Lawrence v. Texas, invaliding state sodomy laws, was one of the decisions in which the U.S. Supreme Court cited a foreign case, and a comment asks whether I think the decision was wrong. I think it would be possible to write a respectable, narrow opinion invalidating those laws, which have come to seem ridiculous, though so inconsequential are they that it hardly seems worth the bother. The trouble is that Justice Kennedy wrote a very broad opinion, with language that armed the Massachusetts Supreme Judicial Court to create a constitutional right of gay marriage, setting off a firestorm. Many liberals now recognize that the Massachusetts court made a serious political mistake. The judges of that court (the judges in the majority, that is) might say that law must not bend to politics, but that would be a ridiculous statement when the issue is gay marriage. Neither the U.S. nor the Massachusetts constitution speaks to gay marriage. The decision that there shall be a right to such marriage is a political decision, in the sense that it rests--it has to rest, there is no other foundation for it--on the ideology, values, temperament, and so forth of the judge, rather than on some "observer independent" source of guidance. When such a decision is contrary to strong public opinion, its undemocratic character is unmistakable, and the decision arouses indignation and strengthens the opponents of (a certain kind of) judicial activism.
In light of several of the comments, I need to clarify what I mean by saying that judges can properly (in my view) cite foreign cases as source of relevant information, but not as precedents. What I mean by citing a case as a precedent is recognizing that its mere existence, as distinct from the cogency of its reasoning or the relevance of information revealed in it, is a reason for deciding the present case the same way: "a" reason, not necessarily the reason, or a controlling reason. No one supposes that a foreign decision would be a binding precedent on an issue of domestic law. My target is citing a foreign decision that, say, announces a constitutional right to gay marriage and saying or implying that the bare fact that a respectable foreign court has announced such a right is a reason why we should do likewise. That is to accord authority to a foreign decision, rather than to treat such a decision as one might treat a law review article--as a source, possibly, of excellent arguments or persuasive facts, but not as an authority.
One commenter asks, wouldn't it have been a good thing if, before the Civil War, the Supreme Court had taken note of the fact that the other nations of what we regarded as the civilized world had outlawed slavery? Could not the Court have used that international consensus to outlaw slavery in the United States? It could not have, because the preservation of slavery was the essence of the compromise that enabled the U.S. Constitution to be ratified, and because a decision outlawing slavery would have precipitated the secession of the southern states even quicker than the election of Lincoln did--he campaigned only to limit the spread of, and not to abolish, slavery. Abolition had to await the Emancipation Proclamation (of uncertain constitutionality) and the enactment of the Thirteenth Amendment.
Slavery is a good example of a practice believed to be contrary to natural law; and one comment notes the affinity between the concept of natural law and the idea of international legal norms as a source of guidance in interpreting local constitutional law. I have a qualified sympathy for the idea of natural law. If a novel case arises--one that cannot be decided by subsumption under clear statutory or constitutional language or precedent--the judge will have to look elsewhere, and if one wants to call the elsewhere "natural law" I have no strong objection, as long as it is understood not to be Thomas Aquinas's concept of natural law. The vaguer, less consistent, more anachronistic, more gap-ridden, and more absurd the orthodox materials of judicial decision (constitutional and statutory text, precedent, etc.) are, the more the judges will be on their own in deciding cases. And that is the situation in which American judges, especially appellate judges, often find themselves. Whether they draw on economic theory or political principles, or on some inarticulate notion of what is fair or right, to decide cases in the broad open area of American law, they will be going outside the positive law in any useful sense of that term--and, as I say, if you want to call where they are going natural law, that is all right with me. Yet in the context of pre-Civil War U.S. law, natural-law arguments for the unconstitutionality of slavery would have collapsed in the face of the constitutional text and history and the political balance of power. And given the cultural heterogeneity of the world, we must recognize, in very sharp contrast to advocates of Catholic natural law, that natural law is primarily a national rather than an international body of thought.
Several comments ask how one state should treat the decisions of another state, and how a court should treat its old decisions. I recognize analogies here to the citation of foreign courts, but the differences seem to me more important. A court in one state will not feel itself bound by a decision of another state, but will recognize that decision as an authority, and for the very good reason that the legal cultures of the states are, today at any rate, pretty similar in most areas. Similarly, a court will not consider it absolutely bound by its old decisions, but will treat them as authorities, in part because there is a considerable continuity in legal culture within a state over time, and in part because of the reliance interests that judicial decisions generate.