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We examine the roots of variation in corruption across societies, and we argue that marriage practices and family structure are an important, overlooked determinant of corruption. By shaping patterns of relatedness and interaction, marriage practices influence the relative returns to norms of nepotism/favoritism versus norms of impartial cooperation. In-marriage (e.g. consanguineous marriage) generates fractionalization because it yields relatively closed groups of related individuals and thereby encourages favoritism and corruption. Out-marriage creates a relatively open society with increased interaction between non-relatives and strangers, thereby encouraging impartiality. We report a robust association between in-marriage practices and corruption both across countries and within countries. Instrumental variables estimates exploiting historical variation in preferred marriage practices and in exposure to the Catholic Church’s family policies provide evidence that the relationship could be causal.