Humans often cooperate with strangers, despite the costs involved. A long tradition of theoretical modeling has sought ultimate evolutionary explanations for this seemingly altruistic behavior. More recently, an entirely separate body of experimental work has begun to investigate cooperation's proximate cognitive underpinnings using a dualprocess framework: Is deliberative self-control necessary to reign in selfish impulses, or does self-interested deliberation restrain an intuitive desire to cooperate? Integrating these ultimate and proximate approaches, we introduce dual-process cognition into a formal gametheoretic model of the evolution of cooperation. Agents play prisoner's dilemma games, some of which are one-shot and others of which involve reciprocity. They can either respond by using a generalized intuition, which is not sensitive to whether the game is oneshot or reciprocal, or pay a (stochastically varying) cost to deliberate and tailor their strategy to the type of game they are facing. We find that, depending on the level of reciprocity and assortment, selection favors one of two strategies: intuitive defectors who never deliberate, or dual-process agents who intuitively cooperate but sometimes use deliberation to defect in one-shot games. Critically, selection never favors agents who use deliberation to override selfish impulses: Deliberation only serves to undermine cooperation with strangers. Thus, by introducing a formal theoretical framework for exploring cooperation through a dual-process lens, we provide a clear answer regarding the role of deliberation in cooperation based on evolutionary modeling, help to organize a growing body of sometimesconflicting empirical results, and shed light on the nature of human cognition and social decision making.dual process | cooperation | evolutionary game theory | prisoner's dilemma | heuristics C ooperation, where people pay costs to benefit others, is a defining feature of human social interaction. However, our willingness to cooperate is puzzling because of the individual costs that cooperation entails. Explaining how the "selfish" process of evolution could have given rise to seemingly altruistic cooperation has been a major focus of research across the natural and social sciences for decades. Using the tools of evolutionary game theory, great progress has been made in identifying mechanisms by which selection can favor cooperative strategies, providing ultimate explanations for the widespread cooperation observed in human societies (1).In recent years, the proximate cognitive mechanisms underpinning human cooperation have also begun to receive widespread attention. For example, a wide range of experimental evidence suggests that emotion and intuition play a key role in motivating cooperation (2-5). The dual-process perspective on decision making (6-8) offers a powerful framework for integrating these observations. In the dual-process framework, decisions are conceptualized as arising from competition between two types of cognitive processes: (i) auto...