Rationalizable Bidding in General First-Price Auctions

We wish to analyze the consequences of strategically sophisticated bidding without assuming equilibrium behavior. As a first step, we characterize interim rationalizable bids in first-price auctions with interdependent values and affiliated signals. We show that (1) every non-zero bid below the equilibrium is rationalizable, (2) some bids above the equilibrium are rationalizable, (3) the upper bound on rationalizable bids of a given player is a continuous, non-decreasing function of her signal/valuation. In the special case of symmetric bidders with independent signals and quasi-linear valuation functions, (i) the least upper bound on rationalizable bids is increasing and concave; hence (ii) rationalizability is consistent with substantial shading for high valuations, but only little shading for low valuations. Our main technical contribution is to show that the set of rationalizable bids is essentially determined by iteratively solving a simple one-dimensional optimization problem. We argue that our theoretical analysis may shed some light on experimental findings about deviations from the risk-neutral Nash equilibrium.