This paper analyzes the impact of intermediary concentration on the allocation of revenue in online platforms. We study sponsored search documenting how advertisers increasingly bid through a handful of specialized intermediaries. This enhances automated bidding and data pooling, but lessens competition whenever the intermediary represents competing advertisers. Using data on nearly 40 million Google keyword auctions, we first apply machine learning algorithms to cluster keywords into thematic groups serving as relevant markets. Using an instrumental variable strategy, we estimate a decline in the platform’s revenue of approximately 11 percent due to the average rise in concentration associated with intermediary merger and acquisition activity. (JEL C45, D44, G34, L13, L81, M37)

The efficiency of publicly-subsidized, privately-provisioned social insurance programs depends on the interaction between strategic insurers and the subsidy mechanism. We study this interaction in the context of Medicare's prescription drug coverage program. We find that the observed mechanism is successful in keeping "raise-the-subsidy" incentives relatively low, acts much like a flat voucher, and obtains a level of welfare close to the optimal voucher. Across a range of counterfactuals, we find that more efficient subsidy mechanisms share three features: they retain the marginal elasticity of demand, limit the exercise of market power, and preserve the link between prices and marginal costs.