Game theory
The mathematical study of strategic interaction between rational agents, where each player's best move depends on what the others are doing. Foundational results like the prisoner's dilemma show how individually rational choices can produce collectively bad outcomes, and why credible third-party enforcement (a Leviathan, a mafia don, a state) can change the payoff structure and unlock cooperation. Originating with John von Neumann and Oskar Morgenstern in the 1940s and developed further by John Nash, it has become a standard lens for thinking about everything from nuclear deterrence to evolutionary biology.