Show v. Ford Motor Co., slip op., Nos. 10-2428 & 10-2637 (Sep. 19, 2011), issued recently by the Seventh Circuit, provided Chief Judge Easterbrook with the opportunity to explore the question whether the need for expert testimony is one of substantive or procedural law, so that in a diversity case the court should follow state or federal law.

Ultimately, because neither party pressed the issue, but assumed that Illinois law applied in this design-defect products case, the court decided it under state law. But the discussion strongly suggests that the court would apply a federal law of expert testimony if the issue were pressed. Rules governing expert evidence “are found in the Federal Rules of Evidence, which like other rules adopted under the Rules Enabling Act control federal litigation notwithstanding contrary state law.” It’s an interesting issue to keep in mind—and one that could arise in a variety of settings.