bility of that provision to cases solely involving causes of action under state law and where the jurisdiction of the federal courts was based on diversity of citizenship, Justice Brandeis for the majority in Erie at 78 said: "there is no federal common law." In Erie, a state procedural rule which conflicted with a federal rule was applied for two reasons: (1) to preserve an appropriate allocation of sovereign authority between the state and federal governments; and (2) to prevent forum shopping by plaintiffs. Brown says that "Erie was a blatant example of successful federal state forum-shopping by a private law plaintiff, until Justice Brandeis thwarted it."
In Guaranty Trust Company v. York (1945) 326 U.S. 99, the Court upheld the use of a conflicting state statute of limitations in another diversity case and emphasized the importance of deterring forum-shopping and ensuring reasonable certainty and consistency in the interpretation of state substantive law, especially in cases where the choice between state and federal rules would be outcome-determinative. In the present case, the choice between state and federal rules of evidence on the admissibility of RMs expert testimony might well be outcome determinative. As discussed below, such testimony is clearly inadmissible under California rules of evidence but it might be admissible under the Federal Rules of Evidence. In the absence of Dr. Smith's testimony, plaintiff-appellant has no case. Beginning with Byrd v. Blue Ridge Electric Corporation (1958) 356 U.S. 525, the Supreme Court began to weigh the respective federal and state interests involved, but it has been reluctant to apply a conflicting federal rule when to do so would "present a usurpation of a regulatory prerogative [which] belongs to the state." Some confusion arose after Hanna v. Plumer (1985) 380 U.S. 525, when the Supreme Court began to the supremacy of federal rules of procedure in some diversity cases. However...