Cognitive biases

Cognitive biases are systematic errors in thinking that shape the judgements and decisions we make every day. Each entry below explains what the bias is, why it happens, real examples, and concrete ways to counter it.

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Confirmation bias

Confirmation bias is the tendency to seek, interpret, and remember information that confirms what you already believe, while discounting evidence that contradicts it — making beliefs feel better justified than the evidence actually warrants.

Sunk-cost fallacy

The sunk-cost fallacy is continuing a course of action because of resources already invested — time, money, or effort — even when quitting would be the better choice. Past costs that can’t be recovered should never drive future decisions, yet they do.

Dunning–Kruger effect

The Dunning–Kruger effect is the tendency for people with low competence in a domain to overestimate their ability — because the very skills needed to perform well are also the skills needed to recognize poor performance. Experts, conversely, often underrate themselves.

Anchoring bias

Anchoring bias is the tendency to rely too heavily on the first piece of information encountered — the “anchor” — when making decisions. Later judgements adjust away from it but rarely far enough, so the initial number quietly skews the outcome.

Survivorship bias

Survivorship bias is focusing on the people or things that made it through a selection process while overlooking those that didn’t — usually because the failures are invisible. It makes success look more achievable, and its causes clearer, than they really are.

Availability heuristic

The availability heuristic is judging how likely or frequent something is by how easily examples come to mind. Vivid, recent, or emotionally charged events feel more probable than they are, which systematically distorts our sense of risk.

Hindsight bias

Hindsight bias is the tendency, after an event, to see it as having been predictable all along. Once you know the outcome, your memory reshapes your earlier beliefs to fit it, making the past feel more obvious and inevitable than it was.

Halo effect

The halo effect is letting one positive trait — often attractiveness, likability, or success in one area — color your overall judgement, so a good impression in one dimension spills over into unrelated ones you haven’t actually assessed.

Loss aversion

Loss aversion is the tendency to feel the pain of a loss about twice as strongly as the pleasure of an equivalent gain. Losing €100 hurts more than gaining €100 delights — so we take irrational risks to avoid losses.

Framing effect

The framing effect is when the way information is presented — not its content — changes your decision. “Ninety percent survive” and “ten percent die” state the same fact, yet the first feels far more reassuring and shifts the choice.

Negativity bias

Negativity bias is the tendency for negative events, emotions, and information to affect us more strongly than equally intense positive ones. Bad feedback, losses, and threats grab more attention, weigh more heavily, and linger longer in memory.

Recency bias

Recency bias is the tendency to give the most recent events disproportionate weight in judgements and predictions — assuming what just happened will keep happening, while older but relevant information quietly fades from view.

Fundamental attribution error

The fundamental attribution error is the tendency to explain other people’s behaviour by their character but our own by circumstance. When someone else errs they’re “careless”; when we err, the situation forced our hand.

Self-serving bias

Self-serving bias is the tendency to take credit for successes but blame failures on outside forces. A win proves your skill; a loss was bad luck, other people, or circumstance — protecting self-esteem at the cost of accurate self-assessment.

Bandwagon effect

The bandwagon effect is the tendency to adopt beliefs or behaviours because many others already have. As something gains popularity, the rate of adoption rises — people follow the crowd, often regardless of the underlying evidence.

Optimism bias

Optimism bias is the tendency to overestimate the likelihood of good outcomes and underestimate the bad — believing you’re personally less at risk than others of negative events, from illness to project overruns.

The planning fallacy

The planning fallacy is the tendency to underestimate how long a task will take, how much it will cost, and how likely it is to go wrong — even when you know similar past tasks ran over. We plan for the smooth best case, not the messy real one.

The gambler’s fallacy

The gambler’s fallacy is the belief that past random events change the odds of future ones — that after a run of reds, black is "due." For independent events the probability resets every time; the coin has no memory.

Status-quo bias

Status-quo bias is the preference for things to stay the same — sticking with the current option or default simply because it’s the current one. Change feels like a risk and a loss, so we leave things as they are even when switching would help.

Authority bias

Authority bias is the tendency to over-trust and obey an authority figure regardless of the content of what they say. A title, uniform, or credential can override our own judgement — even when the authority is wrong or speaking outside their field.

Base-rate neglect

Base-rate neglect is the tendency to ignore how common or rare something is (the base rate) and judge by specific, vivid details instead. It’s why a positive test for a rare disease feels alarming even when most positives are false alarms.

Choice overload

Choice overload is the finding that too many options can make us less likely to decide, less satisfied with what we pick, and more prone to regret. Past a point, more choice subtracts from well-being rather than adding to it.

The peak-end rule

The peak-end rule is the finding that we judge an experience largely by how it felt at its most intense moment (the peak) and at its end — not by the sum or average of the whole. Its duration barely registers.

The curse of knowledge

The curse of knowledge is the difficulty experts have imagining what it’s like not to know what they know. Once you understand something you can’t easily un-understand it — so you overestimate how obvious it is to everyone else.

The spotlight effect

The spotlight effect is the tendency to overestimate how much others notice and judge you. We feel under a spotlight — our mistakes, our outfit, our stumble — but everyone else is too busy under their own spotlight to pay much attention.

The decoy effect

The decoy effect is when adding a third, deliberately inferior option changes which of the original two you prefer. The "decoy" isn’t meant to be chosen — it’s there to make one option look like obviously better value.

The mere-exposure effect

The mere-exposure effect is the tendency to like things simply because they’re familiar. Repeated exposure — to a song, a face, a brand, an idea — increases preference for it, even with no positive new information.

The IKEA effect

The IKEA effect is the tendency to place disproportionately high value on things we partly made ourselves. The labour we put in inflates our sense of worth — a wobbly self-assembled shelf can feel more valuable than a sturdier store-bought one.

In-group bias

In-group bias is the tendency to favour people we see as part of our group — and to be more critical or suspicious of outsiders. The group can be anything: nationality, team, politics, even one assigned at random minutes ago.

Illusory superiority

Illusory superiority is the tendency to overestimate your qualities relative to others — to rate yourself above average on skill, ethics, and judgement. Statistically most people can’t be above average, yet most people believe they are.

The ostrich effect

The ostrich effect is the tendency to avoid negative information — to "bury your head in the sand" rather than face something unpleasant. We dodge the bank balance, the medical result, the hard feedback, hoping not-knowing keeps the problem away.

The Zeigarnik effect

The Zeigarnik effect is the tendency to remember unfinished or interrupted tasks better than completed ones. Open loops nag at the mind — the half-written email, the unresolved argument — while finished tasks quietly fade.

Endowment effect

The endowment effect is the tendency to value something more highly simply because you own it. Once a thing is yours, parting with it feels like a loss — so you demand more to sell it than you would have paid to buy the very same thing.

Cognitive dissonance

Cognitive dissonance is the mental discomfort of holding two conflicting beliefs, or of acting against your beliefs. To relieve the tension we change one of them — usually by rationalising the action rather than admitting we were wrong.

Illusion of control

The illusion of control is the tendency to overestimate how much influence you have over outcomes that are largely or entirely down to chance. We feel more in control when we are personally involved, even when our actions change nothing.

Hyperbolic discounting

Hyperbolic discounting is our tendency to prefer smaller rewards that arrive sooner over larger rewards that come later — and to do so far more steeply for near-term choices. The present feels disproportionately urgent compared with the future.

Backfire effect

The backfire effect is when being shown evidence that contradicts a deeply held belief makes you hold that belief even more strongly, rather than revising it. Correction can entrench the very error it was meant to fix.

Just-world hypothesis

The just-world hypothesis is the tendency to believe the world is fundamentally fair — that people get what they deserve and deserve what they get. It leads us to assume victims must have done something to bring misfortune on themselves.

Bias blind spot

The bias blind spot is the tendency to recognise cognitive biases in other people while failing to see them in yourself. We readily spot others’ flawed thinking but feel our own judgements are objective and bias-free.

Frequency illusion

The frequency illusion is the experience of noticing something everywhere right after you first encounter it — a word, a car model, an idea. The thing isn’t actually more common; your attention has simply been primed to notice it.

Representativeness heuristic

The representativeness heuristic is judging the probability of something by how closely it matches a mental prototype, rather than by actual statistics. If someone "looks like" a category, we assume they belong to it — ignoring how common that category really is.

Overconfidence effect

The overconfidence effect is the gap between how accurate people think their judgements are and how accurate they actually are. When people say they are "99% sure," they turn out to be wrong far more than 1% of the time.

False-consensus effect

The false-consensus effect is the tendency to overestimate how much other people share your beliefs, values, and behaviours. We assume our own views are more common and "normal" than they actually are, and that most reasonable people agree.

Hot-hand fallacy

The hot-hand fallacy is the belief that someone who has succeeded several times in a row is "on a streak" and more likely to succeed again — even when each attempt is statistically independent. We see momentum in what is really just chance.

Barnum effect

The Barnum effect is the tendency to accept vague, general personality descriptions as uniquely accurate about yourself. Statements broad enough to fit almost anyone feel personally revealing — the mechanism behind horoscopes, fortune-telling, and many personality tests.

Actor–observer bias

The actor–observer bias is our tendency to attribute our own actions to the situation but other people’s actions to their character. When you stumble, the pavement was uneven; when someone else stumbles, they’re clumsy.

Affect heuristic

The affect heuristic is the mental shortcut of judging something — its risks, benefits, and merits — by the emotion it triggers rather than by analysis. If something feels good, we judge it low-risk and high-benefit; if it feels bad, the reverse.

Illusory correlation

Illusory correlation is perceiving a relationship between two things when none exists, or seeing one far stronger than it really is. We notice and remember the cases that fit our expectation, and overlook the many that don’t.

Primacy effect

The primacy effect is the tendency to remember and be most influenced by the first items in a sequence. First impressions, opening arguments, and the top of a list carry disproportionate weight — they set the frame everything after is judged against.

Clustering illusion

The clustering illusion is the tendency to see patterns, streaks, or clusters in what is actually random data. Genuine randomness produces more apparent "runs" than we expect, so we mistake normal statistical noise for a meaningful signal.

Pessimism bias

The pessimism bias is the tendency to overestimate the likelihood of negative outcomes and underestimate positive ones — expecting things to turn out worse than they statistically do. It is the mirror image of optimism bias, often linked to anxiety and low mood.

Reactance

Reactance is the urge to do the opposite of what you’re told when you feel your freedom of choice is being threatened. A heavy-handed rule, ban, or hard sell can trigger resistance — making the forbidden option suddenly more appealing.

Omission bias

The omission bias is the tendency to judge harmful actions as worse than equally harmful inactions. We feel more responsible for damage we cause by doing something than for the same damage we allow by doing nothing.

Action bias

The action bias is the tendency to favour doing something over doing nothing, even when action is no better — or worse — than waiting. Under pressure or uncertainty, acting feels productive and responsible, while inaction feels like failure.

Contrast effect

The contrast effect is when our judgement of something shifts depending on what we compare it to. The same option seems better or worse, bigger or smaller, more or less expensive, based purely on the reference point placed beside it.

Illusion of transparency

The illusion of transparency is the tendency to overestimate how well others can perceive your inner states — your nervousness, feelings, or thoughts. You feel your emotions are written on your face when they are largely invisible to everyone else.

Availability cascade

An availability cascade is a self-reinforcing cycle in which an idea gains plausibility through sheer repetition in public discourse. The more often a claim is repeated, the more available and believable it becomes — regardless of its actual evidence.