People vary in how easily they feel ashamed, that is, in their shame proneness. According to the information threat theory of shame, variation in shame proneness should, in part, be regulated by features of a person's social ecology. On this view, shame is an emotion program that evolved to mitigate the likelihood or costs of reputation-damaging information spreading to others. In social environments where there are fewer possibilities to form new relationships (i.e., low relational mobility), there are higher costs to damaging or losing existing ones. Therefore, shame proneness toward current relationship partners should increase as perceived relational mobility decreases. In contrast, individuals with whom one has little or no relationship history are easy to replace, and so shame-proneness towards them should not be modulated by relational mobility. We tested these predictions cross-culturally by measuring relational mobility and shame proneness towards friends and strangers in Japan, the United States, and the United Kingdom. Japanese subjects were more shame-prone than their British and American counterparts. Critically, lower relational mobility was associated with greater shame proneness towards friends (but not strangers), and this relationship partially mediated the cultural differences in shame proneness. Shame proneness appears tailored to respond to relevant features of one's social ecology.