Most theories and studies of decision making are a-developmental. However, there is ample evidence that there are differences in behavior on many decision-making tasks between children and adults. This article asserts that within adults there are differences in behavior on many decision-making tasks and discusses investment as a decisionmaking task where differences in adult behavior can be analyzed. It presents an argument that stage theory can predict investment behavior. The major properties of investment behavior are (a) how many variables a person can look at and (b) whether a person can compare systems and understand that regulations are incomplete and not consistent. We propose that the rational theories of investing fail because most economic theories assume perfectly rational players in the market place. One of the major reasons that private investors do terribly in managing and investing money is the inadequate stage development of the investors on the task of investing.