Admissibility of Expert Testimony in Diversity Actions
A question often arises regarding whether state or federal law is applicable when determining the compentency or qualification of an expert to give testimony. The 6th Circuit largely answered this question in 2002 with its decision in Legg v. Chopra, 286 F.3d 286. The Court noted the common proposition that in federal diversity actions, state law governs substantive issues and federal law governs procedural issues. Accordingly, rules of evidence are generally deemed rules of procedure, and therefore, the Federal Rules of Evidence, rather than state evidentiary laws, are held to apply in federal diversity proceedings. For this reason, the 6th Circuit categorically held that the admissibility of expert testimony is a matter of federal, rather than state, law.
However, the Court also noted a limited exception where state evidentiary law is intimately intertwined with a state substantive rule. In Legg for example, the Court held that the Tennessee medical malpractice competency statute is linked to the substantive issue of “standard of care” and that it therefore remains applicable in diversity actions. In cases such as this, both state and federal law may be applicable to different aspects of the admissibility determination.