Mate choice is one of the most important decisions an animal makes and it promotes the evolution of some of the most elaborate traits in nature. Mate choice models derive from traditional microeconomic decision theory, implicitly assuming that individuals behave rationally by maximizing their Darwinian fitness, the logical evolutionary analogy to economic utility(1). Rational choices exhibit the property of regularity, requiring that the relative strength of preferences between objects remain stable when additional options are available. Human consumers routinely violate this assumption(2-3) and there is evidence that nonhuman animals make irrational foraging decisions(4-5). The long held assumption of rationality in mate choice, however, has yet to be rejected. Here we show that female frogs reverse their mate decisions in a systematic manner known as a “decoy effect,” violating the axiom of regularity and therefore the assumption of rationality in mate choice. We tested preferences for three mating calls in which their total subjective value to females was a function of static acoustic characters and the rate at which the call was presented. In binary choice tests, the females’ preferences for these three stimuli were transitive and favored stimulus B (B>A, B>C, A>C). In trinary tests, in which C is by definition the inferior decoy, females’ preferences showed a significant change in preference from B to A, whether the females could actually choose the decoy or only hear it. Decoy effects in mate decisions reveal that the relative valuation of two mates is not independent of inferior alternatives in the choice set and therefore cannot be explained with rational choice models such as those we currently utilize for understanding mate choice.