Since the XIXth century incest has been a central although controversial topic in Anthropology. Genetics and biology, psychology and psychoanalysis, history and colonization, demography and social structures, and many others are the academic and scientific fields invoked to support divergent theoretical theses and different analyses of concrete cases. In this paper, after a short historical account and a a brief ethnographic contextualization, I’ll show how the Arara (a small Karib-speaking people living in the Xingu-Iriri rivers basin, Amazonia, Brazil) managed to maintain a culturally prescribed matrimonial tradition through limited cases of incest (or “incest-like” marriages). In so doing I’ll confront different theories and analytical possibilities to grasp the matrimonial decisions that helped them to increase their population from only a hundred souls (at the time of the contact with the Brazilians in the 1980s) to more than 350 people nowadays.