The Cognitive Bias Library

The interactive guide to cognitive biases, a Decision-DNA test, a daily spot-the-bias puzzle, and deep, research-grounded pages on 130 biases.

Decision Making

• Anchoring Bias, The tendency to rely too heavily on the first piece of information offered (the 'anchor') when making decisions.
• Sunk Cost Fallacy, The phenomenon where a person is reluctant to abandon a strategy or course of action because they have invested heavily in it, even when it is clear that abandonment would be more beneficial.
• Framing Effect, The bias whereby people decide on options based on whether the options are presented with positive or negative connotations; e.g. as a loss or as a gain.
• Zero-Risk Bias, A tendency to prefer the complete elimination of a risk in a sub-part of a problem even when the overall risk reduction is smaller.
• Decoy Effect, The phenomenon where consumers will tend to have a specific change in preference between two options when also presented with a third option that is asymmetrically dominated.
• Endowment Effect, The tendency for people to ascribe more value to things merely because they own them.
• Hyperbolic Discounting, The tendency for people to increasingly choose a smaller-sooner reward over a larger-later reward as the delay occurs sooner rather than later.
• IKEA Effect, A cognitive bias in which consumers place a disproportionately high value on products they partially created.
• Status Quo Bias, A preference for the current state of affairs. The current baseline (or status quo) is taken as a reference point, and any change from that baseline is perceived as a loss.
• Paradox of Choice, The observation that having many options to choose from, rather than making people happy and ensuring they get what they want, can cause them stress and problematize decision-making.
• Planning Fallacy, The tendency to overestimate the time, costs, and risks of future actions and at the same time overestimate the benefits of the same actions.
• Choice-Supportive Bias, The tendency to retroactively ascribe positive attributes to an option one has selected and/or demote the forgone options.
• Information Bias, The tendency to seek information even when it cannot affect action.
• Ambiguity Effect, The tendency to avoid options for which missing information makes the probability seem 'unknown'.
• Bike-Shedding Effect, The tendency to give disproportionate weight to trivial issues. Also known as Parkinson's Law of Triviality.
• Unit Bias, The tendency to want to complete a unit of a given item or task.
• Loss Aversion, The tendency to prefer avoiding losses to acquiring equivalent gains.
• Outcome Bias, You judge the quality of a decision by how it turned out, not by whether it was smart when it was made.
• Impact Bias, Your tendency to overestimate how intensely and how long a future event will affect your emotions, in both directions.
• Empathy Gap, You badly misjudge how a different emotional or physical state (yours later, or someone else's now) reshapes decisions, because your current state feels like the only real one.
• Focusing Illusion, You wildly overweight whatever your attention lands on when predicting how it will affect your happiness, because the very act of thinking about it makes it loom larger than it ever will in real life.
• Omission Bias, You judge harm you caused by doing something as worse than the identical harm you caused by doing nothing, so you default to inaction even when acting would leave everyone better off.
• Regret Aversion, You make the choice that protects you from feeling stupid later, not the choice with the best expected outcome.
• Mental Accounting, You treat money differently depending on where it came from, where you keep it, or what you plan to spend it on, even though every dollar is identical and fully interchangeable.
• Restraint Bias, You overestimate how well you can control your own impulses (hunger, cravings, arousal, fatigue, the urge to spend), so you walk straight into temptation you should have avoided.
• Pseudocertainty Effect, The tendency to treat an outcome as guaranteed when it is actually only conditional, because a multi-stage problem was framed to hide the risk sitting one step earlier. You see "certain" where there is only "certain if you get lucky first," and you pay a premium for a certainty that does not exist.
• Distinction Bias, The tendency to overvalue small differences between options when you compare them side by side, then discover the "better" one barely moves your happiness once you actually live with it alone.
• Contrast Effect, The contrast effect is your brain judging something by comparison to whatever came right before it, not on its own merits. Put an average option next to an extreme one and the average shifts: a $40 shirt feels cheap after you looked at the $200 one, and painful after the $15 one, even though it never changed.
• Less-Is-Better Effect, When you judge options one at a time instead of side by side, you sometimes rate the objectively worse option higher, because you fixate on whatever attribute is easy to feel (fullness, generosity, "no broken bits") and ignore the attribute that actually counts (total amount).
• Denomination Effect, You spend a pile of small bills faster than a single large bill worth exactly the same amount, because the big bill feels harder to break.
• Present Bias, Present bias is your tendency to overweight rewards and costs that hit right now and heavily discount anything that arrives later, so your preferences flip the moment "later" becomes "now." You genuinely plan to start the diet Monday, then Monday you eat the donut, because the near-term payoff balloons in value the instant it is within reach.
• Default Effect, The tendency to stick with whatever option is pre-selected for you, accepting the default instead of actively choosing, even when switching costs almost nothing and a different option would serve you better.
• Extremeness Aversion, Your tendency to reject the options at the edges of a set and grab the one in the middle, because "middle" feels safe, reasonable, and easy to defend, even when nothing about the options themselves changed.

Probability & Belief

• Confirmation Bias, The tendency to search for, interpret, favor, and recall information in a way that confirms or supports one's prior beliefs or values.
• Availability Heuristic, The tendency to overestimate the likelihood of events with greater 'availability' in memory, which can be influenced by how recent, unusual, or emotionally charged they may be.
• Survivorship Bias, The logical error of concentrating on the people or things that made it past some selection process and overlooking those that did not, typically because of their lack of visibility.
• Gambler's Fallacy, The mistaken belief that, if something happens more frequently than normal during a given period, it will happen less frequently in the future.
• Base Rate Fallacy, If presented with related base rate information (i.e. generic, general information) and specific information (information only pertaining to a certain case), the mind tends to ignore the former and focus on the latter.
• Optimism Bias, The tendency to overestimate the likelihood of positive events and underestimate the likelihood of negative events happening to oneself.
• Ostrich Effect, The tendency to avoid dangerous or negative information. We bury our heads in the sand.
• Negativity Bias, The notion that, even of equal intensity, things of a more negative nature (e.g. unpleasant thoughts, emotions, or social interactions) have a greater effect on one's psychological state than neutral or positive things.
• Clustering Illusion, The tendency to erroneously consider the inevitable 'streaks' or 'clusters' arising in small samples from random distributions to be non-random.
• Conjunction Fallacy, The fallacy that occurs when it is assumed that specific conditions are more probable than a single general one.
• Barnum Effect, The tendency to accept certain information as true, such as character assessments or horoscopes, even when the information is so vague as to be worthless.
• Illusion of Control, The tendency for people to overestimate their ability to control events.
• Placebo Effect, A beneficial effect produced by a placebo drug or treatment, which cannot be attributed to the properties of the placebo itself, and must therefore be due to the patient's belief in that treatment.
• Belief Bias, The tendency to judge the strength of arguments based on the plausibility of their conclusion rather than how strongly they support that conclusion.
• Frequency Illusion, The phenomenon where the thing you just learned suddenly appears everywhere. Also known as the Baader-Meinhof phenomenon.
• Overconfidence Effect, The Overconfidence Effect is the systematic gap between how sure you feel about your judgments and how often you are actually right.
• Backfire Effect, The backfire effect is when a correction to a false belief makes you hold that false belief even more strongly than before.
• Illusory Truth Effect, The more often you hear a claim, the truer it feels, regardless of whether it is actually true.
• Hot-Hand Fallacy, The belief that a person on a streak of successes is "hot" and more likely to succeed on the next attempt, when the outcomes are actually independent.
• Illusory Correlation, Seeing a relationship between two things that are actually unrelated, because a few memorable co-occurrences stuck in your head and the non-events did not.
• Declinism, The gut conviction that things used to be better and are now sliding downhill, usually held with total confidence and almost no data.
• Neglect of Probability, You judge a risky choice by how bad (or how good) the outcome feels, and barely adjust for how likely it actually is.
• Semmelweis Reflex, The reflex-like tendency to reject new evidence because it contradicts what you already believe, no matter how strong the data is.
• Availability Cascade, A self-reinforcing loop where a belief becomes more believable simply because more people are repeating it, until "everyone knows it" replaces "anyone checked it."
• Normalcy Bias, The tendency to assume that because a disaster has never happened to you, it never will, so you underestimate both the odds of a catastrophe and how bad it will get once it starts.
• Belief Perseverance, Your conviction sticks around long after the evidence that built it gets torn down. Someone proves the foundation was fake, and you keep the house standing anyway.
• Automation Bias, Your tendency to trust an automated system's output over your own judgment, and over contradictory evidence sitting right in front of you, treating "the computer said so" as the end of the inquiry instead of the start.
• Regression Fallacy, The regression fallacy is when you credit (or blame) some intervention for a change that was going to happen anyway because of regression toward the mean. Extreme results tend to be followed by more average ones for purely statistical reasons, but you invent a cause: the treatment, the punishment, the pep talk, the new coach.
• Insensitivity to Sample Size, You judge how likely a result is by how it looks, and ignore how many data points produced it. A 60% coin-flip streak feels equally surprising whether it came from 10 flips or 10,000, even though the second is basically a miracle and the first is a Tuesday.
• Congruence Bias, The tendency to test your favorite hypothesis directly (does the evidence I expect show up?) while skipping the tests that would rule it out or point to a rival explanation. You run experiments designed to say yes, not experiments designed to decide.
• Subadditivity Effect, You judge the probability of a whole category to be less than the sum of the probabilities you assign to its parts. Break "death by natural causes" into cancer, heart attack, and other, and suddenly the pieces add up to way more than the lump did.
• Pessimism Bias, Pessimism bias is your tendency to overestimate the odds of bad outcomes and underestimate the odds of good ones, so your gut forecast for the future runs darker than the actual probabilities warrant.
• Hard-Easy Effect, The tendency to be overconfident when a task is hard and underconfident when it is easy. Your confidence barely moves when difficulty changes, so on brutal questions you claim to be far more accurate than you are, and on trivial ones you sell yourself short. Confidence stays roughly flat while actual accuracy swings wildly, and the gap is the bias.
• Nocebo Effect, The nocebo effect is when negative expectations alone produce real, physical harm: you feel worse, hurt more, or suffer side effects because you believed you would, even though the "cause" is inert or harmless. It is the placebo effect's evil twin, running in reverse.
• Anthropomorphism, Anthropomorphism is your tendency to attribute human minds, intentions, emotions, and personalities to things that do not have them: your car, your Roomba, the weather, the stock market, a chatbot, God. You do not just say the printer is "acting up" as a figure of speech. On some level you believe it is out to get you.

Social

• Dunning-Kruger Effect, A cognitive bias stating that people with low ability at a task overestimate their ability.
• Halo Effect, The tendency for positive impressions of a person, company, brand, or product in one area to positively influence one's opinion or feelings in other areas.
• Bandwagon Effect, The tendency to do (or believe) things because many other people do (or believe) the same.
• Fundamental Attribution Error, The tendency for people to under-emphasize situational explanations for an individual's observed behavior while over-emphasizing dispositional and personality-based explanations.
• Authority Bias, The tendency to attribute greater accuracy to the opinion of an authority figure (unrelated to its content) and be more influenced by that opinion.
• Groupthink, A psychological phenomenon that occurs within a group of people in which the desire for harmony or conformity in the group results in an irrational or dysfunctional decision-making outcome.
• False Consensus Effect, A cognitive bias whereby a person tends to overestimate the extent to which their opinions, beliefs, preferences, values, and habits are normal and typical of those of others.
• In-Group Bias, The tendency to give preferential treatment to others they perceive to be members of their own group.
• Spotlight Effect, The phenomenon in which people tend to believe they are being noticed more than they really are.
• Ben Franklin Effect, A person who has performed a favor for someone is more likely to do another favor for that person than they would be if they had received a favor from that person.
• Bystander Effect, The phenomenon where individuals are less likely to offer help to a victim when other people are present.
• Curse of Knowledge, A cognitive bias that occurs when an individual, communicating with other individuals, unknowingly assumes that the others have the background to understand.
• Self-Serving Bias, The tendency to attribute positive events to one's own character but attribute negative events to external factors.
• Just-World Hypothesis, The cognitive bias to assume that actions will have morally fair and fitting consequences; that 'you get what you deserve'.
• Identifiable Victim Effect, The tendency to offer greater aid when a specific, identifiable person ('victim') is observed under hardship, as compared to a large, vaguely defined group with the same need.
• Mere Exposure Effect, The tendency to like something more just because you have seen or heard it before, with no new information and no reason attached.
• Actor-Observer Bias, The tendency to blame your own bad behavior on the situation while blaming other people's identical behavior on their character.
• Reactance, Reactance is the urge to do the opposite of what you are told, because a rule or push feels like a threat to your freedom.
• Pygmalion Effect, The Pygmalion Effect is when someone performs better or worse because a person in power expected them to, and that expectation quietly reshaped how they were treated.
• Naive Realism, The conviction that you see the world exactly as it is, so anyone who disagrees with you must be uninformed, irrational, or biased.
• Moral Licensing, Doing one good thing gives you psychological permission to do a bad thing right after, because your "good person" account feels topped up.
• Social Desirability Bias, You shade your answers toward what makes you look good, so self-reports drift toward the flattering version of you instead of the real one.
• Third-Person Effect, You think media, ads, and propaganda barely touch you but heavily warp everyone else, so "they" need protection you don't.
• Outgroup Homogeneity Bias, You see your own group as full of distinct individuals and the other group as a blur of interchangeable copies. "We" are diverse; "they" are all the same.
• Reactive Devaluation, You rate a proposal as worse the moment you learn your opponent is the one offering it. Same terms, same numbers, different messenger, and suddenly the deal stinks.
• Ultimate Attribution Error, The tendency to explain away an entire group's behavior in a way that always keeps your side looking good: when one of "them" does something bad, it proves they are all like that; when one of "them" does something good, it was luck, a fluke, or an exception that doesn't count.
• Horn Effect, The horn effect is when one negative trait you notice about a person poisons your judgment of everything else about them. Spot a weak handshake, a typo, or an unfashionable jacket, and your brain quietly downgrades their intelligence, competence, and character too.
• Pratfall Effect, The tendency to find a competent person MORE likable after they commit a small blunder, and a mediocre person LESS likable after the exact same slip.
• Cheerleader Effect, The tendency to rate a face as more attractive when it appears in a group than when that same face is seen alone. Your brain averages the group and drags each member toward that flattering average.
• Golem Effect, When someone in authority expects little of you, they treat you in ways that quietly wreck your performance, and you sink to meet the low bar. It is the evil twin of the Pygmalion effect: a negative self-fulfilling prophecy delivered by a boss, teacher, or coach.
• Not-Invented-Here Syndrome, The tendency to reject, downgrade, or ignore an idea, tool, or solution mainly because it came from outside your group, even when the outside version is cheaper, faster, and better. You are not evaluating the thing. You are evaluating who made it, and marking down anything with someone else's fingerprints on it.
• Trait Ascription Bias, The tendency to see yourself as flexible, situational, and hard to pin down, while seeing everyone else as a walking bundle of fixed traits you can predict on sight.
• Illusion of Asymmetric Insight, Your conviction that you see through other people, and through opposing groups, more clearly than they see through you, and more clearly than they even see themselves.
• Bias Blind Spot, The bias blind spot is your tendency to spot cognitive and motivational biases in everyone but yourself. You readily admit that other people are swayed by self-interest, first impressions, and tribal loyalty, yet you rate yourself as unusually objective, as if you got a factory upgrade that skipped the rest of the species.
• Better-Than-Average Effect, The tendency to rate yourself as better than the typical person on desirable traits and abilities, even when the math makes that impossible for most of the people claiming it.
• False Uniqueness Effect, The tendency to underestimate how many other people share your desirable traits and abilities, so you convince yourself your good qualities are rarer than they actually are. You recycle less than you think you do relative to everyone else, you assume fewer people are as honest or as generous as you, and you quietly rate yourself in a smaller, more special club than the data supports. It is the mirror image of the false consensus effect, which does the same self-flattering trick for your bad habits by making them feel common and normal.
• Naive Cynicism, Naive cynicism is your tendency to assume other people are more selfishly biased and self-serving than they actually are, while assuming you yourself see the world straight. You expect everyone else to grab credit, dodge blame, and rig their judgments in their own favor, and you are usually wrong about how much they do it.

Memory

• Hindsight Bias, The common tendency for people to perceive events that have already occurred as having been more predictable than they actually were.
• Peak-End Rule, People judge an experience largely based on how they felt at its peak (i.e., its most intense point) and at its end, rather than based on the total sum or average of every moment of the experience.
• Google Effect (Digital Amnesia), The tendency to forget information that can be found readily online by using Internet search engines.
• Misinformation Effect, The impairment in memory for the past that arises after exposure to misleading information.
• Zeigarnik Effect, The tendency to remember uncompleted or interrupted tasks better than completed ones.
• Recency Bias, The tendency to weigh the latest information more heavily than older data.
• Primacy Effect, The tendency to recall the first items in a list better than items in the middle.
• Rosy Retrospection, The psychological phenomenon of people sometimes judging the past disproportionately more positively than they judge the present.
• Cryptomnesia, Occurs when a forgotten memory returns without it being recognized as such by the subject, who believes it is something new and original.
• Context-Dependent Memory, The improved recall of specific information when the context present at encoding and retrieval are the same.
• Spacing Effect, The phenomenon whereby learning is greater when studying is spread out over time, as opposed to studying the same amount of content in a single session.
• Von Restorff Effect, Your memory grabs whatever breaks the pattern. Put nine identical items next to one oddball, and the oddball is the one you'll recall later.
• Testing Effect, The Testing Effect is the finding that retrieving information from memory (taking a test, using a flashcard, recalling an answer) locks it in far better than passively re-reading it, even though re-reading feels like the smarter move.
• Telescoping Effect, You misjudge when past events happened, dragging recent ones farther back and pulling old ones closer to now.
• Tip-of-the-Tongue Phenomenon, The maddening state where you know you know a word, feel its retrieval is imminent, and can grab everything about it except the word itself. You get the first letter, the syllable count, the rhythm, a wrong word that keeps butting in, but the target stays just out of reach.
• Generation Effect, You remember things you produced yourself far better than the identical things you just read. Wrestle the answer out of your own head and it sticks; let someone hand it to you and it slides off.
• Picture Superiority Effect, You remember pictures far better than words. Show someone an image and a label for the same thing, and the image wins at recall by a wide margin, often more than double.
• Flashbulb Memory, A vivid, confident, seemingly photographic memory of the moment you learned about a shocking public event. The catch: it is your memory of hearing the news, not the event itself, and it is far less accurate than it feels.
• Next-in-Line Effect, You remember almost nothing that happened in the roughly 10 seconds before it was your turn to speak, because your brain hijacked its own attention to rehearse what you were about to say.
• Self-Reference Effect, The Self-Reference Effect is your tendency to remember information much better when you relate it to yourself than when you process it any other way. Ask "does this word describe me?" and you will recall it later; ask "is this word in capital letters?" and it evaporates.
• Modality Effect, Your memory for the last item or two in a list is noticeably better when you heard the list than when you read it. Sound sticks around in your head a beat longer than print, and that leftover echo props up the tail end of what you just took in.
• Levels-of-Processing Effect, The tendency to remember information far better when you engage with its meaning than when you skim its surface. How deeply you process something at the moment you encounter it, not how long you stare at it or how hard you "try," predicts whether you will recall it later.
• Suggestibility, Suggestibility is the tendency for your memory of an event to get quietly rewritten by information that arrives after the fact: a leading question, a confident bystander, a suggestive interview, a news report. You do not just misremember. You incorporate the suggestion into the memory itself and then recall it with full confidence, as if you saw it with your own eyes.
• Serial Position Effect, Your memory for a list is lopsided: you reliably recall the first few items and the last few items, and the stuff in the middle falls into a hole. Position on the list, not content, decides what survives.
• Humor Effect, The tendency to remember funny material far better than the boring, worthy stuff sitting right next to it.