US legal principles and confidentiality of the peer review process

When drawn into other parties' litigation or investigations, journals often receive requests or subpoenas for confidential peer review documents. We describe reasons for such requests and reasons that journals resist producing confidential documents, ways that journals respond, and steps that journals can take to minimize third-party breaches of journals' confidential processes. We discuss legal principles on which resistance may be based in the United States, including the journalist's privilege, the scholar's privilege, the burdensomeness of response, and the Federal Rules of Civil Procedure, which protect against attempts to obtain expert opinion testimony without compensation. Illustrative cases are described in which courts affirmed the confidentiality of peer review conducted by journals.