The significance of the market portfolio

Arguments for creating a market to allow trading the portfolioof all endowments in the entire world, the 'market portfolio',are considered. This world share market would represent a radicalinnovation, since at the present time only a small fractionof world endowments are traded. Using a stochastic endowmenteconomy where preference are mean variance, it is shown thatcreating such a market may be justified in terms of its contributionto social welfare. It is also argued that creating a marketfor world shares is attractive for certain reasons of robustnessand simplicity.