One of the most heated debates of the last two decades in US legal academia centers on customary international law's domestic status after Erie Railroad v. Tompkins. At one end, champions of the "modern position" support CIL's wholesale incorporation into post-Erie federal common law. At the other end, "revisionists" argue that federal courts cannot apply CIL as federal law absent federal political branch authorization. Scholars on both sides of the Erie debate also make claims about what sources judges cite to when discerning CIL, which they then use to support their arguments regarding CIL's domestic status. Interestingly, neither side of this great debate has done anything in the way of empirically looking at what US federal courts actually do. In this article, we take a first cut at doing that, and what we find suggests that the US federal courts have, for the most part, been doing something that neither revisionism nor the modern position has focused on – following themselves. After tracking the sources cited to as evidence of CIL in both pre-Erie and post-Erie case law, it turns out that, at all times before and after 1938, US federal judges have relied primarily on domestic case law when making CIL determinations. Not only does this finding yield thought-provoking implications for the Erie debate, but it also forces all of us to circle back to the bigger questions about CIL: namely, what is customary international law, is it even legitimate "law", and if so, just how "international" does it have to be? 




G. Mitu Gulati & Nikki C. Gutierrez, Custom in Our Courts: Reconciling Theory with Reality in the Debate About Erie Railroad and Customary International Law, 27 Duke Journal of Comparative & International Law, 243–290 (2017).