Problems…

1)    How far can a state go in binding up its procedural rules with its causes of action? If it bound up trivial procedural rules with its cause of action, would it really be the case that federal courts entertaining the action could not apply a contrary federal procedural common law rule? That seems unlikely.

2)    SCOTUS has never answered whether it is constitutional for a state court to favor its procedural law over a sister state’s procedural rule that is bound up with the sister state’s cause of action. If it can, why can’t federal courts constitutionally do the same? If there is a difference between federal court’s and state court’s powers, a puzzle could arise. Assume a federal court in New York entertains a Massachusetts negligence action and state courts in New York would ignore the Massachusetts burden of proof rule. If the federal court respects the rule, there will be vertical disuniformity between a New York federal court’s treatment of the case and a New York state court’s treatment.

3)    What about the procedural rule of another nation that is bound up with the other nation’s cause of action? Does the Constitution require respect for that bound-up rule too?

4)    What about rules that a state wants applied in federal court but does not bind up with its cause of action? For example, Massachusetts may want its attorney-client privilege rule to apply in federal (or sister-state) court concerning attorney-client communications that occurred in Massachusetts, even when the federal (or sister-state) court is entertaining a non-Massachusetts cause of action. What are the federal court’s powers to create procedural common law that displace the Massachusetts law in such a case?

5) In addition, may the bare fact that the federal court is within a state give that state a sufficient regulatory interest to extend the state's procedural rule to that federal court (even when the federal court is not entertaining the state's causes of action)? For example, could Massachusetts extend a Massachusetts procedural rule to all federal courts in Massachusetts (even when those courts are not entertaining Massachusetts causes of action)? And if it could, is this the sort of interest that a federal court would have to respect?