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.

Allegations of Bidder collusion at Forest Service timber sales in the Pacific Northwest were common in the 1970s. Of course, prices may be low for reasons other than collusion. We formulate an empirical model that allows for both bidder collusion and supply effects and in which we control for demand conditions. Noncooperative behavior in which a single unit is sold (the standard auction model) is a special case: it is found to be definitively outperformed by a model of collusion. We also find that supply effects are dominated by collusion in determining the winning bids in the market.