Sex determination occurs early in development for most animals, at which time sex is fixed for life. Many teleost fishes, however, exhibit remarkable sexual plasticity throughout their life history, ranging from multiple morphs within a sex to functional adult sex reversal. To understand the development and evolution of adult sex reversal, I examined behavioral, endocrine, and genetic contributions to the regulation of functional sex in adult animals, using the bluebanded goby (Lythrypnus dalli) as an experimental model. This species was found to be equally capable of sexual transitions from female to male (protogyny) as from male to female (protandry). Throughout adult life, sexual phenotype is determined by social status, an emergent property of agonistic behavioral interactions that follows a relatively simple social convention: if dominant become or remain male, or if subordinate, become or remain female. The translation of social status into a change in sexual phenotype in the protogynous direction requires a rapid drop in circulating estrogens and an increase in the gonadal expression of a testis differentiating gene dmrt1. Steroid hormones do not play a significant role in modulating status, but the androgen 11-ketotestosterone does positively correlate with the expression of paternal behavior. Taken together, these findings suggest an evolutionary mechanism in sexually plastic species that has linked the conserved molecular cascades of sexual differentiation to a novel signal that varies over life history, social status, thereby allowing for lifelong phenotypic plasticity.