe modality effect is the tendency for memory performance to differ depending on whether information is presented visually, auditorily, or…Next-in-Line EffectThe next-in-line effect is the tendency to remember less about the person or event immediately before your own turn in a sequence.Part-set cueing effectThe part-set cueing effect is the paradox that giving people some items from a studied list as cues can make them recall the remaining…Peak-End RuleThe peak-end rule is the tendency to judge an experience mainly by its most intense moment and how it ended, rather than by its total…PrejudicePrejudice is a pre-judgment about a person or group that persists even when the specific evidence is limited, mixed, or contradictory.Primacy EffectThe primacy effect is the tendency to remember items presented early in a sequence better than those in the middle.Recency EffectThe recency effect is the tendency to remember the most recently presented items better than earlier ones, especially in immediate recall.Serial-Position EffectThe serial-position effect is the overall tendency to remember items at the beginning and end of a sequence better than items in the middle.Source ConfusionSource confusion is a memory error in which you correctly remember information but cannot accurately identify whether it came from direct…Spacing EffectThe spacing effect is the finding that information is remembered better when study or exposure is spread out over time rather than massed…Suffix EffectThe suffix effect is the drop in recall for the last items in an auditory list when an extra, irrelevant sound or word is added at the end.SuggestibilitySuggestibility is the tendency for memory to be altered by leading questions, social cues, or post-event information.Testing EffectThe testing effect is the finding that actively retrieving information from memory strengthens later retention more than simply restudying…Tip of the TongueThe tip of the tongue phenomenon is the feeling that a word or name is known and almost retrievable but temporarily inaccessible.
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