The bias library
95 biases behind real products
Each one in a sentence, with the original source, an honest evidence rating and, where we have them, real examples. Note: nudge effects shrink after correcting for publication bias (Mertens et al. 2022; Maier et al. 2022).
How we rate evidence
- Strong: several independent replications, or a meta-analysis or multi-lab replication that supports it; no credible large failed replication; an empirical source is cited, not only a theory paper.
- Mixed: credible evidence both ways, such as a large or pre-registered failed replication, a meta-analysis near zero or heavily affected by publication bias, or a live dispute that it’s an artefact. The critique is always cited.
- Limited: one paper, or a few from one lab; no independent replication found by us; or only lab or hypothetical evidence for the marketing use described.
- Provisional: any rating without a cited replication or review. A rating based only on a theory paper can’t be Strong.
Principles and techniques (e.g. Hick’s law, door-in-the-face) aren’t biases. Each page names the bias it relies on.
Start here
The 30 with real teardowns
Price
Price & attribute framing
The same price feels smaller in the right unit.
Social
Social proof
When unsure, we copy what people like us do.
Price
Zero-price effect
Free feels different from cheap, not just cheaper.
Social
Authority
We lean on experts and badges when we can't judge for ourselves.
Principle
Incentive (principle)
A reward for doing something changes what we do.
Anchoring
Reference price
A price feels cheap or dear only next to another price.
Time
Urgency (deadline effect)
A deadline makes acting now feel necessary.
Risk
Regret aversion (risk reversal)
We pick the option we're least likely to regret.
Principle
Friction (principle)
Every extra step loses people; remove steps to help, add them to protect.
Choice architecture
Default effect / status quo bias
We tend to keep whatever is pre-chosen for us.
Price
Mental accounting
We sort money into mental jars and spend each differently.
Risk
Loss aversion
Losing something can sting more than gaining the same thing pleases.
Directory
Every bias, A–Z
95 biases
A
Aesthetic-usability effectWe perceive attractive designs as easier to use.ExperienceLimitedNo teardown yetAffect heuristicOur feelings about something shape our judgement of its risks and benefits.JudgementStrong1 teardownAmbiguity aversionWe prefer known risks to unknown ones, even when the unknown might be better.RiskStrong1 teardownAnchoringThe first number we see sets the scale for everything after.AnchoringStrong6 teardownsAuthorityWe lean on experts and badges when we can't judge for ourselves.SocialStrong19 teardownsAvailability heuristicWe judge how likely or common something is by how easily examples come to mind.JudgementStrong2 teardownsC
Certainty effectWe overweight outcomes that are certain compared with ones that are merely likely.RiskStrongNo teardown yetChange blindnessWe often fail to notice large changes to a scene, especially across a break or page load.Memory & attentionStrongNo teardown yetChoice blindnessWe can fail to notice when what we get is not what we chose, and still justify it.JudgementLimitedNo teardown yetChoice overload (simplification)Too many options can make choosing harder.Choice architectureMixed6 teardownsChoice-supportive biasAfter choosing, we remember the chosen option as better than it was.JudgementLimitedNo teardown yetCognitive dissonanceHolding conflicting beliefs or actions is uncomfortable, so we adjust our attitudes to fit what we did.JudgementMixedNo teardown yetCognitive loadMental effort spent on processing a task leaves less capacity for learning or judging it.PrincipleStrongNo teardown yetCommitment & consistencyA small yes makes a bigger yes easier.MotivationStrong4 teardownsCompromise effectWe favour the middle option when choices range from low to high.Choice architectureMixed3 teardownsConfirmation biasWe look for, notice and remember evidence that fits what we already believe.JudgementStrongNo teardown yetConformityUnder direct group pressure we may go along with a majority, even against what we can see is true (Asch-style conformity).SocialStrongNo teardown yetCredit-card premiumPeople may be willing to pay more when using a card than cash.PriceLimitedNo teardown yetCurse of knowledgeOnce we know something, it's hard to imagine not knowing it.JudgementLimitedNo teardown yetD
Decoy effectA third, worse option can make one of the others look better.Choice architectureMixedNo teardown yetDefault effect / status quo biasWe tend to keep whatever is pre-chosen for us.Choice architectureStrong11 teardownsDenomination effectWe are less likely to spend money held in a large denomination than the same amount in smaller units.PriceLimitedNo teardown yetDistinction biasComparing options side by side makes small differences seem bigger than they will feel in use.Choice architectureLimitedNo teardown yetDiversification biasWhen choosing several items at once, we pick more variety than we'll actually want.Choice architectureStrongNo teardown yetDoor-in-the-faceAfter refusing a big request, we're more likely to agree to a smaller one.TechniqueStrongNo teardown yetDunning–Kruger effectPeople with low skill in an area tend to overestimate their ability.JudgementMixedNo teardown yetE
Effort justificationWe value things more when we've worked or suffered to get them.ExperienceLimitedNo teardown yetEndowment effectOnce something feels like ours, we value it more.RiskStrong2 teardownsF
Fitts's lawThe time to hit a target depends on its size and distance.PrincipleStrongNo teardown yetFocusing illusionWhatever we focus on seems more important than it is.JudgementLimitedNo teardown yetFresh start effectTemporal landmarks like New Year, a birthday or a Monday make us more likely to pursue goals.MotivationLimitedNo teardown yetFriction (principle)Every extra step loses people; remove steps to help, add them to protect.PrincipleStrong12 teardownsH
Halo effectOne positive trait colours our judgement of unrelated traits.SocialStrongNo teardown yetHick's lawThe time it takes to choose grows with the number and complexity of options.PrincipleStrongNo teardown yetHot–cold empathy gapIn a calm state we underestimate how much hunger, anger or desire will change our choices, and vice versa.TimeStrongNo teardown yetI
Identifiable victim effectWe tend to help one named person more than many statistical victims.SocialMixedNo teardown yetIdentity labellingDescribing a behaviour as part of who we are ("be a voter") can motivate it.Self & identityMixedNo teardown yetIKEA effectWe value what we helped build.ExperienceStrongNo teardown yetIllusion of controlWe overestimate how much we can influence outcomes that are largely down to chance.JudgementStrongNo teardown yetIllusory truth effectRepeated statements feel more true, whether or not they are.Memory & attentionStrongNo teardown yetImplementation intentionsPlanning exactly when, where and how we'll act makes us more likely to follow through.TechniqueStrongNo teardown yetIn-group biasWe favour people who belong to our own group, even when the groups are trivially defined.SocialStrongNo teardown yetInattentional blindnessWhen focused on one thing, we can miss something unexpected in plain sight.Memory & attentionStrongNo teardown yetIncentive (principle)A reward for doing something changes what we do.PrincipleStrong19 teardownsInformation gap (curiosity)An open question begs to be closed.ExperienceMixed1 teardownIsolation (Von Restorff) effectAn item that stands out from the rest is more likely to be noticed and remembered.Memory & attentionStrongNo teardown yetL
Labour illusionWe value a service more when we can see the effort that goes into it.ExperienceLimitedNo teardown yetLeft-digit effectWe judge prices largely by their first digit, so £2.99 feels much cheaper than £3.00.PriceStrongNo teardown yetLess-is-better effectA lower-value option can be valued more when judged alone than a higher-value one.Choice architectureLimitedNo teardown yetLicensing effectAfter doing something good, we feel licensed to indulge or behave less well.Self & identityMixedNo teardown yetLoss aversionLosing something can sting more than gaining the same thing pleases.RiskMixed8 teardownsLow-ball techniqueOnce committed to a deal, we often accept it even when the cost goes up.TechniqueStrongNo teardown yetM
Marketing placebo effectMarketing such as price can change how well a product actually works for us.PriceLimitedNo teardown yetMental accountingWe sort money into mental jars and spend each differently.PriceMixed9 teardownsMere-exposure effectWe tend to like things more the more often we've seen them.Memory & attentionStrongNo teardown yetMere-measurement effectBeing asked about our intentions can change our behaviour.TechniqueLimitedNo teardown yetN
Name-letter effectWe tend to prefer the letters in our own name.Self & identityLimitedNo teardown yetNegativity biasBad events and information affect us more than equally good ones.RiskStrongNo teardown yetO
Optimism biasWe think good things are more likely, and bad things less likely, to happen to us than to others.JudgementStrongNo teardown yetOstrich effectWe avoid information we expect to be unpleasant.RiskLimitedNo teardown yetOverconfidenceWe are more sure of our judgements than our accuracy justifies.JudgementStrongNo teardown yetOverjustification effectExternal rewards can reduce our intrinsic motivation for an activity we already enjoyed.MotivationMixedNo teardown yetP
Pain of payingPaying feels unpleasant, and that pain depends on how and when we pay.PriceStrong2 teardownsPartitioned pricingSplitting a price into a base price plus surcharges can make the total seem lower.TechniqueStrong4 teardownsPeak-end ruleWe remember the best moment and the ending, not the average.ExperienceStrong1 teardownPlanning fallacyWe underestimate how long our own tasks will take, even when similar tasks ran late before.JudgementStrongNo teardown yetPratfall effectA small blunder can make a highly competent person more likeable.SocialLimitedNo teardown yetPresent biasNow beats later, even when later is worth more.TimeStrong1 teardownPrice & attribute framingThe same price feels smaller in the right unit.PriceStrong32 teardownsPrice-quality heuristicWe use price as a cue to quality, and a higher price can even make something taste better to us.PriceLimited1 teardownProcessing fluencyInformation that's easy to process feels more true, likeable and familiar.Memory & attentionStrongNo teardown yetProjection biasWe assume our future preferences will match our current ones.TimeLimitedNo teardown yetR
ReactanceWhen we feel our freedom is threatened, we push back and want the restricted thing more.SocialStrongNo teardown yetReason-giving ("because")Requests that come with a reason are more often granted, even when the reason is thin.TechniqueLimited1 teardownReciprocityA gift creates a quiet urge to give something back.SocialStrong4 teardownsReference priceA price feels cheap or dear only next to another price.AnchoringStrong18 teardownsRegret aversion (risk reversal)We pick the option we're least likely to regret.RiskStrong12 teardownsRhyme-as-reason effectRhyming statements can seem more accurate than non-rhyming ones.Memory & attentionLimitedNo teardown yetS
ScarcityThings seem more valuable when they might run out.TimeMixed2 teardownsScope insensitivityOur willingness to pay or help barely changes with the size of the problem.JudgementLimitedNo teardown yetSelf-reference effectWe remember information better when it relates to ourselves.Self & identityStrongNo teardown yetSerial position effectWe remember the first and last items in a list best.Memory & attentionStrongNo teardown yetSimilarity–likingWe like people who are similar to us, including in their attitudes.SocialStrongNo teardown yetSocial comparisonWe evaluate ourselves by comparing with others.SocialStrongNo teardown yetSocial proofWhen unsure, we copy what people like us do.SocialStrong31 teardownsSpacing effectWe remember better when practice is spread out over time rather than crammed.PrincipleStrongNo teardown yetSunk cost effectWe keep investing in something because of what we've already put in, not what we'll get.RiskMixedNo teardown yetT
Temptation bundlingPairing something we should do with something we want to do helps us do the first.TechniqueLimitedNo teardown yetThat's-not-all techniqueAdding a bonus or discount before the person responds makes them more likely to accept.TechniqueStrongNo teardown yetNo biases match. Try clearing a filter.