Jump Bidding and Overconcentration in Decentralized Simultaneous Ascending Auctions

A model of English auction that allows jump bidding is proposed. When two objects are sold separately via such English auctions, I construct an equilibrium such that bidders signal via jump bids, thereby forming rational expectations of the prices without relying on any central mediator. This equilibrium eliminates the exposure problem for a bidder whose valuation function is superadditive. Consequently, the auction game overly concentrates the goods to a multi-item bidder and never overly diffuses them to single-item bidders.