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 selection into polygyny, greater propensity for cooperation among women, or household heads enforcing others’ cooperative agreements.