John von Neumann 1903-1957, was a Hungarian-born American mathematician, who made important contributions to numerous fields. Regarding economics, he is well known for his contribution to game theory and for the development, along with Oskar Mogenstern, of expected utility theory.
Von Neumann was one of the earliest creators of the game theory field, thanks to his mathematical abilities. As early as 1928 he developed theorems in this field, but his greatest contribution came as a result of his collaboration with Morgenstern which is reflected in their joint book “Theory of Games and Economic Behavior”, 1944. In this book they formulated the expected utility hypothesis regarding risk aversion, which is analysed using an expected utility function and describes the individual decision-making process. They developed a set of axioms under which the expected utility hypothesis holds, the so called von Neumann–Morgenstern utility theorem.
John von Neumann made also another great contribution to decision theory, which is nowadays known as the minimax theorem. This theorem states that each player in a game will try to minimise the maximum payoff possible for the other players, which will in turn maximise his or hers own utility.