This paper provides an economic rationale for the practice of consanguineous marriages observed in parts of the developing world. In a model of incomplete marriage markets, dowries are viewed as ex ante transfers made from the bride's family to the groom's family when the promise of ex post gifts and bequests is not credible. Consanguineous unions join families between whom ex ante pledges are enforceable ex post. The model predicts a negative relationship between consanguinity and dowries and higher bequests in consanguineous unions. An empirical analysis based on data from Bangladesh delivers results consistent with the model.