Where do the twin aims come from? They are not constitutionally compelled. It would not be unconstitutional for federal courts to make federal common law in a way that led to forum shopping between federal and forum state courts. The usual answer is that their source is the Rules of Decision Act, 28 U.S.C. § 1652. I disagree. The most likely source is the statute that gives the federal court jurisdiction. The usual theory about why Congress allowed federal courts to entertain diversity cases is that it protects out of state parties from bias in the state court system. If federal courts aggressively created federal procedural common law that differed from forum state law, that would frustrate the purposes of diversity. An out of state plaintiff who was worried about state court bias might nevertheless refrain from suing in federal court because the federal procedural common law would disfavor him compared to the procedural rules in state court. Conversely an out of state plaintiff who was not worried about state court bias might choose federal court because he liked the federal procedural common law there, compared to forum state procedural law, thereby wasting federal resources on cases that didn’t implicate the purposes of diversity. I discuss this in my twin aims article (pp. 1887-1904).

Vertical uniformity and the twin aims would not be an issue however concerning federal statutes and Federal Rules of Civil Procedure. The reason is that these are congressional directives or are created pursuant to congressional directives. Although federal statutes or FRCPs that diverge from forum state law could frustrate the purposes of diversity, Congress is free to frustrate the purposes of earlier legislation through subsequent legislation if it wishes.