"Looking at foreign law for support is like looking out over a crowd and picking out your friends."

So go Roberts's comments on the subject. Here's what a commenter at Althouse had to say:

T]he implied opprobrium in this text on the unwillingness of most American Judges to allow international law to bleed into constitutional interpretation, is predicated on the assumption that the use of foreign materials is relevant to the task of an American Judge. This predicate is highly dubious.

There are two great schools of thought regarding the American constitution and how it should be interpreted. One, subscribed to on the Supreme Court currently by Justices Scalia and Thomas, holds that in order for a constitution that is ratified by the people, and by its own terms can only be amended by their elected representatives (which Judges are not, and should not be), must be given a fair reading in light of what the document says, and what that text meant when it was adopted.

The other, "the living constitution", subscribed to by most of the other Justices, holds that in order for the constitution to remain meaningful to society, its provisions must be interpreted in light of "the evolving standards of decency of American society", meaning in effect that the Constitution has no fixed meaning, and that Chief Justice Marshall was wrong when he wrote in Marbury v. Madison that "The powers of the legislature are defined and limited; and that those limits may not be mistaken or forgotten, the constitution is written", and correct when he added that "[t]he distinction between a government with limited and unlimited powers is abolished, if those limits do not confine the persons on whom they are imposed."

In the interests of full disclosure, I am a subscriber to the former theory. But what's important is that, within either paradigm, it is hard to see what value foreign precedent might be. If you're an originalist, you're looking for what a statute or the constitution meant when they were adopted. Obviously, if you have that theory, subsequent foreign legal precedent is irrelevant, because it can't change the meaning of the text. But, even if you buy into the other school of thought, even if you're a living constitutionalist, what relevance would foreign law have to your view of your task? If you're looking to give the text the best meaning it could have, in light of the contemporary standards of decency in American society, what are you looking for? The standards of decency of AMERICAN society. What does looking to foreign court decisions tell you about the contemporary standards of decency in American society? Nothing.

So if you're an originalist, foreign law is irrelevant. If you're an honest living constitutionalist, foreign law is irrelevant. The only way it's useful, the only context in which it's relevant, is if you have a third approach to interpreting the law. And that approach, I fear, is exactly what the living constitution tends to be in practise: judicial paternalism. It is the approach that calls on a Judge to determine, "what is the best answer to this case? What do I think is the fairest outcome in this case? What is the best for society? And how can I remove the legal and precedential barriers to the imposition of that outcome?" Having asked those questions, the paternalist needs to write an opinion that looks like the product of a judge; and thus, they turn to doctrines like substantive due process, and increasingly, they turn to foreign law. They are, in the words of Clinton administration Solicitor-General Seth Waxman, "attracted by any notion, or principle, the logic of which carries us to a result we think is just."

And so it is. In the context of the modern court - and it is clearly on display in Roper v. Simmons and Lawrence v. Texas, for example - foreign sources are cited selectively to support the result desired by the author (in both examples above, Justice Kennedy; although Justices Breyer and Ginsburg most often fall back on foreign sources). In Printz, for example, Justice Breyer cited the example of Switzerland to show how Switzerland had approached the Federal system. This might be very valid if he was writing for a Swiss court, but America is not Switzerland! Maybe their system is better, I don't know - but the point is, their system isn't ours! Justice Breyer claims that "we might learn something" if we look at foreign precedent. That's true, as far as it goes; we might also learn something from taking a class in automobile engine maintenance, a PBS special on the chemical composition of Jupiter, a class in foreign & comparative law, or an hour watching Music TeleVision (strapped, presumably into a contraption similar to that used in A Clockwork Orange), but while these endeavors might all be very enlightening on some level, I would find discussion of those subjects somewhat out of place in a judicial opinion on the meaning of the constitution of the United States. "Interesting" is no more the criterion for "relevant" than "people feel passionately about this" is the criterion for "is this constitutional?"

The Constitution has a mechanism for changing its meaning; it is located in Article V, and it does not include the passing of laws by the legislature, or the precedents of courts American or otherwise. The Constitution also has a mechanism for importing foreign law into American law: the treaty process. Neither of these are to be found within the power of the federal courts; not only is the use of foreign materials irrelevant, but it is also borderline unconstitutional.

For a Judge in a common law jurisdiction, this might be a good idea; but in a system governed by a written instrument, it is indefensible. It is easy, to the point of lazy, to casually opine that the American legal system is insular. But what is missing, and what cannot be missing for the argument to be rescued from the depths of meaningless rhetoric, is a coherent explanation as to why foreign precedent should be of any relevance to the U.S. system.