AbstractLead donorship is a condition of sustained outsized responsibility by one foreign aid donor country for giving aid to a developing country. Prior research argues that lead donorship can either be a product of donors collaborating to carve out spheres of influence to maximize non-development foreign policy goals or else of donors buck-passing responsibility for addressing recipient needs. Which is it? This study seeks to answer this question using theory-driven expectations about when and where lead donorship should arise if donors cooperate or systematically fail to do so. It further develops novel measures of foreign aid’s public goods characteristics and donors’ marginal returns from giving aid that should help triangulate conditions under which we can expect lead donorship to arise due to cooperation or else its absence. Estimates from a flexible semi-parametric modeling strategy show that when and where lead donorship has the highest probability of occurring is consistent with systematic cooperation failure in the allocation of foreign aid. Various robustness checks are done and alternative explanations are also discussed. These findings provide insight into ongoing policy debates about donor cooperation and aid effectiveness and raise questions about what deeper strategic logics preclude donor cooperation.