The Court of Appeals for the Tenth Circuit held that a court sitting in diversity still must consider substantive state policy when determining evidence admissibility. In Sims v. Great American Life Insurance Co., Tenth Cir. November 7, 2006 469 F.3d 870 the issue was whether plaintiff’s decedent’s death was as a result of an accident or as a result of suicide. Oklahoma has a statute directing that evidence of the use or non-use of seatbelts is inadmissible “in any civil suit in Oklahoma.” Defendant sought to introduce the evidence of non-use to reflect his mental state and his intent to commit suicide. His wife, the plaintiff, told the police that there was an argument and that when he left the house he mentioned something about driving off a cliff. The Court held that the Federal Rules of Evidence can give way to a substantive state policy and ruled that the district court should have admitted the evidence. The Court said:
“Great American does not argue that Sims was negligent for failing to wear his seat belt or otherwise insinuate fault for this omission. It argues he was suicidal. Whether a mandatory seat belt law exists in Oklahoma is irrelevant to this argument. In a negligence case, the existence of a seat belt law could be critical to a damages assessment because it imparts a duty on the injured party to wear a seat belt. See, e.g., Gardner ex rel. Gardner v. Chrysler Corp., 89 F.3d 729, 733 (10th Cir. 1996) (noting that the Kansas legislature modified earlier law in order to clarify the extent of the duty owed under a mandatory seat belt law). But here duty is not an element of the claim. Great American frankly is unconcerned whether Sims could have avoided his injuries had he been wearing his seat belt. Great American is only concerned with how his failure to wear a seat belt reflects on his mental state. In this sense, the evidence does not speak to the conduct of the driver but instead to the driver’s state of mind.
Because Great American introduced this evidence for purposes of showing Sim’s state of mind, not to insinuate fault, we hold that Oklahoma’s Mandatory Seat Belt Act is inapplicable to the present case, and the district court should have admitted this evidence at trial. Cf. Barron, 965 F.2d at 198-200 (holding that North Carolina’s seat belt exclusionary rule, while a substantive law, is inapplicable to the present case because the use of evidence “was not to show that [Plaintiff] failed to mitigate the consequences of the accident but to show that [Defendant] had been reasonable”).”