Motivated by a secular increase in the concentration of the U.S. banking industry, I develop a new macroeconomic model with oligopolistic financial intermediaries and heterogeneous firms. Market power allows banks to price discriminate and charge firm-specific markups, exerting greater market power over productive and more financially constrained firms. This dampens capital accumulation and amplifies the effects of macroeconomic shocks. During a crisis, banks exploit the higher share of financially constrained firms to extract higher markups, inducing a larger decline in real activity. When a large bank fails, the remaining banks use their increased market power to restrict credit supply, worsening and prolonging the downturn.

Adverse selection in procurement arises when low-cost bidders are also low-quality suppliers. We propose a mechanism called LoLA (lowball lottery auction) which, under some conditions, maximizes any combination of buyer’s and social surplus, subject to incentive compatibility, in the presence of adverse selection. The LoLA features a floor price, and a reserve price. The LoLA has a dominant strategy equilibrium that, under mild conditions, is unique. In a counterfactual analysis of Italian government auctions, we compute the gain that the government could have made, had it used the optimal procurement mechanism (a LoLA), relative to a first-price auction (the adopted format).