Independence of irrelevant alternatives

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It has been suggested that Luce's choice axiom be merged into this article. (Discuss) Proposed since July 2024.

Independence of irrelevant alternatives (IIA), also known as binary independence, the independence axiom, is an axiom of decision theory and economics describing a necessary condition for rational behavior. The axiom says that a choice between ${\displaystyle A}$ and ${\displaystyle B}$ should not depend on the quality of a third, unrelated outcome ${\textstyle C}$.

The axiom is deeply connected to several of the most important results in social choice, welfare economics, ethics, and decision theory. Among these results are Arrow's impossibility theorem, Harsanyi's utilitarian theorem, and the Dutch book theorems.

Violations of IIA in individual behavior (caused by irrational behavior) are called menu effects or menu dependence. Violations of IIA in social choice are called spoiler effects.