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The evolution of secondary sexual floral traits may be driven by selection through male or female reproductive success. Even so, the gender-biased function of a floral trait is often unapparent because secondary sexual traits and primary sexual organs of both genders co-occur within most bisexual flowers. Within dichogamous plants, however, secondary sexual traits may be unambiguously expressed in association with the primary sexual organs of one gender, making these species uniquely suited to studies of natural and sexual selection on floral traits. The objectives of this article are to summarize patterns of gender-biased nectar production and to critically explore theories relevant to its evolution. We list 41 species with gender-biased nectar production and provide two sets of adaptive hypotheses for the trait: sexual selection hypotheses and inbreeding avoidance hypotheses. We formulate these hypotheses …