In all cases, determining what privilege law applies should be near the top of every lawyer's to-do list. In federal question cases, federal courts apply federal privilege common law (essentially generic textbook privilege principles). But federal courts sitting in diversity must apply a choice of law analysis to determine which state's privilege law applies.
In Resolute Forest Products, Inc. v. Greenpeace International, Case No. 17-cv-02824-JST (KAW), 2022 U.S. Dist. LEXIS 54308, at *4 (N.D. Cal. Mar. 25, 2022), plaintiffs originally asserted "both federal question and diversity jurisdiction." The court dismissed the federal question RICO claim, so "the only remaining basis for jurisdiction is diversity jurisdiction" – requiring the court's application of "California statutory [privilege] law." Id. at *4-5. Six days later, in Steward v. Roppe Corp., Case No. 3:18-cv-2905, 2022 U.S. Dist. LEXIS 60658, at *15 (N.D. Ohio Mar. 31, 2022), plaintiffs "assert[ed] claims under the Americans with Disabilities Act and pendent claims under Ohio disability discrimination law." The court applied the majority rule that federal privilege common law applies "in federal question cases in which pendent state law claims are also raised." Id. at *15-16 (citation omitted).
Federal court practitioners must familiarize themselves with the choice of law analysis when asserting or defending both federal and state law claims.