Many species have extreme within-sex morphological and behavioral polymorphisms, most commonly different male phenotypes that practice different reproductive strategies. Although much is known about the role of hormones in sexual differentiation, little is known about what role hormones might play in within-sex differentiation. The relative plasticity hypothesis is derived from the classical organization-activation model of hormone action. It distinguishes between two types of polymorphic systems: a fixed system in which individual males assume one phenotype for their adult lives and a plastic system in which individual males can change phenotypes at least once. By analogy to sexual differentiation, the relative plasticity hypothesis generally predicts that organizational influences of hormones will be more important in fixed systems and activational influences of hormones will be more important in plastic systems. A review of our knowledge of the role of hormones in differentiation of within-sex polymorphisms indicates that the relative plasticity hypothesis accounts for otherwise diverse and contradictory results. This further supports the hypothesis that the organizational-activational model of hormone action derived from sexual differentiation generalizes to within-sex polymorphisms. However, studies of the effects of hormone manipulations on within-sex differentiation are rare but are desperately needed to further our understanding of this problem. Further studies of discontinuous behavioral variation characteristic of polymorphic species may further our understanding of the physiological basis of within-sex behavior variation in all species.