Jonah, I agree with your correspondents who have suggested that judicial decisions on common law questions in other common law countries are potnetially relevant to the resolution of common law questions in American courts, but this cannot excuse the reference to foreign court opinions by the Supreme Court. Common law questions almost exclusively arise in the context of state law, rather than federal law. As a result, the U.S. Supreme Court is rarely called upon to resulve common law questions. Rather, almost every Supreme Court case turns on either the Constitution or federal statutory law. In these contexts, the decisions of common law courts in other countries are irrelevant.

One could argue that some common law decisions pre-dating the Constitution’s ratification or predating the enactment of a particular statute may inform a particular question, but only if it would have informed the Constitution’s Framers or the statute’s drafters. So, even with this concession, the Supreme Court’s eagerness to cite foreign nations fails the test.

The only other context in which the Supreme Court would be justified in citing foreign law is where a case turns upon the interpretation of ambiguous treaty language. In such cases, the Court might be justified in considering how the treaty’s language has been interpreted by other signatories to the treaty. Again, however, this is a very small set of cases — I seem to remember one from last year involving airliner liability for harms to passengers on international flights — and cannot justify some of the current Justices fixation on foreign legal opinions.