Independence of irrelevant alternatives

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Independence of irrelevant alternatives (IIA) is a major axiom of decision theory which codifies the intuition that a choice between ${\displaystyle A}$ and ${\displaystyle B}$ should not depend on the quality of a third, unrelated outcome ${\textstyle C}$. There are several different variations of this axiom, which are generally equivalent under mild conditions. As a result of its importance, the axiom has been independently rediscovered in various forms across a wide variety of fields, including economics,^[1] cognitive science, social choice,^[1] fair division, rational choice, artificial intelligence, probability,^[2] and game theory. It is closely tied to many of the most important theorems in these fields, including Arrow's impossibility theorem, the Balinski-Young theorem, and the money pump arguments.

In behavioral economics, failures of IIA (caused by irrationality) are called menu effects or menu dependence.^[3] Violations of IIA in social choice, elections, and sports are called spoiler effects.