Independence of Irrelevant Alternatives, IIA
Updated 2026-08-13
INTRODUCTION
English translation pending.
CORE DEFINITION
A condition of rational choice and social choice theory: the preference between two options A and B should not be affected by the presence or absence of a third, irrelevant option C. The axiom is normatively appealing but regularly violated in reality, as decoy effects demonstrate, so it functions as a diagnostic test for the consistency of a decision process or voting rule rather than as a description of actual behavior.
SCAFFOLDING EFFECT
Reduce cognitive load
- Consistency test: adding or removing an irrelevant option reveals whether underlying preferences are stable - Decoy detector: a sudden preference flip when a third option appears signals framing manipulation - Model audit: check whether the decision rule or voting model respects IIA before trusting its output
Anchor fast decisions
The axiom isolates one variable: if C is truly irrelevant, removing it should leave the A-versus-B ranking untouched. Real choice violates this because options are compared on relative attributes rather than absolute value, which is exactly how a well-designed decoy can shift the comparison frame and reverse a preference without changing either real option.
MINIMUM ACTION
In progress 0/1Practice this model in one real situation:
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Source support: Explicit
- en.wikipedia.orghttps://en.wikipedia.org/wiki/Independence_of_irrelevant_alternativesverified
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