We would like to thank the editor, five anonymous referees, and numerous individuals and seminar participants for helpful comments; and ESRC Grant RES-062-23-2586 (Crawford), the NYU Stern Center for Global Economy and Business (Lee), and the Toulouse Network for Information Technology and the NSF (Whinston) for support. All errors are our own. The views expressed herein are those of the authors and do not necessarily reflect the views of the National Bureau of Economic Research. At least one co-author has disclosed a financial relationship of potential relevance for this research. Further information is available online at http://www.nber.org/papers/w21832.ack NBER working papers are circulated for discussion and comment purposes. They have not been peer-reviewed or been subject to the review by the NBER Board of Directors that accompanies official NBER publications.

The impact of insurer competition on welfare, negotiated provider prices, and premiums in the U.S. private health care industry is theoretically ambiguous. Reduced competition may increase the premiums charged by insurers and their payments made to hospitals. However, it may also strengthen insurers' bargaining leverage when negotiating with hospitals, thereby generating offsetting cost decreases. To understand and measure this trade‐off, we estimate a model of employer‐insurer and hospital‐insurer bargaining over premiums and reimbursements, household demand for insurance, and individual demand for hospitals using detailed California admissions, claims, and enrollment data. We simulate the removal of both large and small insurers from consumers' choice sets. Although consumer welfare decreases and premiums typically increase, we find that premiums can fall upon the removal of a small insurer if an employer imposes effective premium constraints through negotiations with the remaining insurers. We also document substantial heterogeneity in hospital price adjustments upon the removal of an insurer, with renegotiated price increases and decreases of as much as 10% across markets.

We consider the effect of mergers between firms whose products are not viewed as direct substitutes for the same good or service, but are bundled by a common intermediary. Focusing on hospital mergers across distinct geographic markets, we show that such combinations can reduce competition among merging hospitals for inclusion in insurers' networks, leading to higher prices (or lower‐quality care). Using data on hospital mergers from 1996–2012, we find support that this mechanism operates within state boundaries: cross‐market, within‐state hospital mergers yield price increases of 7%–9 % for acquiring hospitals, whereas out‐of‐state acquisitions do not yield significant increases.

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