Textbook of Usability

Anchoring is a cognitive bias in which the first piece of information encountered (the "anchor") exerts a disproportionate influence on subsequent judgements, even when the anchor is arbitrary or irrelevant.

Classic experiments by Amos Tversky and Daniel Kahneman showed that people asked to estimate a quantity (e.g., the number of African countries in the UN) produced higher estimates after seeing a higher arbitrary anchor (a spun wheel showing 65) than after seeing a lower anchor (a wheel showing 10). Even when participants knew the anchor was random, it moved their answers.

In interface design, anchoring makes defaults extraordinarily powerful. A pre-filled form field, a highlighted recommended plan, or a suggested tip amount all anchor the user's thinking. Users tend to adjust insufficiently from the anchor, accepting values close to the default even when a different value might better serve them.

Designers can use anchoring ethically to guide users toward safe, sensible choices (safe defaults, reasonable dose suggestions, privacy-preserving settings), or exploit it for manipulation (default opt-ins, inflated "regular" prices used to make discounts look larger). Awareness of anchoring is central to ethical choice architecture.