 In the 1950s, more well-defined models built on discounted utility theory and approached the question of inter-temporal consumption as a lifetime income optimization problem.Solving this problem mathematically, assuming that individuals are rational and have access to complete markets, Modigliani & Brumberg (1954), Albert Ando, and Milton Friedman (1957) developed what became known as the life-cycle model.See Intertemporal choice#Modigliani's life cycle income hypothesis for details.