Widely regarded as the cornerstone of justice (Rawls, 1971),fairness constitutes one of the pillars of human morality. TheUltimatum Game (UG), extensively studied in behavioral eco-nomics, is the canonical task for studying fairness. In sharpcontrast to the predictions of normative standards in game the-ory, people typically reject low offers in UG. In this work,we present the first resource-rational process model of UG.Concretely, by taking into account people’s expectations, weshow that Nobandegani et al.’s (2018) resource-rational pro-cess model, sample-based expected-utility, provides a unifiedaccount of several experimental findings in UG, namely, theeffects of expectation, competition, and time pressure. Assum-ing that expectation serves as a reference point for subjectivevaluation of an offer, we show that the rejection of low offers inUG can arise from purely self-interested expected-utility max-imization. We conclude by discussing the implication of ourwork for moral decision-making and, more broadly, human ra-tionality.