One of the most heated debates of the last two decades in U.S. legal academia focuses on customary international law’s domestic status after Erie Railroad v. Tompkins. At one end, champions of the “modern position” support customary international law’s (“CIL”) 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 legislative authorization. Scholars on both sides of the Erie debate also make claims about the sources judges reference when discerning CIL. They then use these claims to support their arguments regarding CIL’s domestic status. Interestingly, neither side of the debate has conducted an empirical analysis of what U.S. federal courts have actually done. This Article undertakes such an analysis and suggests that U.S. federal courts have, for the most part, behaved in a manner unanticipated by revisionists and modernists alike—the courts have followed themselves. After tracking the sources considered as evidence of CIL and cited in both pre-Erie and post-Erie case law, it turns out that, at all times before and after Erie in 1938, U.S. federal judges have relied primarily on domestic case law when making CIL determinations. Put starkly, the great Erie debate about CIL determinations in U.S. federal courts—and the authority the judiciary ought to attach to certain international sources—may have been occurring somewhat orthogonally to the fact that U.S. courts do not seem to pay much attention to these sources in practice.

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