Bias comparison · evidence reviewed
Both can change choices without changing the core product, but they work through different choice structures. A decoy changes the comparison between options by adding another alternative; a default changes what happens when the person does not actively choose something else.
The shortest distinction
| Question | Decoy Effect | Default Effect |
|---|---|---|
| Choice structure | An additional option changes the relative comparison among alternatives already under consideration. | One option is pre-selected or applies automatically if the person does nothing. |
| Does the influential option need to be chosen? | No. The decoy can affect preference even when few people choose the decoy itself. | The default is often the option people end up with unless they actively switch. |
| Classic test | Compare target-versus-competitor choice shares before and after adding the suspected decoy. | Compare the same options under different defaults or an active-choice condition. |
| Main caution | Not every third tier is a decoy, and the effect depends on the relative position of the options and existing preferences. | Default uptake does not by itself prove strong preference or improved welfare. |
Decision diagnostic
Write the original options without the suspected decoy or default treatment.
For a decoy, check whether the extra option is inferior to one alternative on the relevant attributes and whether its presence changes choice between the original options.
For a default, identify exactly what happens if the person takes no action and compare with another default or active choice when possible.
Check whether anchoring, framing or switching friction is also present before attributing the whole result to one effect.
Same outcome, different bias
Decoy EffectBasic and Pro are difficult to compare. Adding a third plan that is almost as expensive as Pro but clearly worse on features makes Pro easier to justify.
Default EffectPro is pre-selected during checkout, so customers receive Pro unless they actively change the plan.
Decoy EffectA middle storage tier is priced so poorly relative to the largest tier that the largest tier looks more attractive than it did in the original two-option comparison.
Default EffectExisting customers renew automatically at their current storage tier unless they change it before renewal.
Decoy EffectA weak third vendor makes one of two serious candidates look superior on the dimensions where the third vendor is clearly dominated.
Default EffectThe incumbent vendor remains selected unless the review team actively submits a migration decision.
Retrospective review protocol
Evidence
Comparison reviewed 2026-08-18. The individual bias pages contain their full evidence status and boundary conditions.
