Nothing from college made more of an impact on me than learning Game Theory— a branch of economics focused first on modeling behavior of perfectly rational humans making decisions with different, interconnected payoffs, then on exploring what happens when you try to apply these models to real people in real life. It's fascinating stuff, and has helped me succeed as a schoolteacher (I even taught lessons on it), as a product manager, and in understanding social and political situations as they unfold around me.
One of the core concepts in Game Theory is Nash Equilibrium, a state where a bunch of players have choices, everybody knows everybody else's choices and strategies, and given their current choice, nobody has an incentive to change their decision. It comes out in problems like the Prisoner's Dilemma or Tragedy of the Commons, where the rational, individually preferred Nash Equilibrium ends up creating a bad global outcome. Once you see this model, it's hard to unsee, and is a great explanation of why global problems like pollution, arms races, and energy conservation are so hard to deal with.
The Nash equilibrium is named after John Nash, who, while not the first person to think of the concept, was the first to prove that for any game (at least, for a game with finite available actions), at least one Nash Equilibrium must exist.
John Nash died today in a taxi accident in New Jersey, with his amazing and supportive wife Alicia. Most people actually know John Nash as the subject of the movie "A Beautiful Mind," which dealt with both his work in economics and his struggles with schizophrenia. But I'll always know him as one of the fathers of modern game theory, which gave me a framework to think about the world.
Rest in peace, John.