Cognitive Psychology
Anchoring Bias
Why the first number you hear hijacks every estimate that follows
Anchoring bias is the tendency for an initial piece of information — the "anchor" — to disproportionately influence subsequent judgments, even when the anchor is arbitrary or irrelevant. Tversky and Kahneman demonstrated it in their 1974 Science paper using a rigged wheel of fortune: participants who saw 65 estimated UN African membership at 45%; those who saw 10 estimated 25%. The anchor pulls estimates toward itself by 30-50% on average. It operates through selective accessibility — the anchor primes confirming evidence — and survives warnings, expertise, and incentives.
- DiscoveredTversky & Kahneman (1974), Science
- Classic studyWheel of fortune + UN nations question
- Effect sizeEstimates pull 30-50% toward anchor
- MechanismSelective accessibility; insufficient adjustment
- Robust toWarnings, expertise, incentives, implausibility
- Famous domainsPricing, negotiation, sentencing, real estate
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Why anchoring bias matters
- Negotiation. First offers shift outcomes 15-30% — open or be opened.
- Pricing. Reference prices, decoys, and "compare at" tags exploit it daily.
- Judicial sentencing. Prosecutor demands anchor judges' rulings.
- Real estate. Listing price anchors offers even for trained agents.
- Forecasting. Last quarter's number anchors next quarter's estimate.
- Salary. Stating a number first defines the band.
- Performance reviews. Initial ratings drag subsequent ratings.
Common misconceptions
- Only naive people anchor. Judges, doctors, and traders all show the effect.
- Implausible anchors fail. Even absurd numbers (your phone digits) shift estimates.
- Awareness defeats it. Knowing about anchoring barely reduces its grip.
- It's just adjustment. Selective accessibility, not adjustment, drives most external anchors.
- Incentives fix it. Paying for accuracy reduces but does not erase the bias.
- It's the same as priming. Anchoring requires a numerical comparison; priming is broader.
Frequently asked questions
What's the original anchoring experiment?
Tversky and Kahneman (1974) spun a wheel rigged to land on 10 or 65, then asked participants to estimate the percentage of African nations in the UN. Those seeing 10 averaged 25%; those seeing 65 averaged 45%. The arbitrary number — visibly random — still anchored the estimate. The result kicked off four decades of judgment-and-decision-making research.
Does anchoring affect experts?
Yes. Englich and Mussweiler (2001) had German judges read a hypothetical rape case, then roll loaded dice before sentencing. Judges seeing high rolls (averaging 8 months) gave longer sentences than those seeing low rolls (5 months). Real-estate agents anchor on listing prices even when told the price is uninformed. Expertise reduces but does not eliminate the bias.
How does selective accessibility explain it?
Mussweiler and Strack's account, the leading mechanism, says the anchor triggers a "is the answer X?" hypothesis test. Testing the hypothesis selectively primes evidence consistent with the anchor. By the time you adjust away, anchor-consistent information is more accessible than anchor-inconsistent information. This explains why anchors work even when participants know they're arbitrary.
What's the difference between numeric and self-generated anchors?
Externally provided anchors (a price, a number) work via accessibility. Self-generated anchors — when you start from a known value and adjust — work via insufficient adjustment, as shown by Epley and Gilovich. Asked when George Washington was elected, you anchor on 1776 and adjust up; the adjustment usually stops too early, biasing toward 1776.
How is anchoring used in pricing?
A $200 "compare at" sticker next to a $99 sale price makes $99 feel cheap. Restaurant menus place a $48 entree at the top so $28 entrees seem reasonable. Real-estate listings start above target to anchor offers. Ariely's MIT students bid more for wine and chocolate when their Social Security digits were higher — a demonstration that even pre-existing numbers in working memory can influence willingness to pay.
Can you debias against anchoring?
Partially. Considering the opposite — explicitly listing reasons the anchor is wrong — reduces the effect by roughly half. Forewarning helps marginally. Holding people accountable for accuracy helps slightly. But anchoring is unusually resistant: even when participants are told the anchor is random and given incentives for accuracy, the bias persists. The defense is to generate your own estimate before exposure.
How does anchoring interact with negotiation?
First-offer effects are well documented. Galinsky and Mussweiler (2001) showed negotiators making the first offer in price negotiations achieve outcomes 15-30% better than those who let the counterparty open. The first number defines the bargaining range. Skilled negotiators preempt this by anchoring high or low aggressively, while resilient counterparts ignore the anchor and counter with their own well-prepared figure.