We prove that in competitive market economies with no insurance for idiosyncratic risks, agents will always overinvest in illiquid long term assets and underinvest in short term liquid assets. We take as our setting the seminal model of Diamond and Dybvig (1983), who first posed the question in a tractable model. We reach such a simple conclusion under mild conditions because we stick to the basic competitive market framework, avoiding the banks and intermediaries that Diamond and Dybvig and others introduced.
We offer new sufficient conditions ensuring demand is downward sloping local to equilibrium. It follows that equilibrium is unique and stable in the sense that rising supply implies falling prices. In our setting, there are two goods, which we interpret as consumption in different time periods, and many impatience types. Agents have the same Bernoulli utility function, but the types differ arbitrarily in time preference. Our main result is that if endowments are identical and utility displays nonincreasing absolute risk aversion, then market demand is strictly downward sloping local to equilibrium. We discuss implications for the Diamond-Dybvig literature.