According to Erie Railroad v. Tompkins and its progeny, a federal court interpreting state law must decide as the state's supreme court would. In this Article, I argue that a state court interpreting the law of a sister state is subject to the same obligation. It must decide as the sister state's supreme court would.
Horizontal Erie is such a plausible idea that one might think it is already established law. But the Supreme Court has in fact given state courts significant freedom to misinterpret sister-state law. And state courts have taken advantage of this freedom, by routinely presuming that the law of a sister state is the same as their own-often in the face of substantial evidence that the sister state's supreme court would decide differently. This presumption of similarity to forum law is particularly significant in nationwide class actions. A class will be certified, despite the fact that many states' laws apply to the plaintiffs' actions, on the ground that the defendant has failed to provide enough evidence to overcome the presumption that sister states' laws are the same as the forum's. I argue that this vestige of Swift v. Tyson needs to end.
Applying horizontal Erie to state courts is also essential to preserving federal courts' obligations under vertical Erie. If New York state courts presume that unsettled Pennsylvania law is the same as their own while federal courts in New York do their best to decide as the Pennsylvania Supreme Court would, the result will be the forum shopping and inequitable administration of the laws that are forbidden under Erie and its progeny. As a result, federal courts have often held that they too must employ the presumption of similarity to forum-state law, despite its conflict with their obligations under vertical Erie. Applying horizontal Erie to state courts solves this puzzle.