Introducing more speculators into the market for a given commodity leads to improved risk sharing but can also change the informational content of prices. This inflicts an externality on those traders already in the market, whose ability to make inferences based on current prices will be aff ected. In some cases, the externality is negative: the entry of new s peculators lowers the informativeness of the price to existing trader s. The net result can be one of price destabilization and welfare red uction. This is true even when all agents are rational, risk-averse c ompetitors who make the best possible use of their available informat ion. Copyright 1987 by University of Chicago Press.

Models of collusive bidder behavior at single-object second-price and English auctions are provided. The ind ependent private values model is generalized to permit the formulatio n of coalitions and a strategic response by the auctioneer. Cooperati ve strategies are found to be dominant in these models; coalitions of any size are viable, and the payoff to each member increases with th e size of the coalition. In addition, the collusive strategies of the coalition represent a noncooperative equilibrium. The optimal respon se of the auctioneer is to establish a reserve price that is a functi on of the coalition's size. These and other features of the model are found to be consistent with the essential features of actual behavio r. Copyright 1987 by University of Chicago Press.