To prevent forum shopping of this sort, the courts generally apply state law in diversity cases. According to the clear language of the RDA , the federal Constitution, treaties, and constitutional statutes enacted by Congress, always take precedence, where relevant, over all state provisions. The interesting question, and one on which the RDA is silent, is what the federal court should do where there is no controlling constitutional or statutory provision , federal or state.
P sues D in a diversity action arising out of an automobile accident that took place in Kansas.
The Kansas courts apply common-law contributory negligence. The most important Supreme Court case in all of Civil Procedure is Erie Railroad v. Tompkins . The net result is that in diversity cases, the federal courts must apply state judge-made law on any substantive issue . Kansas law would be favorable to D. Swift v. Tyson , which would allow P to choose federal or state court in Kansas, whichever was more favorable to him, would thus allow P to profit at D's expense. P, a Pennsylvania citizen, was injured while walking on the right of way maintained by D, a New York railroad.