The micro economics of group behavior: General characterization

We study the demand function of a group of S members facing a global budget constraint. Any vector belonging to the budget set can be consumed within the group, with no restriction on the form of individual preferences, the nature of individual consumptions or the form of the decision process beyond efficiency. Moreover, only the group aggregate behavior, summarized by its demand function, is observable. We provide necessary and (locally) sufficient restrictions that fully characterize the group's demand function, with and without distribution factors. We show that the private or public nature of consumption within the group is not testable from aggregate data on group behavior.