Altruism among family members can, in some cases, inhibit cooperation by increasing the utility that players expect to receive in a non-cooperative equilibrium. To test this, we examine agricultural productivity in polygynous households in West Africa. We find that cooperation is greater – production is more efficient – among co-wives than among husbands and wives because co-wives are less altruistic towards each other. The results are not driven by scale effects or self-selection into polygyny. Nor can they be explained by greater propensity for cooperation among women generally or by the household head acting as an enforcement mechanism for others' cooperative agreements.