The article explores the evolutionary mechanisms behind human cooperation, specifically focusing on the role of repeated interactions and intergroup competitions. It challenges the traditional explanations for one-shot cooperation, suggesting that neither repeated interactions nor intergroup competitions alone reliably support cooperation. Instead, it introduces the concept of "ambiguous reciprocity," a strategy that undermines cooperation under repeated interactions. The study uses models and a behavioral experiment in Papua New Guinea to demonstrate that combining repeated interactions with intergroup competitions creates a synergistic effect, leading to super-additive cooperation. This super-additive cooperation is characterized by high initial transfers and escalating reciprocity with ingroup partners, while de-escalating reciprocity is observed with outgroup partners. The findings suggest that both mechanisms must be considered to understand the evolution of cooperation in humans, highlighting the importance of both individual and group-level selection.The article explores the evolutionary mechanisms behind human cooperation, specifically focusing on the role of repeated interactions and intergroup competitions. It challenges the traditional explanations for one-shot cooperation, suggesting that neither repeated interactions nor intergroup competitions alone reliably support cooperation. Instead, it introduces the concept of "ambiguous reciprocity," a strategy that undermines cooperation under repeated interactions. The study uses models and a behavioral experiment in Papua New Guinea to demonstrate that combining repeated interactions with intergroup competitions creates a synergistic effect, leading to super-additive cooperation. This super-additive cooperation is characterized by high initial transfers and escalating reciprocity with ingroup partners, while de-escalating reciprocity is observed with outgroup partners. The findings suggest that both mechanisms must be considered to understand the evolution of cooperation in humans, highlighting the importance of both individual and group-level selection.