Another question about the twin aims of Erie (the goal in diversity cases of uniformity between federal procedural common law and the law used in a forum state court, to avoid “forum shopping” and the “inequitable administration of the laws"). My question is this: Where do the twin aims come from? They do not seem constitutionally required. Congress has the power to regulate the procedure of federal courts in diversity cases even if vertical forum shopping results. Indeed, sometimes forum shopping is just what it wants. The twin aims must have a statutory or federal common law source.
John Hart Ely said the twin aims came from the Rules of Decision Act. I'm skeptical. First of all, there is a growing consensus that Brandeis's reading of the Act in Erie was mistaken. The Act did not prohibit or disfavor the creation of federal common law. Indeed, it can be understood as a direction to federal courts to apply federal common law.
But even if Brandeis's reading of the Act in Erie was right, the twin aims can't follow from the Act. This is what the Act says:
The laws of the several states, except where the Constitution or treaties of the United States or Acts of Congress otherwise require or provide, shall be regarded as rules of decision in civil actions in the courts of the United States, in cases where they apply.
Assume that Brandeis is right and the Act tells federal courts to favor state law over federal common law, if state law applies. The problem is that the twin aims recommend that forum state law be used even if it does not apply - that is, even if the forum state's supreme court would not hold that its law should be used in federal court. It looks like the twin aims recommend that state law be borrowed rather than applied. And nothing in the Rules of Decision Act suggests that state law (much less the law that would be used by a forum state court) should be borrowed by federal courts sitting in diversity.
So, where do the twin aims come from?
(Parallel posted on Prawfsblawg.)