The current research proposes a novel explanation for previously demonstrated findings that East Asians disclose less personal information to others than do Westerners. We propose that both between- and within-culture differences in self-disclosure toward close friends may be explained by the construct of "relational mobility" - the general degree to which individuals in the society have the opportunities to form new and terminate old relationships. In Study 1, we found that cross-cultural differences (Japan vs. U.S.) in self-disclosure toward a close friend were mediated by individuals' perceptions of relational mobility. In Study 2, two separate measures of relational mobility predicted self-disclosure within a single culture (Japan), and this relationship was mediated by the motivation to strengthen interpersonal relationships. We conclude that societies and social contexts high in relational mobility (where relationships can be formed and dissolved relatively easily) produce stronger incentives for self-disclosure as a social commitment device.