Building on anthropological evidence, we develop a model of intra-household decision making on fertility and child survival within the framework of the collective household model. We carry out a test of the implications of this framework with data from Demographic and Health Surveys in rural Mali, where polygyny rates among married women are close to 50 per cent. The econometric tests reject the implications of efficient intrahousehold allocations for junior wives in bigynous households and fail to reject for senior wives in bigynous households as well as for wives in monogamous households. These findings are consistent with existing narrative evidence according to which co-wife rivalry is responsible for resource-consuming struggle and junior wives are the adults with the weakest bargaining position in the household.