The 10 Commandments cases also nicely illustrate the fundamental problem with the Supreme Court's "now you see it, now you don't" approach to looking to international sources for guidance on interpreting the U.S. Constitution. If the Court is going to engage in a free-range analysis of policy, history, psychology, etc., about the accommodation of religion in society and the public sphere, wouldn't the development and experience of Europe be at least as relevant here as in the other cases where the Court has looked to those sources for guidance? I can't see of any reason why world experience on interpreting the Establishment Clause would be any less relevant than for interpreting, say, the Cruel and Unusual Punishment Clause in the context of the juvenile death penalty. Furthermore, my impression is that public displays of religion are much more permissible in Europe, for instance, than what would be permitted here (not to mention Israel and Islamic societies). Maybe someone can explain to me why it would be relevant in interpreting one clause and not the other because I can't see the difference.
Leaving aside all of the intellectual arguments for whether the Court should or should not rely on international law for constitutional guidance, it is hard to escape the conclusion that the Court's periodic reliance on world legal opinion is purely strategic rather than sincere, perhaps to dress up the Court's personal predilections in the guise of legal authority. I am not aware of any principle that the Court has articulated which would permit litigants or the public to predict when international law or practice would be relevant to the interpretation of the U.S. Constitution. Nor am I aware of any scholar who has defended reliance on international sources who has tried to distinguish when international law is relevant to constitutional interpretation versus when it is not.
A reader has offered the amendment that the term "foreign law" rather than "international law" is the more accurate term to use here. I think the context makes clear what I had in mind, but if not, I offer this amendment as well.