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Much contemporary political discourse centers on preserving valued cultural norms given perceived threats from interaction with culturally-distinct others, such as immigrants or colonial majority populations. Cultural maintenance or change in such contexts likely depends crucially on individuals with the ability to interact successfully across cultural boundaries (cross-cultural competence), mirroring the important role of bilingualism in language dynamics. However, the effects of cross-cultural competence on cultural change remain largely unknown, due primarily to difficulties of measurement. Here, simple field methods and Bayesian item-response theory models are used to derive the first experience-level measure of cross-cultural competence. This new measure brings into focus two under-studied developmental paths with profoundly different implications for cultural sustainability: Cross-cultural competence may emerge as a side effect of adopting out-group cultural norms, or it may be acquired while maintaining in-group norms. Ethnographic analysis of a uniquely-suited Amazonian population suggests that the path taken is a function of relative power differences during interaction with co-ethnics and out-group members. Counterintuitively, cross-cultural competence can contribute to cultural loss, especially if it develops via the former path. However, if it can be achieved via the latter developmental path, cross-cultural competence can potentially play a key role in the sustainability of valued cultural diversity within a society. Recognition of the path-dependent role of cross-cultural competence in cultural sustainability is vital to effective policy promoting both cultural diversity and constructive inter-ethnic interaction.