This study investigates the evolutionary mechanisms underlying one-shot cooperation in humans, challenging the traditional views of repeated interactions and intergroup competition as the primary explanations. The research shows that neither repeated interactions alone nor intergroup competition alone reliably support cooperation. Instead, the combination of both mechanisms leads to super-additive cooperation, where the combined effect is greater than the sum of their individual effects. The study uses models and a behavioral experiment in Papua New Guinea to demonstrate that cooperative strategies evolve when both repeated interactions and intergroup competition are present. These strategies involve cooperative reciprocity with ingroup partners and uncooperative reciprocity with outgroup partners, which aligns with the observed behavior of the Ngenika and Perepka people. The findings suggest that cooperation in one-shot social dilemmas is not solely explained by repeated interactions or intergroup competition, but rather by the joint influence of both mechanisms. The study highlights the importance of considering the interaction between these two mechanisms in understanding the evolution of cooperation. The results also indicate that repeated interactions alone are insufficient to explain cooperation, as they can lead to uncooperative reciprocity. Similarly, intergroup competition alone does not reliably support cooperation. The study concludes that the combination of repeated interactions and intergroup competition provides a robust explanation for the evolution of one-shot cooperation, as it leads to the evolution of cooperative reciprocity with ingroup partners and uncooperative reciprocity with outgroup partners. The findings have implications for understanding human social behavior and the evolution of cooperation in general.This study investigates the evolutionary mechanisms underlying one-shot cooperation in humans, challenging the traditional views of repeated interactions and intergroup competition as the primary explanations. The research shows that neither repeated interactions alone nor intergroup competition alone reliably support cooperation. Instead, the combination of both mechanisms leads to super-additive cooperation, where the combined effect is greater than the sum of their individual effects. The study uses models and a behavioral experiment in Papua New Guinea to demonstrate that cooperative strategies evolve when both repeated interactions and intergroup competition are present. These strategies involve cooperative reciprocity with ingroup partners and uncooperative reciprocity with outgroup partners, which aligns with the observed behavior of the Ngenika and Perepka people. The findings suggest that cooperation in one-shot social dilemmas is not solely explained by repeated interactions or intergroup competition, but rather by the joint influence of both mechanisms. The study highlights the importance of considering the interaction between these two mechanisms in understanding the evolution of cooperation. The results also indicate that repeated interactions alone are insufficient to explain cooperation, as they can lead to uncooperative reciprocity. Similarly, intergroup competition alone does not reliably support cooperation. The study concludes that the combination of repeated interactions and intergroup competition provides a robust explanation for the evolution of one-shot cooperation, as it leads to the evolution of cooperative reciprocity with ingroup partners and uncooperative reciprocity with outgroup partners. The findings have implications for understanding human social behavior and the evolution of cooperation in general.